COURSE TITLE:Fundamentals and Control of D.C. Generators and Motors

DUTY TITLE: Operation of D.C. Motors & Generators

POS #:1800

TASK:Operation of a D.C. Motor

PURPOSE:To Understand the Concept, Control, and Troubleshooting Techniques of the Various Types of D.C. Motors.

TASKS:

1801

Demonstrate knowledge of basic direct current circuits.

1802

Explain the theory of operation of a direct current motor.

1803

Operate and test a direct current motor.

1804

Operate and test a direct current shunt motor.

1805

Perform calculations for horsepower, speed and torque for direct current motors.

1806

Measure performance and efficiency of a direct current motor.

1807

Demonstrate knowledge of technical terms and units used in a basic direct current circuit.

1808

Demonstrate knowledge of the basic operations of variable speed control for direct current motors.

REVISION: 2019

ENGLISH LANGUAGE ARTS

CC.1.2.11-12.J Acquire and use accurately general academic and domain-specific words and phrases, sufficient for reading, writing, speaking, and listening at the college and career readiness level; demonstrate independence in gathering vocabulary knowledge when considering a word or phrase important to comprehension or expression

CC.1.3.11-12.I Determine or clarify the meaning of unknown and multiple-meaning words and phrases based on grade level reading and content, choosing flexibly from a range of strategies and tools.

MATH

CC.2.1.HS.F.4 Use units as a way to understand problems and to guide the solution of multi-step problems.

CC.2.1.HS.F.6 Extend the knowledge of arithmetic operations and apply to complex numbers.

READING IN SCIENCE & TECHNOLOGY

CC.3.5.11-12.B. Determine the central ideas or conclusions of a text; summarize complex concepts, processes, or information presented in a text by paraphrasing them in simpler but still accurate terms.

CC.3.5.11-12.C. Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks; analyze the specific results based on explanations in the text.

WRITING IN SCIENCE & TECHNOLOGY

CC.3.6.11-12.E. Use technology, including the Internet, to produce, publish, and update individual or shared writing products in response to ongoing feedback, including new arguments or information.

CC.3.6.11-12.F. Conduct short as well as more sustained research projects to answer a question (including a self generated question) or solve a problem; narrow or broaden the inquiry when appropriate; synthesize multiple sources on the subject, demonstrating understanding of the subject under investigation

*ACADEMIC STANDARDS *

READING, WRITING, SPEAKING & LISTENING

1.1.11.A Locate various texts, assigned for independent projects before reading.

1.1.11.D Identify strategies that were most effective in learning

1.1.11.E Establish a reading vocabulary by using new words

1.1.11.F Understanding the meaning of, and apply key vocabulary across the various subject areas

1.4.11.D Maintain a written record of activities

1.6.11.A Listen to others, ask questions, and take notes

MATH

2.2.11.A Develop and use computation concepts

2.2.11.B Use estimation for problems that don’t need exact answers

2.2.11.C Constructing and applying mathematical models

2.2.11.D Describe and explain errors that may occur in estimates

2.2.11.E Recognize that the degree of precision need in calculating

2.3.11.A Selecting and using the right units and tools to measure precise measurements

2.5.11.A Using appropriate mathematical concepts for multi-step problems

2.5.11.B Use symbols, terminology, mathematical rules, Etc.

2.5.11.C Presenting mathematical procedures and results

SCIENCE

3.1.12.A Apply concepts of systems, subsystems feedback and control to solve complex technological problems

3.1.12.B Apply concepts of models as a method predict and understand science and technology

3.1.12.C Assess and apply patterns in science and technology

3.1.12D Analyze scale as a way of relating concepts and ideas to one another by some measure

3.1.12.E Evaluate change in nature, physical systems and man-made systems

3.2.12.A Evaluate the nature of scientific and technological knowledge

3.2.12.B Evaluate experimental information for appropriateness

3.2.12.C Apply elements of scientific inquiry to solve multi – step problems

3.2.12.D Analyze the technological design process to solve problems

3.4.12.A Apply concepts about the structure and properties of matter

3.4.12.B Apply energy sources and conversions and their relationship to heat and temperature

3.4.12.C Apply the principles of motion and force

3.8.12.A Synthesize the interactions and constraints of science

3.8.12.B Use of ingenuity and technological resources to solve specific societal needs and improve the quality of life

3.8.12.C Evaluate the consequences and impacts of scientific and technological solutions

ECOLOGY STANDARDS

4.2.10.A Explain that renewable and non-renewable resources supply energy and material.

4.2.10.B Evaluate factors affecting availability of natural resources.

4.2.10.C Analyze the use of renewable and non-renewable resources.

4.2.12.B Analyze factors affecting the availability of renewable and non-renewable resources.

4.3.10.A Describe environmental health issues.

4.3.10.B Explain how multiple variables determine the effects of pollution on environmental health, natural processes and human practices.

4.3.12.C Analyze the need for a healthy environment.

4.8.12.A Explain how technology has influenced the sustainability of natural resources over time.

CAREER & EDUCATION

13.1.11.A Relate careers to individual interest, abilities, and aptitudes

13.2.11.E Demonstrate in the career acquisition process the essential knowledge needed

13.3.11.A Evaluate personal attitudes that support career advancement

ASSESSMENT ANCHORS

M11.A.3.1.1 Simplify expressions using the order of operations

M11.A.2.1.3 Use proportional relationships in problem solving settings

M11.A.1.2 Apply any number theory concepts to show relationships between real numbers in problem solving

STUDENT

The student will be able to identify, connect and control the output of all three types of D.C. motors. (Series-Shunt-Compound)

TERMINAL PERFORMANCE OBJECTIVE

Given all the electrical tools and materials required, the student will be able to identify, connect and control the output of all three types of D.C. motors. (Series-Shunt-Compound)

SAFETY

· Always wear safety glasses when working in the shop.

· Always check with the instructor before turning power on.

· Always use tools in the correct manner.

· Keep work area clean and free of debris.

· Make sure guard is over motor couplings.

· Never wire a project without the correct wiring diagram.

RELATED INFORMATION

1. Attend lecture by instructor.

2. Obtain handout.

3. Review chapters in textbook.

4. Define vocabulary words.

5. Complete all questions in this packet.

6. Complete all projects in this packet.

7. Complete K-W-L Literacy Assignment by Picking an Article From the

“Electrical Contractor” Magazine Located in the Theory Room. You can pick any article you feel is important to the electrical trade.

EQUIPMENT & SUPPLIES

1. Safety glasses 11. Various light bulbs

2. Hammer 12. Light fixture bank

3. Screw driver 13. Alligator clips

4. Awl 14. D.C. motor module

5. Wire strippers 15. D.C. generator

6. Side cutters 16. Prime mover (A.C. or D.C. motor)

7. Cable rippers 17. Couplings

8. Lineman pliers 18. Power supply

9. Needle nose pliers 19. THHN wire

10. Multimeter 20. In line meters

VOCABULARY

CC.1.3.11-12.I Determine or clarify the meaning of unknown and multiple-meaning words and phrases based on grade level reading and content, choosing flexibly from a range of strategies and tool

CC.3.5.11-12.D. Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific or technical context relevant to grades 11–12 texts and topics.

· Stator:

· Riser:

· C.E.M.F:

· Rheostat:

· Shunt field:

· Laminations:

· Neutral plane:

· Armature loop:

· Permanent magnet:

· E.M.F:

· Electromagnet:

· Brushless DC motors:

· Compound motor:

· Constant-speed motors:

· Counter-electromotive force (CEMF) (back-EMF):

· Cumulative-compound motors:

· Differential-compound motors:

· Field-loss relay:

· Motor:

· Permanent magnet motors:

· Printed-circuit motor:

· Series motor:

· Servomotors:

· Shunt motor:

· Speed regulation:

· Torque:

OUTLINE

DC Motor Principles

Compare DC motors with DC generators, pointing out how similar they are. Explain how the DC motor functions

Torque

Describe what torque is and describe how it is formed by the magnetic .eld of the pole pieces and the magnetic field of the loop or armature. Discuss the factors that determine how much torque is produced.

Increasing the Number of Loops

Discuss the advantage of armatures constructed with many turns of wire per loop and many loops.

The Commutator

Compare the use of the commutator in a DC generator and its use in a DC motor. Emphasize that in the DC motor, it forces the direction of current flow to remain constant through sections of the rotating armature.

Shunt Motors

Describe the construction of the shunt motor and discuss why it is such a good motor to use when constant speed is required.

Counter-Electromotive Force

Explain what CEMF is and what three factors it is proportional to.

Speed-Torque Characteristics

Discuss what happens when a DC motor is started without a load. Then discuss what happens when a load is added to the motor. Be sure to talk about the level of torque needed to overcome the various types of losses.

Speed Regulation

Have students write the speed regulation rule into their notes. Be sure that students understand why a lower armature resistance will result in better speed regulation.

Series Motors

Describe the DC series motor and compare the characteristics of the series motor with those of the DC shunt motor. Be sure students note the relationship of torque to current in the series motor.

Series Motor Speed Characteristics

Emphasize the fact that series motors have no speed limitations and, therefore, need to be coupled with a load at all times. Explain what can happen if a series motor is run without a load. Give examples of where series motors are used.

Compound Motors

Explain that just like with the compound generator, the compound motor is a combination of the shunt and series motors. Discuss the benefits gained by utilizing the best of each motor, which makes the compound motor the most widely used in industry. Go through the steps for setting up a compound motor, and emphasize how important it is to not set up a differential-compound motor. Demonstrate how to check for rotation direction on a display motor.

Terminal Identification for DC Motors

Referring to Figure 30-12, and displaying a DC motor, guide students through the various terminal identifications.

Determining the Direction of Rotation of a DC Motor

Explain how reversing the connections of the armature leads or field leads changes the rotation direction of the motor. Also, discuss how this is done differently in a compound motor.

Speed Control

Discuss how reducing the armature current, resulting in less torque, causes the speed to reduce. Then, describe ways of reducing the armature. Also, discuss why some ways of reducing the armature are not the best.

The Field-Loss Relay

Explain how the field loss relay is used to protect the compound motor and the load. Also, discuss the use of two separate shunt fields in compound motors. Have students note that most large DC motors have voltage applied to the shunt field at all times to prevent moisture build-up.

Horsepower

Review the basic units of power and have students add these to their notes, along with the horsepower formula.

Work out the two example problems on the board.

Brushless DC Motors

Describe this motor and compare it to previously studied motors. Explain the use of the converter in supplying power to the brushless motor, and discuss the difference between sine waves and trapezoidal waves. Explain the use of multiple stator poles to achieve a low speed and high torque output.

Inside-Out Motors

Describe and display a rotor wound inside. Make sure students note that motors with a large amount of inertia exhibit superior speed regulation.

Differences between Brush-Type and Brushless Motors

Discuss the advantages and disadvantages of each type of motor.

Converters

Elaborate on the use of converters.

Permanent Magnet Motors

Explain the make-up of a permanent magnet motor and discuss why it is more efficient than a field wound motor.

Operating Characteristics

Discuss how PM motors function, and compare them to separately excited shunt motors.

DC Servomotors

Describe what servomotors are and how they are used.

DC ServoDisc® Motors

Describe this printed-circuit motor and compare it to a servomotor, emphasizing the difference in the armature.

Explain how the torque and tangential forces are produced.

Characteristics of ServoDisc® Motors

Explain why the absence of iron in the armature of the disc motor eliminates the cogging effect that the PM DC motors experience. Discuss pulse-width modulation and how it is used.

The Right-Hand Motor Rule

Describe how to use the right hand to determine the direction of flow of thrust, field flux, and current through the armature, just as you did previously with the left-hand generator rule. Compare this to that left-hand rule, and make sure students don’t get the two confused.

Unit Round Up

Go over the summary together, checking for understanding, as you have students elaborate on each point in the summary.

Have students complete the review questions on their own, and go over these during your next class session.

ANSWER TO PRACTICAL APPLICATIONS

The motor has probably been connected differential compound instead of cumulative compound. To test the motor:

1. Uncouple the motor from the load

2. Disconnect the shunt field winding (F1 and F2)

3. Momentarily apply power (bump the motor) and check for direction of rotation. The motor is now a series motor so the application of power must be short. If the motor turns in the incorrect direction, change S1 and

S2.

4. Reconnect the shunt field winding.

5. Apply power to the motor. If it turns in the incorrect direction change F1 and F2. When a compound motor

is operated at no load, the shunt field should determine the direction of rotation.

6. When the motor turns in the same direction for both tests it is connected cumulative compound.

PROCEDURE

CC.2.1.HS.F.4 Use units as a way to understand problems and to guide the solution of multi-step problems.

CC.3.5.11-12.C. Follow precisely a complex multistep procedure when carrying out experiments, taking measurements, or performing technical tasks; analyze the specific results based on explanations in the text.

1.6.11A Listen to others, ask questions, and take notes

3.4.12.B Apply energy sources and conversions and their relationship to heat and temperature

EXPERIMENT # 1 D.C. SERIES MOTOR

1. Connect D.C. series motor to work station. (Diagram “A”)

2. Connect D.C. shunt motor to the generator (Prime mover).

3. Connect variable D.C. power supply to the motor so you can variate the speed of the motor.

4. Connect a D.C. ammeter in series with the armature and the field.

5. Turn on power and notice that you can vary the speed of the motor by increasing or decreasing the D.C. power supply.

6. Adjust the speed of the motor by 10 levels to the highest speed, and record all of the readings (Voltage and Current).

7. Explain why the current and voltages are increasing with the RPM increase:

8. Reverse the motor by either switching the field or the armature leads. (Diagram “B”)

9. Install the aluminum coupling on the shaft of the D.C. motor.

10. Install the prony brake on the work station make sure the coupling fits inside the belt.

(TO AVOID OVERHEATING OF THE COUPLING PUT A SMALL AMOUNT OF WATER INSIDE THE COUPLING.)

11. Put a small load on the motor using the prony brake and repeat step #6, record your readings:

12. Explain why the readings are different:

13. Put the input voltage at a constant speed and increase the load slightly. Record your readings:

14. Explain why the load affected the current of the motor, and what area it affected. (Field current or Armature current).

DIAGRAM “A” DIAGRAM “B”

EXPERIMENT # 2 D.C. SHUNT MOTOR

PROCEDURE

1. Connect the D.C. shunt motor in the self excited form. (Diagram “A”)

2. Connect the power supply to the motor.

3. Install ammeters in series with the fields and the armature.

4. Turn on power and notice that you can vary the speed of the motor by increasing or decreasing the D.C. power supply.

5. Adjust the speed of the motor by 10 levels to the highest speed, and record all of the readings (Voltage and Current).

6. Explain what the currents and voltages are doing with the RPM increase:

7. Reverse the motor by either switching the field or the armature leads.

8. Install the aluminum coupling on the shaft of the D.C. motor.

9. Install the prony brake on the work station make sure the coupling fits inside the belt. (TO AVOID OVERHEATING OF THE COUPLING PUT A SMALL AMOUNT OF WATER INSIDE THE COUPLING.)

10. Put a small load on the motor using the prony brake and repeat step #5, record your readings:

11. Explain why the readings are different:

12. Put the input voltage at a constant speed and increase the load slightly. Record your readings:

13. Explain why the load affected the current of the motor, and what area it affected. (Field current or Armature current).

14. Connect the motor as a separately excited control. (Diagram “B”).

15. Repeat step # 5, was there any difference in the readings using this type of control?

16. What are some advantages and disadvantages of using a separately excited type of control?

DIAGRAM “A”

DIAGRAM “B”

EXPERIMENT # 3 D.C. COMPOUND MOTOR

PROCEDURE

1. Connect the D.C. compound motor in the self excited form. (Diagram “A”)

2. Connect the power supply to the motor.

3. Install ammeters in series with the fields and the armature.

4. Turn on power and notice that you can vary the speed of the motor by increasing or decreasing the D.C. power supply.

5. Adjust the speed of the motor by 10 levels to the highest speed, and record all of the readings (Voltage and Current).

6. Explain what the currents and voltages are doing with the RPM increase:

7. Reverse the motor by either switching the field or the armature leads.

8. Install the aluminum coupling on the shaft of the D.C. motor.

9. Install the prony brake on the work station make sure the coupling fits inside the belt. (TO AVOID OVERHEATING OF THE COUPLING PUT A SMALL AMOUNT OF WATER INSIDE THE COUPLING.)

10. Put a small load on the motor using the prony brake and repeat step #5, record your readings:

11. Explain why the readings are different:

12. Put the input voltage at a constant speed and increase the load slightly. Record your readings:

13. Explain why the load affected the current of the motor, and what area it affected. (Field current or Armature current).

14. Hook the motor up as a separately excited control. (Diagram “B”).

15. Repeat step # 5, was there any difference in the readings using this type of control?

16. What are some advantages and disadvantages of using a separately excited type of control on this unit?

DIAGRAM “A”

DIAGRAM “B”

CC.3.5.11-12.B. Determine the central ideas or conclusions of a text; summarize complex concepts, processes, or information presented in a text by paraphrasing them in simpler but still accurate terms.

CC.3.6.11-12.E. Use technology, including the Internet, to produce, publish, and update individual or shared writing products in response to ongoing feedback, including new arguments or information.

3.1.12.B Apply concepts of models as a method predict and understand science and technology

ANSWER THE FOLLOWING QUESTIONS

1. Why would the operation of a single coil armature be “jerky”?

2. Name the three parts of a D.C. motor:

3. How is a megger used to test a D.C. motor?

4. What first item to receive power in a D.C. motor to start it?

5. What is the last item to have power removed from the D.C. motor to stop it?

6. Explain the left hand rule?

7. What are the two ways to change direction of a D.C. motor?

8. What would happen if the fields were lost while the motor was running at full RPM? (EXPLAIN!)

WRITE A SUMMARY ON THE ADVANTAGES AND DISADVANTAGES OF USING D.C. MOTORS, GIVE EXAMPLES.

REFERENCE PAGES

Nikola Tesla (Serbian Cyrillic: Никола Тесла) (10 July 1856 – 7 January 1943) was an inventor, physicist, mechanical engineer, and electrical engineer. Born in Smiljan, Croatian Krajina, Military Frontier, he was an ethnic Serb subject of the Austrian Empire and later became an American citizen. Tesla is best known for his many revolutionary contributions to the discipline of electricity and magnetism in the late 19th and early 20th century. Tesla's patents and theoretical work formed the basis of modern alternating current electric power (AC) systems, including the polyphase power distribution systems and the AC motor, with which he helped usher in the Second Industrial Revolution. Contemporary biographers of Tesla have deemed him "the man who invented the twentieth century" and "the patron saint of modern electricity."

After his demonstration of wireless communication (radio) in 1893 and after being the victor in the "War of Currents", he was widely respected as America's greatest electrical engineer. Much of his early work pioneered modern electrical engineering and many of his discoveries were of groundbreaking importance. During this period, in the United States, Tesla's fame rivaled that of any other inventor or scientist in history or popular culture, but due to his eccentric personality and unbelievable and sometimes bizarre claims about possible scientific and technological developments, Tesla was ultimately ostracized and regarded as a "mad scientist". Never having put much focus on his finances, Tesla died impoverished at the age of 86.

The SI unit measuring magnetic flux density or magnetic induction (commonly known as the magnetic field ), the tesla, was named in his honor (at the Conférence Générale des Poids et Mesures, Paris, 1960).

Aside from his work on electromagnetism and engineering, Tesla is said to have contributed in varying degrees to the establishment of robotics, remote control, radar and computer science, and to the expansion of ballistics, nuclear physics, and theoretical physics. In 1943, the Supreme Court of the United States credited him as being the inventor of the radio. Many of his achievements have been used, with some controversy, to support various pseudoscience’s, UFO theories, and early new age occultism. Tesla is honored in Serbia and Croatia, as well as in Czech Republic (he was awarded the highest order of the White Lion by Czechoslovakia) and in unofficial ways in his adopted home, the United States.

NICOLA TESLA

(10 July 1856 – 7 January 1943)

History and development

The principle of conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by the British scientist Michael Faraday in 1821 and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool of mercury. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine (salt water) is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called homopolar motors. A later refinement is the Barlow's Wheel. These were demonstration devices, unsuited to practical applications due to limited power.

The first commutator-type direct-current electric motor capable of a practical application was invented by the British scientist William Sturgeon in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor made with the intention of commercial use was built by the American Thomas Davenport and patented in 1837. Although several of these motors were built and used to operate equipment such as a printing press, due to the high cost of primary battery power, the motors were commercially unsuccessful and Davenport went bankrupt. Several inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues with primary battery power. No electricity distribution had been developed at the time. Like Sturgeon's motor, there was no practical commercial market for these motors.

The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the first electric motor that was successful in the industry.

In 1888 Nikola Tesla invented the first practicable AC motor and with it the polyphase power transmission system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse company.

DC motors

A DC motor is designed to run on DC electric power. Two examples of pure DC designs are Michael Faraday's homopolar motor (which is uncommon), and the ball bearing motor, which is (so far) a novelty. By far the most common DC motor types are the brushed and brushless types, which use internal and external commutation respectively to create an oscillating AC current from the DC source -- so they are not purely DC machines in a strict sense.

Brushed DC motors

The classic DC motor design generates an oscillating current in a wound rotor with a split ring commutator, and either a wound or permanent magnet stator. A rotor consists of a coil wound around a rotor which is then powered by any type of battery.

Brushless DC motors

Many of the limitations of the classic commutator DC motor are due to the need for brushes to press against the commutator. This creates friction. At higher speeds, brushes have increasing difficulty in maintaining contact. Brushes may bounce off the irregularities in the commutator surface, creating sparks. This limits the maximum speed of the machine. The current density per unit area of the brushes limits the output of the motor. The imperfect electric contact also causes electrical noise. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance. The commutator assembly on a large machine is a costly element, requiring precision assembly of many parts.

These problems are eliminated in the brushless motor. In this motor, the mechanical "rotating switch" or commutator/brushgear assembly is replaced by an external electronic switch synchronised to the rotor's position. Brushless motors are typically 85-90% efficient, whereas DC motors with brushgear are typically 75-80% efficient.

Midway between ordinary DC motors and stepper motors lies the realm of the brushless DC motor. Built in a fashion very similar to stepper motors, these often use a permanent magnet external rotor, three phases of driving coils, one or more Hall effect sensors to sense the position of the rotor, and the associated drive electronics. The coils are activated, one phase after the other, by the drive electronics as cued by the signals from the Hall effect sensors. In effect, they act as three-phase synchronous motors containing their own variable-frequency drive electronics. A specialized class of brushless DC motor controllers utilize EMF feedback through the main phase connections instead of Hall effect sensors to determine position and velocity. These motors are used extensively in electric radio-controlled vehicles. When configured with the magnets on the outside, these are referred to by modelists as outrunner motors.

Brushless DC motors are commonly used where precise speed control is necessary, computer disk drives or in video cassette recorders the spindles within CD, CD-ROM (etc.) drives, and mechanisms within office products such as fans, laser printers and photocopiers. They have several advantages over conventional motors:

Compared to AC fans using shaded-pole motors, they are very efficient, running much cooler than the equivalent AC motors. This cool operation leads to much-improved life of the fan's bearings.

Without a commutator to wear out, the life of a DC brushless motor can be significantly longer compared to a DC motor using brushes and a commutator. Commutation also tends to cause a great deal of electrical and RF noise; without a commutator or brushes, a brushless motor may be used in electrically sensitive devices like audio equipment or computers.

The same Hall effect sensors that provide the commutation can also provide a convenient tachometer signal for closed-loop control (servo-controlled) applications. In fans, the tachometer signal can be used to derive a "fan OK" signal.

The motor can be easily synchronized to an internal or external clock, leading to precise speed control.

Brushless motors have no chance of sparking, unlike brushed motors, making them better suited to environments with volatile chemicals and fuels.

Brushless motors are usually used in small equipment such as computers and are generally used to get rid of unwanted heat.

They are also very quiet motors which is an advantage if being used in equipment that is affected by vibrations.

Modern DC brushless motors range in power from a fraction of a watt to many kilowatts. Larger brushless motors up to about 100 kW rating are used in electric vehicles. They also find significant use in high-performance electric model aircraft.

Universal motors

A variant of the wound field DC motor is the universal motor. The name derives from the fact that it may use AC or DC supply current, although in practice they are nearly always used with AC supplies. The principle is that in a wound field DC motor the current in both the field and the armature (and hence the resultant magnetic fields) will alternate (reverse polarity) at the same time, and hence the mechanical force generated is always in the same direction. In practice, the motor must be specially designed to cope with the AC current (impedance must be taken into account, as must the pulsating force), and the resultant motor is generally less efficient than an equivalent pure DC motor. Operating at normal power line frequencies, the maximum output of universal motors is limited and motors exceeding one kilowatt are rare. But universal motors also form the basis of the traditional railway traction motor in electric railways. In this application, to keep their electrical efficiency high, they were operated from very low frequency AC supplies, with 25 Hz and 16 2/3 hertz operation being common. Because they are universal motors, locomotives using this design were also commonly capable of operating from a third rail powered by DC.

The advantage of the universal motor is that AC supplies may be used on motors which have the typical characteristics of DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and short life problems caused by the commutator. As a result such motors are usually used in AC devices such as food mixers and power tools which are used only intermittently. Continuous speed control of a universal motor running on AC is very easily accomplished using a thyristor circuit, while stepped speed control can be accomplished using multiple taps on the field coil. Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with the motor (causing the motor to run on half-wave rectified AC).

Universal motors can rotate at relatively high revolutions per minute (rpm). This makes them useful for appliances such as blenders, vacuum cleaners, and hair dryers where high-speed operation is desired. Many vacuum cleaner and weed trimmer motors exceed 10,000 rpm, Dremel and other similar miniature grinders will often exceed 30,000 rpm. Motor damage may occur due to overspeed (rpm in excess of design specifications) if the unit is operated with no significant load. On larger motors, sudden loss of load is to be avoided, and the possibility of such an occurrence is incorporated into the motor's protection and control schemes. Often, a small fan blade attached to the armature acts as an artificial load to limit the motor speed to a safe value, as well as provide cooling airflow to the armature and field windings.

With the very low cost of semiconductor rectifiers, some applications that would have previously used a universal motor now use a pure DC motor, sometimes with a permanent magnet field.

AC motors

In 1882, Nikola Tesla identified the rotating magnetic field principle, and pioneered the use of a rotary field of force to operate machines. He exploited the principle to design a unique two-phase induction motor in 1883. In 1885, Galileo Ferraris independently researched the concept. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.

Introduction of Tesla's motor from 1888 onwards initiated what is sometimes referred to as the Second Industrial Revolution, making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system, also of Tesla's invention (1888).[1] Before the invention of the rotating magnetic field, motors operated by continually passing a conductor through a stationary magnetic field (as in homopolar motors).

Tesla had suggested that the commutators from a machine could be removed and the device could operate on a rotary field of force. Professor Poeschel, his teacher, stated that would be akin to building a perpetual motion machine.[2] Tesla would later attain U.S. Patent 0,416,194 , Electric Motor (December 1889), which resembles the motor seen in many of Tesla's photos. This classic alternating current electro-magnetic motor was an induction motor.

Michail Osipovich Dolivo-Dobrovolsky later invented a three-phase "cage-rotor" in 1890. This type of motor is now used for the vast majority of commercial applications.

Components

1. A typical AC motor consists of two parts:

2. An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and;

An inside rotor attached to the output shaft that is given a torque by the rotating field.

Slip ring

The slip ring or wound rotor motor is an induction machine where the rotor comprises a set of coils that are terminated in slip rings to which external impedances can be connected. The stator is the same as is used with a standard squirrel cage motor!

By changing the impedance connected to the rotor circuit, the speed/current and speed/torque curves can be altered.

The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range. By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce maximum torque at a relatively low current from zero speed to full speed. A secondary use of the slip ring motor is to provide a means of speed control. Because the torque curve of the motor is effectively modified by the resistance connected to the rotor circuit, the speed of the motor can be altered. Increasing the value of resistance on the rotor circuit will move the speed of maximum torque down. If the resistance connected to the rotor is increased beyond the point where the maximum torque occurs at zero speed, the torque will be further reduced.

When used with a load that has a torque curve that increases with speed, the motor will operate at the speed where the torque developed by the motor is equal to the load torque. Reducing the load will cause the motor to speed up, and increasing the load will cause the motor to slow down until the load and motor torque are equal. Operated in this manner, the slip losses are dissipated in the secondary resistors and can be very significant. The speed regulation is also very poor.

Maintenance and Troubleshooting of Electric Motors

Reliance Electric

Motor Maintenance - SCHEDULED ROUTINE CARE

IntroductionThe key to minimizing motor problems is scheduled routine inspection and service. The frequency of routine service varies widely between applications.

Including the motors in the maintenance schedule for the driven machine or general plant equipment is usually sufficient. A motor may require additional or more frequent attention if a breakdown would cause health or safety problems, severe loss of production, damage to expensive equipment or other serious losses.

Written records indicating date, items inspected, service performed and motor condition are important to an effective routine maintenance program. From such records, specific problems in each application can be identified and solved routinely to avoid breakdowns and production losses.

The routine inspection and servicing can generally be done without disconnecting or disassembling the motor. It involves the following factors:

Dirt and Corrosion

1. Wipe, brush, vacuum or blow accumulated dirt from the frame and air passages of the motor. Dirty motors run hot when thick dirt insulates the frame and clogged passages reduce cooling air flow. Heat reduces insulation life and eventually causes motor failure.

2. Feel for air being discharged from the cooling air ports. If the flow is weak or unsteady, internal air passages are probably clogged. Remove the motor from service and clean.

3. Check for signs of corrosion. Serious corrosion may indicate internal deterioration and/or a need for external repainting. Schedule the removal of the motor from service for complete inspection and possible rebuilding.

4. In wet or corrosive environments, open the conduit box and check for deteriorating insulation or corroded terminals. Repair as needed.

LubricationLubricate the bearings only when scheduled or if they are noisy or running hot. Do NOT over-lubricate. Excessive grease and oil creates dirt and can damage bearings.

Heat, Noise and VibrationFeel the motor frame and bearings for excessive heat or vibration. Listen for abnormal noise. All indicate a possible system failure. Promptly identify and eliminate the source of the heat, noise or vibration.

Winding InsulationWhen records indicate a tendency toward periodic winding failures in the application, check the condition of the insulation with an insulation resistance test. Such testing is especially important for motors operated in wet or corrosive atmospheres or in high ambient temperatures.

Brushes and Commutators (DC Motors)

1. Observe the brushes while the motor is running. The brushes must ride on the commutator smoothly with little or no sparking and no brush noise (chatter).

2. Stop the motor. Be certain that:

· The brushes move freely in the holder and the spring tension on each brush is about equal.

· Every brush has a polished surface over the entire working face indicating good seating.

· The commutator is clean, smooth and has a polished brown surface where the brushes ride. NOTE: Always put each brush back into its original holder. Interchanging brushes decreases commutation ability.

· There is no grooving of the commutator (small grooves around the circumference of the commutator). If there is grooving, remove the motor from service immediately as this is a symptomatic indication of a very serious problem.

3. Replace the brushes if there is any chance they will not last until the next inspection date.

4. If accumulating, clean foreign material from the grooves between the commutator bars and from the brush holders and posts.

5. Brush sparking, chatter, excessive wear or chipping, and a dirty or rough commutator indicate motor problems requiring prompt service. Figure 1. Typical DC Motor Brushes And Commutator

Brushes and Collector Rings (Synchronous Motors)

1. Black spots on the collector rings must be removed by rubbing lightly with fine sandpaper. If not removed, these spots cause pitting that requires regrinding the rings.

Figure 2. Rotary Converter Armature Showing Commutator And Slip Rings.

2. An imprint of the brush, signs of arcing or uneven wear indicate the need to remove the motor from service and repair or replace the rings.

3. Check the collector ring brushes as described under "Brushes and Commutators". They do not, however, wear as rapidly as commutator brushes.

BEARING LUBRICATION

IntroductionModern motor designs usually provide a generous supply of lubricant in tight bearing housings. Lubrication on a scheduled basis, in conformance with the manufacturer's recommendations, provides optimum bearing life.

Thoroughly clean the lubrication equipment and fittings before lubricating. Dirt introduced into the bearings during lubrication probably causes more bearing failures than the lack of lubrication.

Too much grease can overpack bearings and cause them to run hot, shortening their life.

Excessive lubricant can find its way inside the motor where it collects dirt and causes insulation deterioration.

Many small motors are built with permanently lubricated bearings. They cannot and should not be lubricated.

Oiling Sleeve BearingsAs a general rule, fractional horsepower motors with a wick lubrication system should be oiled every 2000 hours of operation or at least annually. Dirty, wet or corrosive locations or heavy loading may require oiling at three-month intervals or more often. Roughly 30 drops of oil for a 3-inch diameter frame to 100 drops for a 9-inch diameter frame is sufficient. Use a 150 SUS viscosity turbine oil or SAE 10 automotive oil.

Some larger motors are equipped with oil reservoirs and usually a sight gage to check proper level.(Figure 3) As long as the oil is clean and light in color, the only requirement is to fill the cavity to the proper level with the oil recommended by the manufacturer. Do not overfill the cavity. If the oil is discolored, dirty or contains water, remove the drain plug. Flush the bearing with fresh oil until it comes out clean. Coat the plug threads with a sealing compound, replace the plug and fill the cavity to the proper level.

When motors are disassembled, wash the housing with a solvent. Discard used felt packing. Replace badly worn bearings. Coat the shaft and bearing surfaces with oil and reassemble.

Figure 3. Cross Section of the Bearing System of a Large Motor

Greasing Ball and Roller BearingsPractically all Reliance ball bearing motors in current production are equipped with the exclusive PLS/Positive Lubrication System. PLS is a patented open-bearing system that provides long, reliable bearing and motor life regardless of mounting position. Its special internal passages uniformly distribute new grease pumped into the housing during regreasing through the open bearings and forces old grease out through the drain hole. The close running tolerance between shaft and inner bearing cap minimizes entry of contaminants into the housing and grease migration into the motor. The unique V-groove outer slinger seals the opening between the shaft and end bracket while the motor is running or is at rest yet allows relief of grease along the shaft if the drain hole is plugged. (Figure 4)

The frequency of routine greasing increases with motor size and severity of the application as indicated in Table 1. Actual schedules must be selected by the user for the specific conditions.

During scheduled greasing, remove both the inlet and drain plugs. Pump grease into the housing using a standard grease gun and light pressure until clean grease comes out of the drain hole.

If the bearings are hot or noisy even after correction of bearing overloads (see "Troubleshooting") remove the motor from service. Wash the housing and bearings with a good solvent. Replace bearings that show signs of damage or wear. Repack the bearings, assemble the motor and fill the grease cavity.

Whenever motors are disassembled for service, check the bearing housing. Wipe out any old grease. If there are any signs of grease contamination or breakdown, clean and repack the bearing system as described in the preceding paragraph.

Figure 4. Cross Section of PLS Bearing System (Positive Lubrication System)

HEAT, NOISE AND VIBRATION

HeatExcessive heat is both a cause of motor failure and a sign of other motor problems.

The primary damage caused by excess heat is to increase the aging rate of the insulation. Heat beyond the insulation's rating shortens winding life. After overheating, a motor may run satisfactorily but its useful life will be shorter. For maximum motor life, the cause of overheating should be identified and eliminated.

As indicated in the Troubleshooting Sections, overheating results from a variety of different motor problems. They can be grouped as follows:

· WRONG MOTOR: It may be too small or have the wrong starting torque characteristics for the load. This may be the result of poor initial selection or changes in the load requirements.

· POOR COOLING: Accumulated dirt or poor motor location may prevent the free flow of cooling air around the motor. In other cases, the motor may draw heated air from another source. Internal dirt or damage can prevent proper air flow through all sections of the motor. Dirt on the frame may prevent transfer of internal heat to the cooler ambient air.

· OVERLOADED DRIVEN MACHINE: Excess loads or jams in the driven machine force the motor to supply higher torque, draw more current and overheat.

Table 1. Motor Operating Conditions

· Light Duty: Motors operate infrequently (1 hour/day or less) as in portable floor sanders, valves, door openers.

· Standard Duty: Motors operate in normal applications (1 or 2 work shifts). Examples include air conditioning units, conveyors, refrigeration apparatus, laundry machinery, woodworking and textile machines, water pumps, machine tools, garage compressors.

· Heavy Duty: Motors subjected to above normal operation and vibration (running 24 hours/day, 365 days/year). Such operations as in steel mill service, coal and mining machinery, motor-generator sets, fans, pumps.

· Severe Duty: Extremely harsh, dirty motor applications. Severe vibration and high ambient conditions often exist.

· EXCESSIVE FRICTION: Misalignment, poor bearings and other problems in the driven machine, power transmission system or motor increase the torque required to drive the loads, raising motor operating temperature.

· ELECTRICAL OVERLOADS: An electrical failure of a winding or connection in the motor can cause other Windings or the entire motor to overheat.

Noise and VibrationNoise indicates motor problems but ordinarily does not cause damage. Noise, however, is usually accompanied by vibration.

Vibration can cause damage in several ways. It tends to shake windings loose and mechanically damages insulation by cracking, flaking or abrading the material. Embrittlement of lead wires from excessive movement and brush sparking at commutators or current collector rings also results from vibration. Finally, vibration can speed bearing failure by causing balls to "brinnell," sleeve bearings to be pounded out of shape or the housings to loosen in the shells.

Whenever noise or vibration are found in an operating motor, the source should be quickly isolated and corrected. What seems to be an obvious source of the noise or vibration may be a symptom of a hidden problem. Therefore, a thorough investigation is often required.

Noise and vibrations can be caused by a misaligned motor shaft or can be transmitted to the motor from the driven machine or power transmission system. They can also be the result of either electrical or mechanical unbalance in the motor.

After checking the motor shaft alignment, disconnect the motor from the driven load. If the motor then operates smoothly, look for the source of noise or vibration in the driven equipment.

If the disconnected motor still vibrates, remove power from the motor. If the vibration stops, look for an electrical unbalance. If it continues as the motor coasts without power, look for a mechanical unbalance.

Electrical unbalance occurs when the magnetic attraction between stator and rotor is uneven around the periphery of the motor. This causes the shaft to deflect as it rotates creating a mechanical unbalance. Electrical unbalance usually indicates an electrical failure such as an open stator or rotor winding, an open bar or ring in squirrel cage motors or shorted field coils in synchronous motors. An uneven air gap, usually from badly worn sleeve bearings, also produces electrical unbalance.

The chief causes of mechanical unbalance include a distorted mounting, bent shaft, poorly balanced rotor, loose parts on the rotor or bad bearings. Noise can also come from the fan hitting the frame, shroud, or foreign objects inside the shroud. If the bearings are bad, as indicated by excessive bearing noise, determine why the bearings failed. (See Troubleshooting Problems D and L.)

Brush chatter is a motor noise that can be caused by vibration or other problems unrelated to vibration. See Troubleshooting Problem M for details.

WINDlNGS

Care of Windings and InsulationExcept for expensive, high horsepower motors, routine inspections generally do not involve opening the motor to inspect the windings. Therefore, long motor life requires selection of the proper enclosure to protect the windings from excessive dirt, abrasives, moisture, oil and chemicals.

When the need is indicated by severe operating conditions or a history of winding failures, routine testing can identify deteriorating insulation. Such motors can be removed from service and repaired before unexpected failures stop production.

Whenever a motor is opened for repair, service the windings as follows:

1. Accumulated dirt prevents proper cooling and may absorb moisture and other contaminants that damage the insulation. Vacuum the dirt from the windings and internal air passages. Do not use high pressure air because this can damage windings by driving the dirt into the insulation.

2. Abrasive dust drawn through the motor can abrade coil noses, removing insulation. If such abrasion is found, the winding should be revarnished or replaced.

3. Moisture reduces the dielectric strength of insulation which results in shorts. If the inside of the motor is damp, dry the motor per information in "Cleaning and Drying Windings".

4. Wipe any oil and grease from inside the motor. Use care with solvents that can attack the insulation.

5. If the insulation appears brittle, overheated or cracked, the motor should be revarnished or, with severe conditions, rewound.

6. Loose coils and leads can move with changing magnetic fields or vibration, causing the insulation to wear, crack or fray. Revarnishing and retying leads may correct minor problems. If the loose coil situation is severe, the motor must be rewound.

7. Check the lead-to-coil connections for signs of overheating or corrosion. These connections are often exposed on large motors but taped on small motors. Repair as needed.

8. Check wound rotor windings as described for stator windings. Because rotor windings must withstand centrifugal forces, tightness is even more important. In addition, check for loose pole pieces or other loose parts that create unbalance problems.

9. The cast rotor rods and end rings of squirrel cage motors rarely need attention. However, open or broken rods create electrical unbalance that increases with the number of rods broken. An open end ring causes severe vibration and noise.

Testing WindingsRoutine field testing of windings can identify deteriorating insulation permitting scheduled repair or replacement of the motor before its failure disrupts operations. Such testing is good practice especially for applications with severe operating conditions or a history of winding failures and for expensive, high horsepower motors and locations where failures can cause health and safety problems or high economic loss.

The easiest field test that prevents the most failures is the ground-insulation, or &127megger," test. It applies DC voltage, usually 500 or 1000 volts, to the motor and measures the resistance of the insulation.

NEMA standards require a minimum resistance to ground at 40 degrees C ambient of 1 megohm per kv of rating plus 1 megohm. Medium size motors in good condition will generally have megohmmeter readings in excess of 50 megohms. Low readings may indicate a seriously reduced insulation condition caused by contamination from moisture, oil or conductive dirt or deterioration from age or excessive heat.

One megger reading for a motor means little. A curve recording resistance, with the motor cold and hot, and date indicates the rate of deterioration. This curve provides the information needed to decide if the motor can be safely left in service until the next scheduled inspection time.

The megger test indicates ground insulation condition. It does not, however, measure turn-to-turn insulation condition and may not pick up localized weaknesses. Moreover, operating voltage peaks may stress the insulation more severely than megger voltage. For example, the DC output of a 500-volt megger is below the normal 625-volt peak each half cycle of an AC motor operating on a 440-volt system. Experience and conditions may indicate the need for additional routine testing.

A test used to prove existence of a safety margin above operating voltage is the AC high potential ground test. It applies a high AC voltage (typically, 65% of a voltage times twice the operating voltage plus 1000 volts) between windings and frame.

Although this test does detect poor insulation condition, the high voltage can arc to ground, burning insulation and frame, and can also actually cause failure during the test. It should never be applied to a motor with a low megger reading.

DC rather than AC high potential tests are becoming popular because the test equipment is smaller and the low test current is less dangerous to people and does not create damage of its own.

Cleaning and Drying WindingsMotors which have been flooded or which have low megger readings because of contamination by moisture, oil or conductive dust should be thoroughly cleaned and dried. The methods depend upon available equipment.

A hot water hose and detergents are commonly used to remove dirt, oil, dust or salt concentrations from rotors, stators and connection boxes. After cleaning, the windings must be dried, commonly in a forced-draft oven. Time to obtain acceptable megger readings varies from a couple hours to a few days.

BRUSH AND COMMUTATOR CARESome maintenance people with many relatively trouble-free AC squirrel cage motors forget that brushes and commutators require more frequent routine inspection and service. The result can be unnecessary failures between scheduled maintenance.

As indicated in Troubleshooting Problem M on Page 27, many factors are involved in brush and commutator problems. All generally involve brush sparking usually accompanied by chatter and often excessive wear or chipping. Sparking may result from poor commutator conditions or it may cause them.

The degree of sparking should be determined by careful visual inspection. The illustrations shown inFigure 5 are a useful guide. It is very important that you gauge the degree number as accurately as possible. The solution to the problem may well depend upon the accuracy of your answer since many motor, load, environmental and application conditions can cause sparking.

It is also imperative that a remedy be determined as quickly as possible. Sparking generally feeds upon itself and becomes worse with time until serious damage results.

Some of the causes are obvious and some are not. Some are constant and others intermittent. Therefore, eliminating brush sparking, especially when it is a chronic or recurring problem, requires a thorough review of the motor and operating conditions. Always recheck for sparking after correcting one problem to see that it solved the total problem. Also remember that, after grinding the commutator and properly reseating the brushes, sparking will occur until the polished, brown surface reforms on the commutator.

NOTE: Small sparks are yellow in color, and the large sparks are white in color. The white sparks, or blue-white sparks, are most detrimental to commutation (both brush and commutator).

Figure 5. Degrees of Generator and Motor Sparking

First consider external conditions that affect commutation. Frequent motor overloads, vibration and high humidity cause sparking. Extremely low humidity allows brushes to wear through the needed polished brown commutator surface film. Oil, paint, acid and other chemical vapors in the atmosphere contaminate brushes and the commutator surface.

Look for obvious brush and brush holder deficiencies:

1. Be sure brushes are properly seated, move freely in the holders and are not too short.

2. The brush spring pressure must be equal on all brushes.

3. Be sure spring pressure is not too light or too high. Large motors with adjustable springs should be set at about 3 to 4 pounds per square inch of brush surface in contact with the commutators.

4. Remove dust that can cause a short between brush holders and frame.

5. Check lead connections to the brush holders. Loose connections cause overheating.

Look for obvious commutator problems:

1. Any condition other than a polished, brown surface under the brushes indicates a problem. Severe sparking causes a rough blackened surface. An oil film, paint spray, chemical contamination and other abnormal conditions can cause a blackened or discolored surface and sparking. Streaking or grooving under only some brushes or flat and burned spots can result from a load mismatch and cause motor electrical problems. Grooved commutators should be removed from service. A brassy appearance shows excessive wear on the surface resulting from low humidity or wrong brush grade.

2. High mica or high or low commutator bars make the brushes jump, causing sparking.

3. Carbon dust, copper foil or other conductive dust in the slots between commutator bars causes shorting and sometimes sparking between bars.

If correcting any obvious deficiencies does not eliminate sparking or noise, look to the less obvious possibilities:

1. If brushes were changed before the problem became apparent, check the grade of brushes. Weak brushes may chip. Soft, low abrasive brushes may allow a thick film to form. High friction or high abrasion brushes wear away the brown film, producing a brassy surface. If the problem appears only under one or more of the brushes, two different grades of brushes may have been installed. Generally, use only the brushes recommended by the motor manufacturer or a qualified brush expert.

2. The brush holder may have been reset improperly. If the boxes are more than 1/8" from the commutator, the brushes can jump or chip. Setting the brush holder off neutral causes sparking. Normally the brushes must be equally spaced around the commutator and must be parallel to the bars so all make contact with each bar at the same time.

3. An eccentric commutator causes sparking and may cause vibration. Normally, concentricity should be within .001" on high speed, .002" on medium speed and .004" on slow speed motors.

4. Various electrical failures in the motor windings or connections manifest themselves in sparking and poor commutation. Look for shorts or opens in the armature circuit and for grounds, shorts or opens in the field winding circuits. A weak interpole circuit or large air gap also generate brush sparking.

DC MOTOR BRUSH HOLDERS AND

THE PERFORMANCE OF CARBON BRUSHES

Introduction

A DC Motor carbon brush is an electrical contact which makes a connection with a moving surface. Optimal performance on motors, generators and other types of moving contact applications will be attained only when the carbon brush, the brushholder and the contact surface are properly designed and maintained. All three components are critical factors in a complex electro-mechanical system. The DC Motor brushholder, as the name suggests, holds the brush so that the brush can perform properly. Holders provide stable support in the proper position in relation to the contact surface and often provide the means for application of the contact force on the brush.For many decades brushholders had received little attention. New rotating equipment was supplied with copies of the same old brushholder designs. Typically, when performance problems occurred the focus had been on the brush as this was the part exhibiting rapid wear. In the early 1980’s Helwig Carbon led the industry towards the consideration of brushholders and particularly spring pressure as a common cause of many brush problems. Further, recent holder developments and the coordination of the designs of constant pressure holders with Red Top brushes have resulted in significant advancements in performance and life.The purpose of this paper is to review the critical areas of consideration for brushholders in relation to the proper functioning of brushes. The most important factors are 1) maximum stability of the carbon in the holder, 2) proper positioning of the brush on the contact surface, and 3) minimum resistance through the brush and holder portion of the electrical circuit.

Holder Size Dimensions

The fit of the carbon portion of the brush in the holder is critical for stable electrical contact. If there is inadequate space between the holder walls and the thickness and width of the brush, there is potential for binding of the brush in the holder particularly with increased temperature and contamination. On the other hand, an excess amount of space between the holder and the carbon will result in an unstable electrical contact as the brush face can move tangentially or axially within the holder. The holder and brush tolerances on the thickness and width therefore must be well coordinated. Brushes are machined undersize per NEMA tolerances or per drawing specifications while brushholders are made oversize. As a general guideline for brushholders, industrial sizes typically should be held oversize to a tolerance of +.002/+.008". Smaller frame units with a brush thickness less than .500" and greater than .125" should have holders with a tolerance of +.001/+.005". Micro size units with brushes of thickness .125" or less should have holders held to a tolerance of +.001/+.003".Over a long period of usage the thickness dimension on a holder can become worn from brush movement or distorted from heat. Therefore, it is important to periodically measure the thickness and width dimensions on the top and bottom of the holders to ensure they are within tolerance and that the brush will have adequate support for a stable electrical contact. When motor and generator brushholders are subjected to high temperatures, it may be necessary to provide extra compensation for thermal expansion depending on the temperature rise and the degree of heat dissipation. In these cases it is easier to reduce the brush thickness and width dimensions slightly to avoid sticking in the holder rather than adjusting holder dimensions. Metal graphite brushes with over 50% metal content by weight are manufactured with an increased undersize tolerance per NEMA standards as they usually carry higher current, generate more heat, and have a higher coefficient of thermal expansion than non-metal grades.Brush and holder length can also have a significant effect on the stability and performance of the brush. Most often the length is limited due to the space available within the frame. There are, however, also practical length limitations due to the excess resistance of a long piece of carbon. As the carbon length is increased the resistance of the current path from the shunt to the contact surface is increased. At the same time the amount of contact area between the carbon and the longer holder is increased and the corresponding contact resistance is decreased. This then creates the potential for distorted current flow directly between the holder and the carbon rather than through the shunting. On the other hand, short brush and holder designs are more susceptible to instability at the contact surface. There is potential for a higher degree of brush tilt in the holder since the length of support is less in relation to the brush thickness.In addition to dimensional concerns the insides of the holder must be smooth and free of all obstructions including burrs. If a used brush has any straight scratches down the sides of the carbon then there are protrusions inside the brush box, which will restrict the brush from making proper electrical contact. Rough handling of brushholders can cause distortion of the metal and effect the critical inside dimensions of the brush cavity. Holders made from metal stampings are particularly susceptible to irregularities on the inside dimensions and on squareness. Broaching is generally accepted as the best manufacturing method for assurance of consistent inside dimensions and a smooth finish.Return to top of page.

Holder Position

The holder position will determine the location of the brush on the moving contact surface. For slip ring applications the holders are usually located around the top portion of the ring for ease of access. In this position the weight of the brush contributes to the contact force. If holders are mounted on the underside of a contact surface then additional spring force may be necessary to compensate for the weight of the brush. On DC machines with commutators proper positioning of the holders in relation to the field poles is critical. The brushes should be equally spaced around the commutator. This spacing can be checked by wrapping a paper tape around the commutator, marking the location of the same edge of each brush, and then measuring the distance between marks on the paper. The brushes must also contact the commutator within the neutral zone where voltage levels are near zero. When the holder position allows the brush to make contact outside the neutral zone there will be higher bar to bar voltages under the brush, circulating currents, bar edge burning, and damage from arcing.

Holder Angle

The most common angle for holder mounting is 0 degrees, i.e. perpendicular to the contact surface. Most slip rings and reversing commutator applications make use of this so-called radial mount. The advantages are ease of holder installation, maximum spring force transferred to the contact surface, and fair stability of brush contact upon reversal of direction. Any brush face movement within the holder will result in a change in the contact surface. The most stable surface contact will occur when the top and bottom of the brush are always held to the same side of the holder regardless of the direction of rotation. Angle holder mountings were developed to increase this stability and the effective area of the brush contact. However stability will occur only when the correct angles are used in relation to the direction of rotation.When the entering edge is the short side of the brush or a trailing position the face angle should be 20 degrees or less. At greater angles the action of the rotation and the spring force wedges the brush into the bottom corner of the holder and causes high friction and an unstable contact. Normally trailing brushes also have a shallow top bevel. When the entering edge is the long side of the brush or a leading position the face angle should be 25 degrees or more. At angles of 20 degrees and less the action of the rotation pulls the bottom of the brush to the opposite side of the holder from the top of the brush. Leading brushes should have a top bevel of 20 to 30 degrees. A stable contact can be maintained in either or both directions of rotation with brush face angles between 20 and 25 degrees. The potential disadvantage of holder angles is the loss of effective downward force of the spring. A portion of the spring force is dissipated in holding the brush stable to one side for the holder. The loss in downward contact force for various angles are as follows:

The spring force should be increased to compensate for the loss of effective downward force from the action of the brush angle in holding the brush to the side of the holder. If a brush has bevels of 20 degrees on the top and 30 degrees on the bottom then the spring force should be increased 6.0% + 13.4% or about 20% to maintain the proper level of effective downward contact force at the brush face.In the special case of post mounted double holders commonly used on slip rings, the best design would allow both brushes to make contact at zero degrees or perpendicular to the ring. Any angle will result in one brush in the pair operating with less contact stability.Return to top of page.

Holder Mounting Height

The vertical position of the holders above the contact surface is very important in assuring proper brush support throughout the wearable length of the rush and for proper positioning on the contact surface. When a brushholder is mounted too high above the contact surface or when the surface has been turned down to a significantly smaller diameter, there will not be adequate support for the carbon as the brush wears to a short length. This will contribute to increased electrical wear due to the instability of the contact. The holder mounting height should be proportional to the size of the unit. On the large frame sizes the holders should be mounted a maximum of .125" above the contact surface. In a few cases units operated with intentional runout of the contact surface which must be taken into consideration. The small micro frame sizes should have a holder mounting height of approximately .032". During holder mounting a flexible mounting pad of the appropriate thickness can be placed on the contact surface to ensure consistent height and spacing. This pad also helps protect the commutator from damage during mounting.There are several common problems related to excess height of the holder. When a commutator has been turned down several times angled brushes will make contact in a different position. With steep bottom bevels and significant decreases in diameter the location of the brush contact could even move outside the neutral zone. There will be a significant increase in wear unless the holder is moved closer to the commutator or the neutral is adjusted.Although single post mounted holders can be rotated to move the holder closer to the commutator, the position of brush contact will change. As above it is very likely that adjustment of the neutral position will be required to avoid edge arcing.On V-shaped toe-to-toe holders which are mounted too high above the commutator the brushes can interfere at the toes. This will result in one or both brushes not making contact with the commutator. It is especially important that these old style holders are mounted sufficiently close to the commutator to avoid this problem.Return to top of page.

Spring Force

Many inventive methods have been used for the application of the contact force on brushes. These included clock type springs, torsion bars, lever springs, helical coil springs, and constant force negator springs. As noted in the graph shown below the brush wear rate will change as the spring pressure changes. This is one of the most important concepts in understanding brush performance. There has always been a problem with an accelerating rate of wear as the brush gets shorter due to the declining spring force and the dramatic increase in electrical wear. The most consistent brush performance will be attained when the spring force is virtually constant at the correct level throughout the wear length of the brush.

The use of the proper constant force springs can be a significant advantage with consistent minimal wear rate of the brushes, reduced wear of the contact surface, less carbon dust, and much lower overall maintenance costs on the unit.Testing and application experience have resulted in the following recommended ranges of spring pressure:

Electrical Connections

The primary function of the brush involves conducting current. In many cases the brush holder is also a part of this electrical circuit. Therefore it is necessary that all electrical connections are of minimal resistance to provide the best path for current flow from the main lead connection to the contact surface. Corrosion, contamination, or electrolytic action over a period of time can cause dramatic increases in resistance which then requires cleaning. Careless installation of the brushes or the holders can lead to loose connections. Any high resistance in the brush circuit will result in excess heat or an undesirable path of current flow and unequal loading of the brushes. On fractional horsepower cartridge style brushholders with captive coil spring type brushes the current should flow from the clip connector at the bottom of the holder up the brass insert to the cap on the end of the brush and then down through the shunt to the carbon. The brushes fail very quickly if the round or eared cap on the end of the brush does not make proper contact with the brass holder insert. When this condition exists current will flow directly from the brass insert to the spring or to the carbon. In either case there will be extreme heat, loss of brush contact, commutator wear, and eventually motor failure.Another problem with larger frame sizes can occur when the holder mounting is part of the electric circuit. If the holder mounting surface becomes dirty, corroded, or even painted over then current will again need to follow another path and thereby cause problems.

Tesla Model S

NAME:

LEVEL: DATE:

CHECK LIST FOR D.C. GENERATOR PACKET

STEPS/TASKS

1) The student completed all vocabulary associated with this learning guide to 100% accuracy.

25

2) The student completed all written work associated with this learning guide to 100% accuracy.

25

3) The student completed the written assessment to 80% accuracy.

25

4) The student recorded all project results.

50

5) The student completed experiment #1

25

6) The student completed experiment # 2

25

7) The student completed experiment # 3

25

8) The student completed the required graph of experiment results.

25

9) The student completed the summary of results of this learning guide.

25

Total Points

250

* ALL STEPS/TASKS MUST MEET THE STANDARDS IN ORDER TO ACHIEVE MASTERY.*

COMMENTS:

INSTRUCTOR SIGNATURE:

DATE:

NAME:

DATE:

DC MOTOR ASSESSMENT POST TEST

True/False

Indicate whether the sentence or statement is true or false.

____1.Counter-electromotive force (CEMF) and back-EMF are names for the voltage induced into the armature of a DC motor.

____2.It is all right to operate a series motor with no load connected.

Multiple Choice

Identify the letter of the choice that best completes the statement or answers the question.

____3.A device that converts electrical input into mechanical output is called

a.

alternator

b.

generator

c.

motor

d.

inverter

____4.When the field winding of a DC motor is connected in parallel with the armature, the motor is called a _____ motor.

a.

series

b.

shunt

____5.The DC motor, known as a “constant speed motor,” is the _____ motor.

a.

series

b.

shunt

____6.The speed regulation of a DC motor is proportional to the

a.

applied voltage

b.

load torque

c.

armature resistance

d.

field resistance

____7.A compound motor has _____ field windings.

a.

series

b.

shunt

c.

series and shunt

d.

combined

____8.The cumulative compound motor connection means that the fields of the series and shunt field windings

a.

add

b.

subtract

c.

multiply

d.

divide

____9.The compound motor connection that is rarely used is the

a.

cumulative

b.

differential

____10.In a compound DC motor control circuit, the field loss relay will disconnect the _____ if power is lost to the _____.

a.

rotor, armature

b.

armature, shunt field

c.

shunt field, armature

d.

armature, rotor

____11.The motor that does not contain a wound armature, commutator, or brushes is the _____ motor.

a.

servo

b.

stepping

c.

selsyn

d.

brushless DC

____12.Permanent magnet motors have _____ efficiency than wound field motors.

a.

higher

b.

lower

____13.Small permanent magnet motors that have lightweight rotors are often used as

a.

stepping motors

b.

automobile starters

c.

servomotors

d.

ceiling fans

____14.The servomotor which has a fiberglass and copper rotor is the _____ motor.

a.

copper

b.

copper-glass

c.

nonferrous

d.

ServoDisc®

____15.Because the rotor of a ServoDisc® motor is made from copper laminated onto a fiberglass disk, the motor is also called a _____ motor.

a.

copper

b.

laminated

c.

printed circuit

d.

nonferrous

____16.Some DC servomotors have square wave input voltage. To vary the motor speed, the on and off time ratio of the square wave is varied. This is called _____ modulation.

a.

phase

b.

pulse-width

c.

frequency

d.

amplitude

____17.A square wave voltage varies between 0 V and 12 V. The positive pulses are 50 s wide and there is a 50 s gap between them. What is the DC average of the waveform?

a.

3 V

b.

4 V

c.

6 V

d.

9 V

____18.A square wave voltage varies between 0 V and 12 V. The positive pulses are 25 s wide and there is a 75 s gap between them. What is the DC average of the waveform?

a.

3 V

b.

4 V

c.

6 V

d.

9 V

____19.A square wave voltage varies between 0 V and 12 V. The positive pulses are 75 s wide and there is a 25 s gap between them. What is the DC average of the waveform?

a.

3 V

b.

4 V

c.

6 V

d.

9 V

Completion

Complete each sentence or statement.

20.A force that tends to cause rotation is known as _______________.

21.Torque is a force that tends to cause _______________.

Residential & Industrial Electricity

K-W-L WORKSHEET

NAME:

LEVEL: DATE:

ARTICLE TITLE:

TIME START:

TIME FINISH:

K What do I already KNOW

about this topic?

W What do I WANT to know

about this topic?

L What did I LEARN after

reading ABOUT this

topic?

I checked the following before reading:

· Headlines and Subheadings

· Italic, Bold, and Underlined words

· Pictures, Tables, and Graphs

· Questions or other key information

I made predictions AFTER previewing the article.

Comments:

· Instructor Signature:

NAME:

DATE:

DC MOTOR ASSESSMENT PRE TEST

True/False

Indicate whether the sentence or statement is true or false.

____1.Counter-electromotive force (CEMF) and back-EMF are names for the voltage induced into the armature of a DC motor.

____2.It is all right to operate a series motor with no load connected.

Multiple Choice

Identify the letter of the choice that best completes the statement or answers the question.

____3.A device that converts electrical input into mechanical output is called

a.

alternator

b.

generator

c.

motor

d.

inverter

____4.When the field winding of a DC motor is connected in parallel with the armature, the motor is called a _____ motor.

a.

series

b.

shunt

____5.The DC motor, known as a “constant speed motor,” is the _____ motor.

a.

series

b.

shunt

____6.The speed regulation of a DC motor is proportional to the

a.

applied voltage

b.

load torque

c.

armature resistance

d.

field resistance

____7.A compound motor has _____ field windings.

a.

series

b.

shunt

c.

series and shunt

d.

combined

____8.The cumulative compound motor connection means that the fields of the series and shunt field windings

a.

add

b.

subtract

c.

multiply

d.

divide

____9.The compound motor connection that is rarely used is the

a.

cumulative

b.

differential

____10.In a compound DC motor control circuit, the field loss relay will disconnect the _____ if power is lost to the _____.

a.

rotor, armature

b.

armature, shunt field

c.

shunt field, armature

d.

armature, rotor

____11.The motor that does not contain a wound armature, commutator, or brushes is the _____ motor.

a.

servo

b.

stepping

c.

selsyn

d.

brushless DC

____12.Permanent magnet motors have _____ efficiency than wound field motors.

a.

higher

b.

lower

____13.Small permanent magnet motors that have lightweight rotors are often used as

a.

stepping motors

b.

automobile starters

c.

servomotors

d.

ceiling fans

____14.The servomotor which has a fiberglass and copper rotor is the _____ motor.

a.

copper

b.

copper-glass

c.

nonferrous

d.

ServoDisc®

____15.Because the rotor of a ServoDisc® motor is made from copper laminated onto a fiberglass disk, the motor is also called a _____ motor.

a.

copper

b.

laminated

c.

printed circuit

d.

nonferrous

____16.Some DC servomotors have square wave input voltage. To vary the motor speed, the on and off time ratio of the square wave is varied. This is called _____ modulation.

a.

phase

b.

pulse-width

c.

frequency

d.

amplitude

____17.A square wave voltage varies between 0 V and 12 V. The positive pulses are 50 s wide and there is a 50 s gap between them. What is the DC average of the waveform?

a.

3 V

b.

4 V

c.

6 V

d.

9 V

____18.A square wave voltage varies between 0 V and 12 V. The positive pulses are 25 s wide and there is a 75 s gap between them. What is the DC average of the waveform?

a.

3 V

b.

4 V

c.

6 V

d.

9 V

____19.A square wave voltage varies between 0 V and 12 V. The positive pulses are 75 s wide and there is a 25 s gap between them. What is the DC average of the waveform?

a.

3 V

b.

4 V

c.

6 V

d.

9 V

Completion

Complete each sentence or statement.

20.A force that tends to cause rotation is known as _______________.

21.Torque is a force that tends to cause _______________.

Name:

Date:

Learning Guide Due Date:

Pre Test Due Date:

Post Test Due Date:

RESIDENTIAL & INDUSTRIAL ELECTRICITY

�

Level 3

Task 1800

Schuylkill Technology Center-

South Campus

15 Maple Avenue

Marlin, Pennsylvania 17951

(570) 544-4748

POS # 1800

Total Hours-68

Level(s)-3

*CORE CURRICULUM STANDARDS*

*ACADEMIC STANDARDS*

� INCLUDEPICTURE "http://upload.wikimedia.org/wikipedia/en/5/56/Tesla3.jpg" \* MERGEFORMATINET ���

Points

Earned

Points

Available

Correct/Out of 250

Grade Percentage

Check One Percentage Task Grade

Below Basic 0%-69% 0-6

Basic 70%-85% 7

Competent 86%-92% 8-9

Advanced 93%-100% 10

NAME:

DATE:

3

10

110

140

7

THE PARTS OF A DC MOTOR / GENERATOR

164

150

130

120

5

4

6

8

9

2

1

PAGE

51