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UNIT-3 DC Machines &AC Machines

Syllabus :Rotating Electrical Machines; DC Machines: Principle of Operation ofDC Machine as Motor and Generator, EMF Equation, Applications of DCMachines.

AC Machines: Principle of Operation of 3-Phase Induction Motor, 3-Phase

Synchronous Motor and 3- Phase Synchronous Generator (Alternator),

Applications of AC Machines.

DC Machines

1. Rotating Electrical Machines:

Mechanical Rotor

Rotor. The revolving part of a rotating electrical machine. The rotor may be either

the field or the armature, depending on the design of the machine. Two different

views are shown below.

Commutator: A component for reversing the electric field, switching the current

flow.

Winding: Wires used to carry current to generate a magnetic field. The wires

could be copper or aluminum, but copper wires would be the most common.

DC Machines, Principles Of Operation:

Generator

In a generator, moving a conductor through a stationary magnetic field generates voltage. If a coil is rotated

through a magnetic field as shown in Figure 4, an alternating voltage will be produced. To make this voltage

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available to a stationary external circuit, two slip rings and brushes must be provided. For the external circuit to

produce DC voltage, it is necessary to reverse the polarity of the external leads at the same time the voltage in the

coil is reversed. This is accomplished by segmenting a slip ring to form what is called a commutator. An elementary

two segment commutator is illustrated in Figure 5. This single coil, two piece commutator will yield an

unidirectional but pulsating voltage as shown in Figure 6. However, when a large number of commutator segments

or bars is used, the resulting voltage will be more uniform as shown in Figure 7.

Figure 4.

Brushes and slip rings provide AC voltage

Figure 5.

Brushes and Commutator provides DC voltage

Figure 7.

Uniform DC Voltage

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As stated above, the generated voltage in a single conductor is:

E = N B v x 10-8

where:

B = flux density in lines per square inch

= length of the conductor in inches

v = velocity in inches per second

This equation can be developed to the following equation for DC machines:

E = (Z / paths) x x poles x (rpm / 60) x 10-8

where:

where:

Z = total number of conductors

TITA= flux per pole in lines

This equation represents the average voltage. For a given machine, it can be reduced to:

E = K1 S

where:

E = flux per pole

S = speed in rpm

K1 = all other factors

Motor

As stated previously, if current is supplied to a conductor in a magnetic field, a force will be produced. The force

developed in a single conductor is:

F = (B I) / 10

where:

F = force in dynes

B = flux density in lines per square centimeter

= length of the conductor in centimeters

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I = current in amperes

DC General Construction

A typical DC generator or motor usually consists of: An armature core, an air gap, poles, and a yoke which form themagnetic circuit; an armature winding, a field winding, brushes and a commutator which form the electric circuit;

and a frame, end bells, bearings, brush supports and a shaft which provide the mechanical support. See figure 8.

Figure 8. Four Pole DC Motor

The armature stack is made up thin magnetic steel laminations stamped from sheet steel with a blanking die. Slots

are punched in the lamination with a slot die. Sometimes these two operations are done as one. The laminations

are welded, riveted, bolted or bonded together.

Armature Winding : The armature winding is the winding, which fits in the armature slots and is eventually

connected to the commutator. It either generates or receives the voltage depending on whether the unit is a

generator or motor. The armature winding usually consists of copper wire, either round or rectangular and is

insulated from the armature stack.

Field Poles :The pole cores can be made from solid steel castings or from laminations. At the air gap, the pole

usually fans out into what is known as a pole head or pole shoe. This is done to reduce the reluctance of the air

gap. Normally the field coils are formed and placed on the pole cores and then the whole assembly is mounted tothe yoke.

Field Coils :The field coils are those windings, which are located on the poles and set up the magnetic fields in the

machine. They also usually consist of copper wire are insulated from the poles. The field coils may be either shunt

windings (in parallel with the armature winding) or series windings (in series with the armature winding) or a

combination of both.

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Yoke :The yoke is a circular steel ring, which supports the field, poles mechanically and provides the necessary

magnetic path between the pole. The yoke can be solid or laminated. In many DC machines, the yoke also serves

as the frame.

Commutator :The commutator is the mechanical rectifier, which changes the AC voltage of the rotating

conductors to DC voltage. It consists of a number of segments normally equal to the number of slots. The

segments or commutator bars are made of silver bearing copper and are separated from each other by mica

insulation.

Brushes and Brush Holders : Brushes conduct the current from the commutator to the external circuit. There are

many types of brushes. A brush holder is usually a metal box that is rectangular in shape. The brush holder has a

spring that holds the brush in contact with the commutator. Each brush usually has a flexible copper shunt or

pigtail, which extends to the lead wires. Often, the entire brush assembly is insulated from the frame and is made

movable as a unit about the commutator to allow for adjustment.

E.M.F. Equation & Armature Resistance of a D.C. Generator

We shall now derive an expression for the e.m.f. generated in a d.c. generator.

Let

= flux/pole in Wb

Z = total number of armature conductors

P = number of poles

A = number of parallel paths = 2 for wave winding

= P for lap winding

N = speed of armature in r.p.m.

Eg= e.m.f. of the generator = e.m.f./parallel path

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Armature Resistance (Ra):

The resistance offered by the armature circuit is known as armature resistance (Ra) and includes:

(i) resistance of armature winding

(ii) resistance of brushes

The armature resistance depends upon the construction of machine. Except for small machines, its value is

generally less than 1.

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AC Machines

As in the DC motor case, a current ispassed through the coil, generating a

torque on the coil. Since the current

is alternating, the motor will run

smoothly only at the frequency of the

sine wave. It is called a synchronous

motor. More common is the

induction motor, where electric

current is induced in the rotating

coils rather than supplied to them

directly.

One of the drawbacks of this kind of AC motor is the high current which must flow through the

rotating contacts. Sparking and heating at those contacts can waste energy and shorten the

lifetime of the motor. In common AC motors the magnetic field is produced by an electromagnet

powered by the same AC voltage as the motor coil. The coils which produce the magnetic field

are sometimes referred to as the "stator", while the coils and the solid core which rotates is called

the "armature". In an AC motor the magnetic field is sinusoidally varying, just as the current in

the coil varies.

Three phase induction motors Operating

principle

Anelectric motorconverts electrical energy into a mechanical energy which is then supplied to

different types of loads. A.c. motors operate on an a.c. supply, and they are classified into

synchronous, single phase and 3 phase induction, and special purpose motors. Out of all types, 3

phase induction motors are most widely used for industrial applications mainly because they do

not require a starting device.

A3 phase induction motorderives its name from the fact that the rotor current is induced by

the magnetic field, instead of electrical connections. The operating principle of a 3 phase

induction motor is based on the production of r.m.f..

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indmot.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indmot.html#c2http://electrical-engineering-portal.com/three-phase-induction-motors-operating-principlehttp://electrical-engineering-portal.com/three-phase-induction-motors-operating-principlehttp://electrical-engineering-portal.com/three-phase-induction-motors-operating-principlehttp://electrical-engineering-portal.com/three-phase-induction-motors-operating-principlehttp://electrical-engineering-portal.com/5-steps-to-choose-best-motor-for-your-applicationhttp://electrical-engineering-portal.com/5-steps-to-choose-best-motor-for-your-applicationhttp://electrical-engineering-portal.com/5-steps-to-choose-best-motor-for-your-applicationhttp://electrical-engineering-portal.com/the-cage-induction-motor-explained-in-detailshttp://electrical-engineering-portal.com/the-cage-induction-motor-explained-in-detailshttp://electrical-engineering-portal.com/the-cage-induction-motor-explained-in-detailshttp://electrical-engineering-portal.com/the-cage-induction-motor-explained-in-detailshttp://electrical-engineering-portal.com/5-steps-to-choose-best-motor-for-your-applicationhttp://electrical-engineering-portal.com/three-phase-induction-motors-operating-principlehttp://electrical-engineering-portal.com/three-phase-induction-motors-operating-principlehttp://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indmot.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indmot.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html#c1

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Synchronous motor

A synchronous electric motor is an AC motor in which the rotation rate of the shaft is synchronized with the

frequency of the AC supply current; the rotation period is exactly equal to an integral number of AC cycles.

Synchronous motors contain electromagnets on the stator of the motor that create a magnetic field which rotates in

time with the oscillations of the line current. The rotor turns in step with this field, at the same rate.

Another way of saying this is that the motor does not rely on "slip" under usual operating conditions, and as a result

produces torque at synchronous speed. Synchronous motors can be contrasted with induction motors, which must

slip in order to produce torque. The speed of the synchronous motor is determined by the number of magnetic poles

and the line frequency.

Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower direct-current excited

industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed

is required. In high-horsepower industrial sizes, the synchronous motor provides two important functions. First, it is

a highly efficient means of converting AC energy to work. Second, it can operate at leading or unity power factor

and thereby provide power-factor correction.

BY: SHUBHAM KHANDELWAL

(FARRE.IN)

http://en.wikipedia.org/wiki/AC_motorhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Statorhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Rotor_%28electric%29http://en.wikipedia.org/wiki/Induction_motor#sliphttp://en.wikipedia.org/wiki/Induction_motor#sliphttp://en.wikipedia.org/wiki/Induction_motor#sliphttp://en.wikipedia.org/wiki/Synchronous_speedhttp://en.wikipedia.org/wiki/Induction_motorhttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Angular_speedhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Power_factorhttp://en.wikipedia.org/wiki/Power_factorhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Angular_speedhttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Induction_motorhttp://en.wikipedia.org/wiki/Synchronous_speedhttp://en.wikipedia.org/wiki/Induction_motor#sliphttp://en.wikipedia.org/wiki/Rotor_%28electric%29http://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Statorhttp://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/AC_motor

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