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Chapter

2Terminology and DefinitionsN.Ghai

2.1Types of Motor

There are many ways in which electric motors may be categorized orclassified. Some of these are presented below and in Fig. 2.1.

2.1.1AC and DC

One way of classifying electric motors is by the type of power they

consume. Using this approach, we may state that all electric motors fall

into one or the other of the two categories, viz., AC or DC. AC motors

are those that run on alternating current or AC power, and DC motors

are those that run on direct current, or DC power.

2.1.2Synchronous and induction

Alternating current motors again fall into two distinct categories,

synchronous or induction. Synchronous motors run at a fixed speed,

irrespective of the load they carry. Their speed of operation is given by

the relationship

where fis the system frequency in Hz andPis the number of poles for

which the stator is wound. The speed given by the above relationship is

called the synchronous speed, and hence the name synchronous motor.

The induction motor, on the other hand, runs very close to but less than

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Source: Electric Motor Handbook

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8 Chapter Two

the synchronous speed. The difference between the synchronous speedand the actual speed is called the slip speed. The slip speed of any

induction motor is a function of its design and of desired performance.

Further, for a given motor, the slip speed and the running speed vary

with the load. The running speed decreases as the load on the motor is

increased.

2.1.3Salient-pole and cylindrical-rotor

Synchronous motors fall into two broad categories defined by theirmethod of construction. These are salient-pole motors and cylindrical-

rotor motors. High-speed motors, those running at 3600 r/min with 60

Hz supply, are of the cylindrical-rotor construction for mechanical

strength reasons, whereas slower speed motors, those running at 1800

r/min and slower, are mostly of the salient-pole type.

2.1.4Single-phase and three-phase motors

All AC motors may also be classified as single-phase and multiphasemotors, depending on whether they are intended to run on single-phase

supply or on multiphase supply. Since the distribution systems are

universally of the three-phase type, multiphase motors are almost always

of the three-phase type. Single-phase motors are limited by the power

they can produce, and are generally available in sizes up to only a few

horsepower, and in the induction motor variety only. Synchronous motors

are usually available in three-phase configurations only.

2.1.5Other variations

Many variations of the basic induction and synchronous motors are

available. These include but are not limited to the synchronous-induction

motor, which is essentially a wound-rotor-induction motor supplied with

DC power to its rotor winding to make it run at synchronous speed; the

Figure 2.1 Classification of AC and DC motors.

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Terminology and Definitions 9

permanent-magnet motor in which the field excitation is provided bypermanent magnets; the reluctance motor in which the surface of the

rotor of a squirrel-cage induction motor is shaped to form salient-pole

structures causing the motor to run up to speed as an induction motor

and pull into synchronism by reluctance action and operate at

synchronous speed; and the ac-commutator motor or universal motor,

which possesses the wide speed range and higher starting torque

advantages of DC motor, to name a few. One could also include here

single-phase induction motor variations based on the method of starting

usedthe split-phase motor, the capacitor-start motor, the resistance-start motor, and the shaded-pole motor.

2.2Insulation System Classes

The classification of winding insulation systems is based on their

operating temperature capabilities. These classes are designated by the

letters A, E, B, F, and H. The operating temperatures for these insulation

classes are shown in Table 2.1.

These temperatures represent the maximum allowable operatingtemperature of the winding at which, if the motor were operated in a

clean, dry, free-from-impurities environment at up to 40 hours per week,

an operation life of 10 to 20 years could be expected, before the insulation

deterioration due to heat destroys its capability to withstand the applied

voltage.

The temperatures in the Table 2.1 are the maximum temperatures

existing in the winding, or the hot spot temperatures, and are not the

average winding temperatures. It is generally assumed that in a

welldesigned motor, the hot spot is approximately 10C higher than theaverage winding temperature. This yields the allowable temperature rises

(average, or rises by resistance) in an ambient temperature not exceeding

40C, that one finds in standards. These are shown in Table 2.2.

Class A insulation is obsolete, and no longer in use. Class E insulation

is not used in the United States, but is common in Europe. Class B is

TABLE 2.2 Allowable Temperature Rises

TABLE 2.1 Operating Temperatures for Insulation System Classes

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10 Chapter Two

the most commonly specified insulation. Class F is slowly winning favor,

although for larger motors in the United States, the users tend to specify

class F systems with class B temperature rises to improve the lifeexpectancy of the windings. Class H systems are widely specified in

synchronous generators up to 5 mW in size.

2.3Codes and Standards

Both national and international standards exist for electric motors. For

the most part, these apply to general purpose motors. However, in the

United States, some definite purpose standards also exist which are

industry or application specific. Examples of the latter are the IEEE841, which applies to medium size motors for petroleum and chemical

applications, American Petroleum Institute standards API 541 (large

induction motors) and API 546 (large synchronous motors), both for

petroleum and chemical industry applications, and the American

National Standards Institute standard ANSI C50.41 for large induction

motors for generating station applications.

In the United States, in general, the Institute of Electrical and

Electronics Engineers (IEEE) writes standards for motor testing and test

methods, and the National Electrical Manufacturers Association (NEMA)writes standards for motor performance. In the international field, the

International Electrotechnical Commission (IEC), which is a voluntary

association of countries, writes all standards applicable to electric motors.

U.S. and international standards that apply to electric motors are:

n NEMA MG1-1993, Rev 4, Motors and Generators.

n IEEE Std 1121996, IEEE Standard Test Procedure for Polyphase

Induction Motors and Generators.

n IEEE Std 1151983, IEEE Guide: Test Procedures for SynchronousMachines.

n IEEE Std 5221992, IEEE Guide for Testing Turn-to-Turn Insula-

tion on Form-Wound Stator Coils for Alternating Current Rotating

Electric Machines.

n IEC 341, 1996, 10th ed., Rotating Electrical Machines, Part 1: Rat-

ing and Performance.

n IEC 341, Amendment 1, 1997, Rotating Electrical Machines, Part

1: Rating and Performance.

n IEC 342, 1972, Rotating Electrical Machines, Part 2: Methods ofDetermining Losses and Efficiency of Rotating Electrical Machinery

from Tests.

n IEC 342, Amendment 1, 1995 and Amendment 2, 1996, Rotating

Electrical Machines, Part 2: Methods of Determining Losses and

Efficiency of Rotating Electrical Machinery from Tests.

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Terminology and Definitions 11

n IEC 345,1991, Rotating Electrical Machines, Part 5: Classification

of Degrees of Protection Provided by Enclosures of Rotating Electri-

cal Machines (IP Code).n IEC 346, 1991, Rotating Electrical Machines, Part 6: Methods of

Cooling (IC Code).

n IEC 349, 1990 and 2/979/FDIS, 1997, Rotating Electrical Machines,

Part 9, Noise Limits.

n IEC 3412, 1980, Rotating Electrical Machines, Part 12: Starting

Performance of Single-speed, Three-phase Cage Induction Motors for

Voltages up to and Including 600 Volts.

n IEC 3414, 1990 and 2/940/FDIS, 1996, Rotating Electrical Machines,

Part 14: Mechanical Vibration of Certain Machines with Shaft Heights56 mm and Larger.

n IEC 3415,1995, Rotating Electric Machines, Part 15: Impulse Volt-

age Withstand Levels of Rotating AC Machines with Form-wound

Coils.

n IEC 38, 1983, IEC Standard Voltages.

n IEC 721, 1991, Dimension and Output Series for Rotating Electri-

cal Machines.

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Terminology and Definitions