Motor Ratings

Horsepower. A basic ratings or motors, both d-c and a-c types, is the horsepower output available at the shaft of the motor. The ratings of motors range from a small fraction of 1 hp to several thousand horsepower. Motors of less than 1 hp are classified as fractional-horsepower motors, and those of one or more horsepower are classed as integral-horsepower motors.

Voltage. The voltage rating of d-c motors ranges from 1.5 to 1.500 volts. The higher voltages are used on only large motors. The voltage rating of a-c motors ranges from 1.5 to several thousand volts.

Frequency. With a-c motors, the frequency rating is also important. For industrial motors, 60 cycles is almost universally used.

Speed. Motors are also rated at the speed of rotation of the motor shaft; it is expressed in revolutions per minute and is abbreviated as rpm. The rated full-load speed for which d-c motors can be designed extends over a wide range. The speed of most types of a-c motors, when operated from 60-cycles power sources, is limited to such basic values as 3.600,1.800, 1.200, 900 and 600 rpm. Because of the effect of centrifugal force, large-size motors are limited to lower speeds then are small-size motors.

Torque. The turning effort produced at the shaft of a motor is called the torque. The torque of a motor is proportional to the force produced on the conductors of the rotating member and to the radial distance through which the force acts.

7. Answer the following questions:

1. What is a basic rating of motors?

2. What is the range of motor ratings?

3. What is meant by fractional-horse-power motors?

4. What is the range of the voltage rating for d-c motors?

5. Is the frequency rating important for a-c motors?

6. What is the frequency rating for industrial motors'

7. How is the speed of rotation of the motor shaft expressed?

8. Can d-c motors be designed for the rated full-load speed?

8. Ask your fellow-student:

if the rated full load speed extends over a wide range;

what the basic values limiting the speed of most types of a-c motors are; why large-size motors are limited to lower speeds than small-size motors;

what is called the torque.

9.

a) Explain

1. the ratio between the torque of a motor and the force produced on the conductors of the rotating member;

2. the ratio between the torque and the radial distance through which the force acts.

b) Define

1. horsepower

2. torque

c) Indicate the abbreviations for "revolutions per minute", for "horsepower".

d) Give a summary for all possible motor ratings.

10. Learn the dialogues by heart:

I

Prof.: May I ask you a few questions on the series motor? Could you explain, Comr. Fyodorov, the determination of the rated torque of a motor?

Comr. F.: Let me see. If I am not mistaken, it is possible to determine the rated torque by the rated power and the speed. Am I right?

Prof.: That's correct. And how can you regulate speed with a resistor introduced into the circuit of the motor?

Comr. Ivan.: May I answer your question? In this case the voltage across the armature drops considerably and so does the speed

of rotation.

Prof.: Right. So much for this. Let's turn to some other problem.

II

Stud. A.: Are you ready for today's laboratory classes?

Stud. В.: No. Yesterday I didn't do a stroke of work because of the wedding-party at the hostel. Could you help me now?

Stud. A.: That's what I'm here for. At least do you know the name of the subject to be discussed?

Stud. B.: Yes. It must be the shunt motor.

Stud. A: So you must know two things very well: the difference between the starting and the regulating rheostats and the so-called ]

"no-load" characteristic.

Stud. B.: Really? That I certainly know.

Stud. A.: But I am certainly joking. As a matter of fact, the subject is too broad and deep to exhaust it on the spot. So go home and learn.

Stud. B.: I see. It can't be helped. I will.

Test Exercises

Write the number of the right choice or choose the right letter:

A.

1. A fractional horsepower motor is:

   1. a motor of one or more horsepower

   2. a motor of less than 1 hp

2. The torque is

   1. inversely proportional to the force produced on the conductors of the rotating member and proportional to the radial distance through which the force acts

   2. proportional to the force and inversely proportional to the radial distance

   3. proportional to the force and to the radial distance

B.

1. revolutions ... minute (1.for, 2.per, 3.in)

2. to limit ... (l.in, 2.at, 3.to)

3. proportional ... ( 1.at, 2.in, 3.to)

4. ... a wide range (1.on, 2.at, 3.over)

5. the voltage rating ranges ... 1.5 ... several thousand volts ( l.up to,

2.from to, 3.out of)

Part III

A. Read the following text without a dictionary:

Types of D-C Motors

Direct-current motors may be classed according to their type of field structure as (1) permanent magnet and (2) electromagnet. The use of permanent magnets for field poles is generally limited to very small motors. All medium and large-size motors use electromagnets for the field poles. When electromagnets are used, the motors are further classified in terms of the manner in which the field windings are connected with respect to the armature. These classifications are (1) shunt, (2) series, and (3) compound. .

The construction features of the shunt, series and compound motors are quite similar. For a given horsepower and speed rating, the same armature might be used for all three types. The strength of the magnetic field, expressed in ampere-turns, for full-load conditions is the same for all three types. The chief difference in construction is in the number of turns and the size of wire used in the field coils to obtain the required ampere-turns.

The operating characteristic of the three types of motors differ considerably.