Three-Phase Motors- Test 1

Question 1

What name is given to the stationary part of the motor?

Answer 1

stator

Question 2

What name is given to the rotating part of the motor?

Answer 2

rotor

Question 3

What is the purpose of the laminations in the cores of both of those parts of the motor?

Answer 3

To reduce the eddy-current losses.

Question 4

Why are the bars of the squirrel cage slightly skewed so they are not parallel with the slots in the stator?

Answer 4

To make the motor run smoothly and quietly.

Question 5

What is the main purpose of the end bells on a motor?

Answer 5

To house the bearings that support the rotor shaft and to keep it aligned with the stator

Question 6

Name two ways motor enclosures are classified.

Answer 6

1. The way they protect the motor from the environment
2. The way they are cooled

Question 7

What name is given to the machine with an enclosure that does not prevent the free exchange of air between the inside and the outside of the machine?

Answer 7

open machine

Question 8

What name is given to the machine with an enclosure that prevents the free exchange of air between the inside and the outside of the machine?

Answer 8

totally enclosed machine

Question 9

The winding of a three-phase winding is made up of three single-phase windings placed electrical degrees apart from each other.

Answer 9

120

Question 10

What are the two purposes of the stator core?

Answer 10

1. Support the stator windings.
2. Provide a low-reluctance path for the magnetic circuit.

Question 11

What does the frame designation tell about the motor?

Answer 11

the physical dimensions of the motor in NEMA standards

Question 12

Does the horsepower rating of the motor refer to input power or to output power?

Answer 12

output power, at rated speed

Question 13

What does the service factor of the motor tell about the motor?

Answer 13

The service factor, when multiplied by the horsepower rating of the motor, gives the maximum power the motor may be required to supply continuously without damaging the motor.

Question 14

What would an electrician use the insulation system designation for when installing a motor?

Answer 14

To determine the insulation (temperature) type of the supply conductors.

Question 15

What does the ambient temperature rating refer to?

Answer 15

The highest temperature allowed for the cooling medium that contacts the heated parts of the motor

Question 16

In what units is the speed of the motor rated?

Answer 16

revolutions per minute (rpm)

Question 17

For a motor to operate successfully under running conditions at full load, by what maximum percentage may the frequency of the supply vary?

Answer 17

It should not vary by more than 5% of the rating on the nameplate.

Question 18

For a motor to operate successfully under running conditions at full load, what is the maximum variation from rated voltage allowed?

Answer 18

It should not vary by more than 10% of the rating on the nameplate.

Question 19

What relationship is expressed by the design letter on a motor nameplate?

Question 20

Define the term full-load current

Answer 20

The full-load current is the current the motor will draw when it operates at rated voltage and supplies rated horsepower.