Line C 3-Phase Motors

Question 1

What are the advantage of a 3-phase motor over a single-phase motor?

Answer 1

• They’re more efficient
• Physical size is less for the same power and voltage rating
• It’s easier to change the direction of rotation

Question 2

What are the two main industry standards for electric motors?

Answer 2

IEC and NEMA

Question 3

What is the purpose of a Service Duty for an electric motor?

Answer 3

To determine if the motor is correct for the application

Question 4

What is the purpose of the Ingress Protection (IP) of an electric motor?

Answer 4

To measure how well the motor is protected from the surrounding environment

Question 5

What is the temperature rating of a Class B electric motor?

Answer 5

130 °C

Question 6

What is the temperature rating of a Class F electric motor?

Answer 6

155 °C

Question 7

Referring to the figure below, what is the temperature rating of the motor?

Answer 7

155 °C

Question 8

Referring to the figure below, what is the amount of overload (in %) the motor can handle for short periods of time when operating within its normal voltage and frequency tolerance?

Answer 8

15%

Question 9

Referring to the figure below, what is the Power Factor of the motor?

Answer 9

93%

Question 10

Referring to the figure below, what would be the calculated RPM of the motor?

Answer 10

1,200 RPM

Question 11

If the synchronous speed of a motor is 1300 RPM and the rated full load speed is 1251 RPM, what is the calculated slip rating in a percentage?

Answer 11

3.77%

Question 12

What is the calculated Synchronous RPM of a 6-pole three-phase induction motor when operated at 60 Hz if the three-phase induction motor has a Full Load RPM rating on the nameplate of 1300 RPM?

Answer 12

1200 RPM

Question 13

What is the calculated Full Load RPM of a 4-pole three-phase induction motor when operated at 50 Hz if the three-phase induction motor has a Full Load RPM rating of 1745 and a synchronous RPM rating of 1800?

Answer 13

1454 RPM

Question 14

Torque induced on the rotor is directly proportional to the amount of slip of the induction motor.

TRUE / FALSE

Answer 14

TRUE

Question 15

What is the driving force of an electric motor?

Answer 15

Torque

Question 16

When is the most torque typically produced on an electric motor?

Answer 16

At start up

Question 17

If an electric motor is rated at 18 horsepower and runs at an RPM of 1900 how much torque can the motor produce?

Answer 17

49.76 ft/lb

Question 18

If an electric motor is rated at 6 KW and runs at an RPM of 1,850, how much torque can the motor produce in newton meters?

Answer 18

30.97 Nm

Question 19

Referring to the figure below, what is the type of electric motor connection?

Answer 19

Delta

Question 20

Referring to the figure below, what is the type of electric motor connection?

Answer 20

Star

Question 21

Referring to the figure below, what would the phase current be if the line voltage is 400 volt and the coil impedance is 15 Ohms?

Answer 21

26.67 Amps

Question 22

Referring to the figure below, what would the phase voltage be if the line voltage is 346 volt and the coil impedance is 30 Ohms?

Answer 22

200 Volts

Question 23

What is the most common causes of motor failure?

Answer 23

bad bearings

Question 24

What can cause excessive current draw on a motor during normal operation?

Answer 24

• damaged bearings
• damaged windings
• broken shaft