Operation of Three-Phase Induction Motors

Question 1

Formula:

n_sync =

Answer 1

120 f / N

–> 120 frequency / Number Poles

Question 2

Formula

%slip =

Answer 2

n_sync - n_rotor / n_sync

Question 3

At no-load condition, the rotor speed of a three-phase induction motor is the same as the synchronous speed because there is no load, so no torque is required.

True or False?

Answer 3

False; The rotor speed always lags the magnetic field speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque.

Question 4

The speed difference between the actual rotor speed and synchronous speed is called ___________.

Answer 4

Slip Speed

Question 5

The slip speed of a rotor (increases, decreases, stays the same) as load is added tot the shaft of a three-phase induction motor.

Answer 5

increases

Question 6

A motor with low percent speed regulation is more efficient than one with high percent speed regulation.

True or False?

Answer 6

True

Question 7

Rotating magnetic fields have a constant magnitude and rotate at a constant speed determined by the ________ of _______ and the __________ of the power supply.

Answer 7

number of poles;

frequency

Question 8

The rotor resistance of SCIM is treated as a ________.

Answer 8

constant

Question 9

The equation to calculate rotor frequency is

Answer 9

Fr = s x Fbr

Question 10

The equation to calculate rotor voltage is

Answer 10

Er = s x Ebr

Question 11

The equation to calculate rotor reactance is

Answer 11

Xr = s x Xbr

Question 12

Once a motor is at running speed, if load counter torque becomes greater than __________ _______ the rotor stalls.

Answer 12

Breakdown torque

Question 13

State three quantities that are directly proportional to slip

Answer 13

Rotor Voltage
Rotor Frequency
Rotor Reactance

Question 14

As a SCIM accelerates from a standstill to normal running speed, active rotor current:

a) decreases continuously
b) increases continuously
c) increases and then decreases
d) decreases and then increases

Answer 14

c) increases and then decreases

Question 15

In North America, a motor running at 90% of syncronous speed has a rotor frequency of:

a) 0 Hz
b) 6 Hz
c) 54 Hz
d) 60 Hz

Answer 15

b) 6 Hz

Question 16

Maximum torque and maximum active current are produced when:

Answer 16

Rotor reactance equals rotor resistance
Rotor power factor angle is 45 degrees
Rotor power factor is 0.707

Question 17

NEMA design A or B rotors produce their maximum torque at about ___% to ___% of synchronous speed.

Answer 17

75% to 80%

Question 18

Mechanical power is the product of what two (2) mechanical quantities?

Answer 18

Torque

Speed (rotational frequency)

Question 19

The NEMA rotor design that produces the highest starting torque is:

a) design A
b) design B
c) design C
d) design D

Answer 19

d) design D

Question 20

The NEMA rotor design that produces the highest starting current is:

a) design A
b) design B
c) design C
d) design D

Answer 20

a) design A

Question 21

The NEMA rotor design that produces the highest breakdown torque is:

a) design A
b) design B
c) design C
d) design D

Answer 21

a) design A

Question 22

The NEMA rotor design that produces the lowest full load slip is:

a) design A
b) design B
c) design C
d) design D

Answer 22

a) design A

Question 23

The NEMA rotor design that is the most common is:

a) design A
b) design B
c) design C
d) design D

Answer 23

b) design B

Question 24

What are four disadvantages of wound-rotor motors?

Answer 24

Higher Cost of the wound rotor
Larger frame needed to accommodate brushes and slip rings
Required auxiliary equipment (the secondary resistors and their controller)
Extra maintenance to monitor and replace the brushes

Question 25

Speed control of a wound-rotor motor is suitable from 0 r/min to half of syncronous speed. True or False?

Answer 25

False; Speed control of a wound-rotor motor is most suitable from 0 r/min to full-load speed.

Question 26

The rotor that performs best in terms of producing high starting torque, low starting current and full-load efficiency is the: a) NEMA design A b) NEMA design B c) NEMA design C d) wound-rotor

Answer 26

d) wound-rotor

Question 27

The main disadvantage of a wound-rotor motor is its: a) higher starting current b) excessive starting torque c) inadequate starting torque d) greater cost cost compared to SCIM and higher maintenance

Answer 27

d) greater cost cost compared to SCIM and higher maintenance

Question 28

The starting torque of a wound-rotor motor can be adjusted by the machine operator. True or False?

Answer 28

True

Question 29

A wound-rotor motor should be started with the resistors disconnected from the rotor circuit (bypassed). True or False?

Answer 29

False; the resistance is switched into the rotor circuit for starting

Question 30

Efficiency is the ratio of ______ energy (power) to ______ energy (power).

Answer 30

output energy to input energy

Question 31

The losses in an induction motor are _________ and __________.

Answer 31

electrical and mechanical

Question 32

Electrical losses have two sources:

Answer 32

Iron Losses | Copper Losses

Question 33

Mechanical losses (which are ultimately converted to heat) are caused by:

Answer 33

Friction | Windage

Question 34

Speed regulation is most related to the percentage of: a) slip the motor has at full speed b) rotational frequency change from no-load to full-load speeds c) motor efficiency at full-load d) motor efficiency as compared to torque output

Answer 34

b) rotational frequency change from no-load to full-load speeds

Question 35

Rotor resistance (Rr) is a constant in the rotor of a SCIM. True or False?

Answer 35

True

Question 36

As the rotor of a SCIM accelerates, the rotor resistance decreases. True or False.

Answer 36

False

Question 37

Once the rotor of a SCIM starts to accelerate, the rotor reactance increases. True or False?

Answer 37

False

Question 38

At full load, the power factor of the rotor in a SCIM is normally (lower than, higher than, about the same as) its power factor at no-load.

Answer 38

lower than

Question 39

As load is added to a SCIM, current will (increase, decrease, stay the same)

Answer 39

Increase

Question 40

Increasing the number of poles in an induction motor will (increase / decrease) the synchronous speed.

Answer 40

decrease 120 x f / Number of poles

Question 41

The torque developed in a WRIM is directly proportional to ______ active current.

Answer 41

rotor

Question 42

The rotor active current of a SCIM can be found by

Answer 42

Multiplying rotor current by rotor power factor.

Question 43

Design A rotors produce a starting current of about _____ % of full-load current

Answer 43

600%

Question 44

Design A rotors produce a starting torque of about _____ % of full-load torque

Answer 44

150%

Question 45

Design A rotors produce a breakdown (maximum) torque of about _____ % of full-load torque

Answer 45

200%

Question 46

Design ___ and ___ rotors are suitable for most loads, but not high inertia loads or other hard to start loads

Answer 46

Design A and B

Question 47

Design B rotors produce a starting current of about _____ % of full-load current

Answer 47

500%

Question 48

Design B rotors produce a starting torque of about _____ % of full-load torque

Answer 48

150%

Question 49

Design B rotors produce a breakdown (maximum) torque of about _____ % of full-load torque

Answer 49

200%

Question 50

Design C rotors produce a starting torque of about _____ % of full-load torque

Answer 50

225%

Question 51

Design ___ and ___ rotors produce a breakdown (maximum) torque at the instant of startup (maximum torque is equal to starting torque)

Answer 51

Design C and D

Question 52

Design C rotors produce a starting current of about _____ % of full-load current

Answer 52

500%

Question 53

Design D rotors produce a starting current of about _____ % of full-load current

Answer 53

400%

Question 54

Design D rotors produce a starting torque of about _____ % of full-load torque

Answer 54

275%

Question 55

Design ___ rotors are most suitable for high inertia loads and for frequently started, hard to start loads that overheat design C rotors.

Answer 55

Design D