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3rd Year ELectrical - Motors Theory > Operation of Three-Phase Induction Motors > Flashcards

Operation of Three-Phase Induction Motors Flashcards

1
Q

Formula:

n_sync =

A

120 f / N

–> 120 frequency / Number Poles

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2
Q

Formula

%slip =

A

n_sync - n_rotor / n_sync

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3
Q

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?

A

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

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4
Q

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

A

Slip Speed

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5
Q

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

A

increases

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6
Q

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

True or False?

A

True

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7
Q

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

A

number of poles;

frequency

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8
Q

The rotor resistance of SCIM is treated as a ________.

A

constant

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9
Q

The equation to calculate rotor frequency is

A

Fr = s x Fbr

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10
Q

The equation to calculate rotor voltage is

A

Er = s x Ebr

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11
Q

The equation to calculate rotor reactance is

A

Xr = s x Xbr

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12
Q

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

A

Breakdown torque

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13
Q

State three quantities that are directly proportional to slip

A

Rotor Voltage
Rotor Frequency
Rotor Reactance

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14
Q

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

A

c) increases and then decreases

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15
Q

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

A

b) 6 Hz

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16
Q

Maximum torque and maximum active current are produced when:

A

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

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17
Q

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

A

75% to 80%

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18
Q

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

A

Torque

Speed (rotational frequency)

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19
Q

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

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

A

d) design D

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20
Q

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

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

A

a) design A

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21
Q

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

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

A

a) design A

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22
Q

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

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

A

a) design A

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23
Q

The NEMA rotor design that is the most common is:

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

A

b) design B

24
Q

What are four disadvantages of wound-rotor motors?

A

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

25
Q

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

True or False?

A

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

26
Q

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

A

d) wound-rotor

27
Q

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

A

d) greater cost cost compared to SCIM and higher maintenance

28
Q

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

True or False?

A

True

29
Q

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

True or False?

A

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

30
Q

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

A

output energy to input energy

31
Q

The losses in an induction motor are _________ and __________.

A

electrical and mechanical

32
Q

Electrical losses have two sources:

A

Iron Losses

Copper Losses

33
Q

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

A

Friction

Windage

34
Q

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

A

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

35
Q

Rotor resistance (Rr) is a constant in the rotor of a SCIM.

True or False?

A

True

36
Q

As the rotor of a SCIM accelerates, the rotor resistance decreases.

True or False.

A

False

37
Q

Once the rotor of a SCIM starts to accelerate, the rotor reactance increases.

True or False?

A

False

38
Q

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.

A

lower than

39
Q

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

A

Increase

40
Q

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

A

decrease

120 x f / Number of poles

41
Q

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

A

rotor

42
Q

The rotor active current of a SCIM can be found by

A

Multiplying rotor current by rotor power factor.

43
Q

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

A

600%

44
Q

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

A

150%

45
Q

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

A

200%

46
Q

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

A

Design A and B

47
Q

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

A

500%

48
Q

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

A

150%

49
Q

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

A

200%

50
Q

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

A

225%

51
Q

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

A

Design C and D

52
Q

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

A

500%

53
Q

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

A

400%

54
Q

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

A

275%

55
Q

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

A

Design D

3rd Year ELectrical - Motors Theory (7 decks)

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