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BASIC NEC 2023

Induction Motor Practice Quiz Questions

Free online induction motor practice test. 25 basic electrical questions on synchronous speed, slip, starting methods, torque-slip curve, squirrel cage vs wound rotor motors, and power factor. Good for electrician exam prep.

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An induction motor is the most common type of AC motor. The stator windings create a rotating magnetic field that induces current in the rotor, which then produces torque — this is why it is also called an asynchronous motor: the rotor never quite reaches the synchronous speed of the rotating field. Key formulas: synchronous speed N_s = 120f/P (f = frequency, P = number of poles), and slip s = (N_s − N_r) / N_s, where N_r is the actual rotor speed. Rotors come in two main types: squirrel cage (simple, rugged, fixed rotor bars) and wound rotor (slip rings let you add external resistance to control starting torque and current).

25

Questions

38 min

Time

70%

Pass

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Induction Motor Practice Quiz Questions — Question List

Question 1: A 6 pole 60 HZ induction motor normally runs at which of the following speed?

1140 rpm
1240 rpm
1220 rpm
1500 rpm

Question 2: When the supply voltage to induction motor is fall by 2% what is percentage increase in torque of an induction motor?

4 %
2 %
8 %
no change

Question 3: The slip of the induction motor is always ..........................

Greater than unity
Less than Zero
Between Zero and Unity

Question 4: Which of the following reason the starters are used in induction motor?

Its starting torque is high
Its starting current is five times rated current.
It drives heavier load during starting

Question 5: When the induction is started on load does not accelerate to full load and runs at lower speed ( 1/7). Under this condition the motor is said to be .............................

locking
plugging
cogging
crawling

Question 6: Which of the following starting method is preferable for smooth starting of squirrel case induction motor?

star delta
pony motor
auto transformer

Question 7: When the induction motor runs at rated load and speed the iron losses are negligible.

True
Flase

Question 8: Which of the following can be varied to change the speed of the induction motor?

Supply voltage only
Supply Frequency only
Either supply frequency or supply voltage

Question 9: In servo applications the motor used is ...........................

universal motor
two phase induction motor
three phase induction motor

Question 10: An induction motor develops torque because the rotating magnetic field of the stator:

Induces current in the rotor bars, which react with the field to produce torque
Is directly wired to the rotor through brushes
Charges a battery mounted on the rotor
Has no effect on the rotor at all

Question 11: What is the synchronous speed of a 4-pole induction motor connected to a 50 Hz supply?

1000 rpm
1500 rpm
3000 rpm
750 rpm

Question 12: What is the main advantage of a wound rotor (slip-ring) induction motor over a squirrel cage motor?

External resistance can be added to the rotor circuit to control starting torque and current
It never needs any starter at all
It has no rotor windings
It runs above synchronous speed

Question 13: On the torque-slip curve of an induction motor, the torque is zero when the slip is:

Zero (rotor at synchronous speed)
One (rotor standing still)
0.5
Torque is never zero

Question 14: In a wound rotor induction motor, adding external resistance to the rotor circuit during starting:

Increases starting torque and reduces starting current
Decreases starting torque and increases starting current
Has no effect on starting torque or current
Stops the motor from starting at all

Question 15: The power factor of an induction motor running at light (no) load is:

Very low and lagging
Unity (1.0)
Very high and leading
Always negative

Question 16: The no-load current of an induction motor is mostly made up of which component?

Magnetizing current
Full-load working current
Short-circuit current
Rotor braking current

Question 17: How do you reverse the direction of rotation of a three-phase induction motor?

Interchange any two of the three supply lines
Increase the supply voltage
Reduce the supply frequency to zero
It is not possible to reverse a three-phase motor

Question 18: A double squirrel cage induction motor uses two sets of rotor bars mainly to get:

High starting torque with low starting current, and good running efficiency
Twice the rated output power
The ability to run on DC supply
Zero starting current

Question 19: "Cogging" (magnetic locking) in a squirrel cage induction motor refers to:

The motor refusing to start due to magnetic locking between stator and rotor slots
The motor running at exactly 1/7th of synchronous speed under load
The motor overheating at full load
The motor reversing direction on its own

Question 20: If a running three-phase induction motor loses one of its three supply phases (single-phasing), the motor will:

Keep running but draw much higher current and overheat
Stop instantly with no damage
Automatically reduce its speed to zero current
Increase its output power safely

Question 21: An induction machine connected to the grid will act as an INDUCTION GENERATOR when it is driven by a prime mover:

Above synchronous speed
Below synchronous speed
Exactly at synchronous speed
Only when stopped completely

Question 22: Which NEMA design class of squirrel cage induction motor is the most common general-purpose type, offering normal starting torque with normal starting current?

NEMA Design B
NEMA Design C
NEMA Design D
NEMA Design A

Question 23: An induction motor is also called an "asynchronous motor" because:

The rotor speed is always a little less than synchronous speed while motoring
It cannot be connected to an AC supply
It only runs on DC current
It has no rotating magnetic field

Question 24: A 4-pole, 50 Hz induction motor runs at 1450 rpm under load. What is the percentage slip?

1.33 %
3.33 %
5 %
10 %

Question 25: As the mechanical load on a running induction motor increases, the slip:

Increases
Decreases
Stays exactly the same
Becomes negative

EXAM CARD BASIC

Questions25

Time limit38 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

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Source: NEC 2026 (NFPA 70)