# Induction Motors MCQs – 1000+ Induction Motor Objective Multiple Choice Questions with Answers

An induction motor is a widely used AC electric motor that finds its applications in various industrial applications. An induction motor operates based on the principle of electromagnetic induction. Induction motors find their practical application in fans, pumps, compressors, conveyors, and numerous other mechanical systems where reliable and robust performance is essential. Given below are 1000+ Electrical Engineering MCQs on Induction Motors.

## An Induction motor is turning at 1200 rpm, the frequency is 60 Hz. The number of poles are

An Induction motor is turning at 1200 rpm, the frequency is 60 Hz. The number of poles are:

1. 2 poles
2. 4 poles
3. 6 poles
4. 8 poles

Correct answer: 3. 6 poles

Explanation: From formula, P = 120 * f / Speed, P = 120 * 60 / 1200 = 6

## The correct statement about the induction motor (IM)

The correct statement about the induction motor (IM):

1. It requires no DC field current
2. It requires only one DC field current
3. It requires multiple DC field currents
4. None of above

Correct answer: 1. It requires no dc field current

Explanation: Unlike other types of electrical motors such as DC motors, which rely on a direct current (DC) field current to create a magnetic field, induction motors operate solely on alternating current (AC). The stator of an induction motor generates a rotating magnetic field through the interaction of the AC supply voltage and the motor windings. This rotating magnetic field induces currents in the rotor, resulting in the generation of torque and motor rotation. Therefore, induction motors do not require any external or separate DC field current for their operation.

## Crawling is a phenomenon associated with squirrel cage design in which IM runs at a very slow speed, this phenomenon occurs due to

Crawling is a phenomenon associated with squirrel cage design in which IM runs at a very slow speed, this phenomenon occurs due to:

1. Low voltage
2. Heavy loads connected to rotor
3. Bad mechanical design of the machine
4. Harmonics

Correct answer: 4. Harmonics

Explanation: The phenomenon of crawling in an induction motor refers to a situation where the motor runs at an abnormally slow speed. This phenomenon is associated with squirrel cage designs. Crawling occurs primarily due to low voltage. When the voltage supplied to the motor is low, the magnetic field produced by the stator weakens. As a result, the rotor experiences insufficient torque, causing it to rotate at a reduced speed or even stall. Heavy loads connected to the rotor or bad mechanical design of the machine can contribute to increased chances of crawling, but the primary cause is the low voltage supply. Harmonics, on the other hand, are not directly related to the crawling phenomenon.

## Fractional slip can be expressed by the equation

Fractional slip can be expressed by the equation:

1. Rotor’s copper loss / Rotor’s input
2. Rotor’s copper loss / Rotor’s output
3. Stator’s copper loss / Stators input
4. Stator’s copper loss / Stators input

Correct answer: 1. Rotor’s copper loss / Rotor’s input

## A single phase motor is running slower than its original speed, the most obvious case which happened can be

A single phase motor is running slower than its original speed, the most obvious case which happened can be:

1. Winding is open
2. Winding is short
3. Bearings are worn
4. Smaller load than usual

Correct answer: 3. Bearings are worn

Explanation: When the bearings in a motor become worn, they can cause increased friction and resistance, leading to a decrease in the motor’s rotational speed. Worn bearings create mechanical inefficiencies that impede the smooth rotation of the motor shaft. This increased friction results in a slower operating speed.

## The motor which has high starting torque

The motor which has high starting torque:

1. Squirrel cage induction motor
2. Slip ring induction motor
3. Both possess same starting torque
4. Starting torque in both motors is zero

Correct answer: 2. Slip ring induction motor

Explanation: A slip ring induction motor, also known as a wound rotor induction motor, typically has a higher starting torque compared to a squirrel cage induction motor. The higher starting torque of a slip ring induction motor is primarily due to its rotor construction.

In a slip ring induction motor, the rotor consists of a set of windings connected to external slip rings. These slip rings allow for the external connection of resistors or additional devices during the starting period. By adjusting the resistance connected to the rotor windings, the motor can achieve higher torque during the starting phase.

On the other hand, a squirrel cage induction motor has a simple rotor design with short-circuited bars or conductors. While squirrel cage motors are efficient and widely used, they typically have lower starting torque compared to slip ring induction motors.

Therefore, in terms of starting torque, the slip ring induction motor is the correct choice as it offers higher torque capabilities during motor startup.

## The synchronous speed of a 4 pole machine operated at 50 Hz is

The synchronous speed of a 4 pole machine operated at 50 Hz is:

1. 200 r/min
2. 1500 r/min
3. 10,000 r/min
4. 50 r/min

Correct answer: 2. 1500 r/min

Synchronous speed = 120 *f/P = 120 * 50 / 4 = 1500 R/min

## Commonly, the power factor of a single phase induction motor is

Commonly, the power factor of a single phase induction motor is:

1. Leading
2. Lagging
3. Unity
4. Zero

Correct answer: 2. Lagging

Explanation: In a single-phase induction motor, the power factor is typically lagging. Power factor refers to the phase relationship between the voltage applied to the motor and the current drawn by the motor. In an inductive load like a single-phase induction motor, the current lags behind the voltage waveform due to the presence of inductance in the motor windings.

The inductive nature of the motor’s windings causes the current to lag behind the voltage, resulting in a lagging power factor. This lagging power factor can lead to a less efficient use of electrical power, as it requires additional reactive power to maintain the motor’s magnetic field.

It is important to note that power factor can be improved through the use of power factor correction techniques such as the addition of capacitors or the utilization of power factor correction devices to bring the power factor closer to unity (1) in order to improve energy efficiency and reduce electricity consumption. However, in its typical operating state, a single-phase induction motor has a lagging power fac

## Which of the following Starting methods has the lowest starting torque associated with it

Which of the following method Starting methods has the lowest starting torque associated with it:

1. ATL
2. Auto-transformer
3. Wye-delta
4. All have equal torque

Correct answer: 3. Wye-delta

Explanation: Among the given options, the wye-delta starting method has the lowest starting torque associated with it.

The wye-delta starting method, also known as the star-delta starting method, is commonly used to reduce the starting current and torque of induction motors. In this method, the motor is initially connected in a star configuration during the starting period, which reduces the voltage applied to the motor windings. This results in lower starting torque compared to the other methods.

On the other hand, the autotransformer starting method (ATL) and auto-transformer starting method provide higher starting torque compared to the wye-delta method. These methods involve the use of an autotransformer to reduce the applied voltage during motor starting, which allows for a higher starting torque.

Therefore, among the given options, the wye-delta starting method is associated with the lowest starting torque.

## The major disadvantage of speed control of a slip ring induction motor using resistance in the circuit is

The major disadvantage of speed control of a slip ring induction motor using resistance in the circuit is

1. High losses are associated with it
2. It is restricted for motors having power of more than 100 kW
3. With reduction in speed, the torque decreases significantly
4. The speed can be controlled only very broadly

Correct answer: 1. High losses are associated with it

Explanation: The correct answer for the MCQ is “High losses are associated with it.”

Using resistance in the circuit to control the speed of a slip ring induction motor does indeed have a major disadvantage, which is high losses. When resistance is introduced in the rotor circuit of a slip ring induction motor to control its speed, it results in increased power losses. The resistance causes a voltage drop and dissipates power as heat, leading to reduced efficiency.

While the other options listed may also have some limitations, such as power restrictions for motors over 100 kW or a decrease in torque with reduced speed, the major disadvantage of using resistance for speed control is the high losses. These losses not only reduce the overall efficiency of the motor but also result in increased heat generation, which can lead to additional cooling requirements and potentially affect the motor’s lifespan.

Additionally, it is worth noting that using resistance for speed control provides limited control range, which means the speed can only be controlled broadly rather than finely adjusted. However, the primary disadvantage is still the high losses associated with this method.

## The statement VFDs never use electronic components is

The statement VFDs never use electronic components is:

1. True
2. False

Correct answer: 2. False

## An induction motor has relatively high power factor at

An induction motor has relatively high power factor at

1. 20 percent load
2. Near full load
3. No load
4. Rated RPM

Correct answer: 2. Near full load

Explanation: An induction motor’s power factor improves as the load increases. At no load and low loads, the power factor is typically low because the motor draws more reactive power relative to the active power. Near full load, the motor operates more efficiently, drawing power that is more in phase with the voltage, thereby increasing the power factor. Rated RPM does not specifically indicate load level and therefore isn’t a direct indicator of power factor. Thus

More MCQs on Induction Motor:

More MCQs on Induction Motors