301 MCQ Questions on Induction Motors

101. What would be the backward field slip of a single phase induction motor running at N r.p.m., when it is given that its synchronous speed is N_s, and slip with respect to forward field is ‘s’? [UPPCL JE – 2014]

(a) s
(b) -s
(c) 1 – s
(d) 2 – s

102. The thermal loading on the motor can be determined by:

(a) Duty/Load cycle
(b) Temperature of the winding
(c) Age of the motor
(d) Ambient conditions [UPPCL JE – 2014]

103. A torque is developed in an electro mechanical energy conversion device, the value of which depends upon:

(a) Stator field strength and torque angle.
(b) Stator field and rotor field strengths.
(c) Stator field and rotor field strengths and the torque angle.
(d) Stator field strength only. [UPPCL JE – 2014]

104. Which of the following methods could be used to start a three-phase cage-type induction motor?

1. Direct-on-line starting
2. Auto-transformer starting

III. Star-delta starting
Choose the correct answer from the options given below.
(a) Only I and II
(b) Only II and III
(c) I, II and III
(d) Only I and III [UPPCL JE 2014]

105. What phenomenon is depicted by cross-field theory in context of single-phase induction motors? [UPPCL JE – 2014]

(a) How the rotor develops torque.
(b) How the stator develops torque.
(c) How the rotor develops current.
(d) How the stator develops current.

106. Which of the following is not a type of single phase induction motor?

(a) Resistance split phase type
(b) Capacitor split phase type
(c) Shaded pole type
(d) Opaque pole type [UPPCL JE 2014]

107. The following graph shows the torque-speed characteristics of a …… motor. [UPPCL JE – 2014]

(a) Single phase synchronous
(b) Capacitor split phase type
(c) Shaded pole type
(d) Opaque pole type

108. What will be the frequency (cycles per second) of AC voltage of a 10 pole AC generator which rotates at 1200 rpm? [UPPCL JE 2014]

(a) 100
(b) 135
(c) 150
(d) 168

109. The synchronous speed of linear induction motor depends on

(a) Width of the pole pitch
(b) Number of poles
(c) Supply frequency
(d) Both (a) and (c)

110. Compensating winding is used in:

(a) Capacitor start motor
(b) Shaded pole motor
(c) AC series motor
(d) Capacitor run motor [NMRC JE-2017]

111. The three phase induction motor stator windings is displaced by …….. from each other.

(a) 45° electrical
(c) 120° electrical
(b) 30° electrical
(d) 90° electrical [NMRC JE-2017]

112. A three phase induction motor is wound for 4 pole and is supplied from 50 Hz system. Calculate synchronous speed.

(a) 1550 rpm
(b) 1500 rpm
(c) 1440 rpm
(d) 1400 rpm [NMRC JE-2017]

113. A 4 pole, 50 Hz induction motor operates at 5% slip. The frequency of emf induced in the rotor will be

(a) 25 Hz
(b) 2 Hz
(c) 2.5 Hz
(d) 50 Hz [NMRC JE-2017]

114. If the rotor emf per phase at standstill is E₂ and the motor is operating at a slip (s), the generated rotor emf per phase under running condition will be:

(a) s²E₂
(b) E₂
(c) sE₂
 (d) E₂/s [NMRC JE-2017]

115. In case of 3 phase induction motors, plugging is done by:

(a) Starting the motor on load which is more than the rated load.
(b) Pulling the motor directly on line without a starter.
(c) Interchanging connections of any two phases of stator for quick stopping.
(d) Locking of rotor due to harmonics. [NMRC JE-2017]

116. When the supply voltage to an induction motor is reduced by 10%, the maximum torque will decrease approximately by:

(a) 10%
(b) 20%
(c) 5%
 (d) 40% [NMRC JE-2017]

117. In a three-phase induction motor, the rotor field runs at the following speed with respect to the stator structure:

(a) At synchronous speed in the direction of stator field
(b) At a slip speed in the direction of stator speed
(c) At synchronous speed in a direction opposite to that of stator field
(d) At zero speed

118. V/f is maintained constant in the following case of speed control of induction motor:

(a) Below base speed with voltage control
(b) Below base speed with frequency control
(c) Above base speed with frequency control
(d) None of these

119. In a double-cage induction motor, which of the following is not true:

(a) Resistance of outer bar is less
(b) Leakage reactance of outer cage is less
(c) Outer bar has smaller cross section
(d) None of these

120. For a single phase capacitor start induction motor which of the following statements is valid?

(a) The capacitor is used for power factor improvement
(b) The direction of rotation cannot be changed
(c) The direction of rotation can be changed by interchanging the supply terminals
(d) The direction of rotation can be changed by reversing the main winding terminals [Coal India – 2017]

121. If a resistance is inserted in the rotor circuit of a slip ring induction motor, then compared to direct on line starting:

(a) Both the starting current and torque reduce.
(b) Both the starting current and torque increase.
(c) The starting current reduces but starting torque increases.
(d) The staring current reduces but starting torque remains same.

122. In case of split phase motor, the phase shift between current in the two windings is around

(a) 30 degree
(b) 70 degree
(c) 90 degree
 (d) 120 degree [LMRC AE – 2016]

123. Induction generator is stable at a:

 (a) Speed below synchronous speed
(b) Speed above synchronous speed
(c) Speed equal to synchronous speed
(d) None of these

124. A capacitor start, capacitor run single phase induction motor is basically a

(a) AC series motor
(b) DC series motor
(c) 2 phase induction motor
(d) 3 phase induction motor

125. In a variable speed induction motor drive V/f kept constant over a wide range of its frequency variation. The motor operates in the following mode:

(a) Constant power mode
(b) Constant torque mode
(c) Constant slip mode
(d) Variable power and variable torque mode

126. At slip equal to infinity, the torque of a three phase induction motor is

(a) Zero
(b) Very small
(c) High
 (d) Very high

127. A 230 V, 50 Hz, 4 pole single phase induction motor is rotating in the clockwise forward direction at the speed of 1425 rpm. If the rotor resistance at standstill is 7.8 ohm, then what will be the effective rotor resistance in the backward branch of the equivalent circuit? [LMRC AE – 2016]

(a) 2 ohm
(b) 4 ohm
(c) 78 ohm
(d) 156 ohm

128. Skew is used in induction motor in order to reduced torque of

(a) Time harmonics
(b) Space harmonics
(c) Slot harmonics
(d) Reverse rotating field

129. An I.M. having T = 60 N-m. When delta connected develops a starting torque of 120 N-m. For the same supply voltage, if the motor is star connected, the starting torque developed will be

(a) 40 N-m
(b) 60 N-m
(c) 90 N-m
(d) 120 N-m

130. A 3-4 slip-ring induction motor is fed from the rotor side with stator winding short circuited. The frequency of the current flowing in the short circuit stator is

(a) Slip frequency
(b) Supply frequency
(c) Frequency corresponding to rotor speed
(d) Zero

131. Which of the following 1-φ motor has highest starting torque?

(a) Split phase motor
(b) Shaded pole motor
(c) Capacitor start motor
(d) Repulsion motor

132. In a 3-φ induction motor, the electrical representation of the variable mechanical load is the resistance of

(a) R₂ x (s – 1)
(b) R₂ x (1/s – 1)
(c) R₂ x (1 – 1/s)
(d) R₂ x (1 – s)

133. The induced e.m.f. will be maximum when a conductor cuts the magnetic field at an angle of

(a) 15°
(b) 30°
(c) 45°
 (d) 90° [UPPCL JE 2007]

134. The frequency of an induction motor having synchronous speed of 600 rpm, is 40 Hz. Find

the number of poles.
(a) 6
(b) 12
(c) 8
(d) 4 [UPPCL JE 2016]

135. Slip rings and brushes are found in:

(a) Wound rotor
(b) Squirrel cage rotor
(c) Both of the above
(d) Neither wound nor squirrel cage rotor

136. If there are no copper losses in the rotor, then

(a) Rotor will not run
(b) Rotor will run at a very low speed
(c) Rotor will run at a very high speed
(d) Rotor will run at synchronous speed [UPPCL JE – 2016]

137. An induction machine works at generating mode when slip is:

(a) Zero
(b) 0 – 1
(c) More than one
(d) Negative [UPPCL JE 2016]

138. ………..braking is generally used in braking of battery operated scooters. [UPPCL JE 2016]

(a) Plugging
(b) Regenerative
(c) Mechanical
(d) Rheostatic

139. In an Induction motor, number of stator slots should never be equal to number of rotor slots in order to prevent:

(a) Crawling
(b) Cogging
(c) Over heating
(d) Humming sound [UPPCL JE 2016]

140. Speed control of single phase series motor (AC) can be done by:

(a) Pole changing
(b) Frequency variation
(c) Voltage variation
(d) Shifting of brushes [UPPCL JE – 2016]

141.  ………are employed for the operation of Jaw Crushers.

(a) DC shunt wound motor
(b) Squirrel cage induction motor
(c) Belted slip ring induction motor
(d) Any DC motor [UPPCL JE 2016]

142. Slip of an induction motor ……… when the rotor starts gaining speed.

(a) Remains same
(b) Increases
(c) Decreases
(d) Increases or decreases [UPPCL JE – 2016]

143. Starting torque of an induction motor is ………… applied voltage. [UPPCL JE – 2016]

(a) Directly proportional to
(b) Inversely proportional to
(c) Directly proportional to the square of
(d) Inversely proportional to the square of

144. The advantage of double squirrel cage induction motor over single cage rotor is to improve

(a) Speed regulation
(b) Power factor
(c) Slip
(d) Starting torque [TNPSC AE 2018]

145. The main advantage of distributing the winding in slots is to

(a) Reduce the size of the machine
(b) Add mechanical strength to the winding
(c) Reduce the amount of copper required
(d) Reduce the harmonics in the generated emf

146. Match List-I with List-II

List-I (Method of speed control of 3 phase wound-type induction motor)

1. Rotor resistance control
2. Constant volts/Hz control
3. Injection of voltage in rotor circuit

List-II (Performance achieved)

1. Both speed and p.f. can be controlled
2. Maximum torque remains constant
3. Starting torque decreases

(a) A-2, B-3, C-1
(b) A-1, B-2, C-3
(c) A-2, B-1, C-3
(d) A-3, B-2, C-1

147. An induction machine works at motoring mode when slip is

(a) Zero
(b) 0 – 1
(c) More than one
(d) Negative [UPPCL AE – 2016]

148. Which motor is used in the centrifugal pump?

(a) Split phase induction motor
(b) Shaded pole induction motor
(c) Squirrel cage rotor
(d) Capacitor start/Capacitor run induction motor

149. Which law is a consequence of the law of conservation of energy? [UPPCL JE 2018]

(a) Lenz’s law
(b) Moore’s law
(c) Faraday’s law
(d) Kepler’s law

150. Select out the advantage of three phase induction motor over single phase inductor motor. [UPPCL JE – 2018]

(i) Self starting motor
(ii) Better power factor
(iii) High efficiency
(a) (i), (ii) and (iii)
(b) (ii) and (iii)
(c) (i) and (ii)
(d) (i) and (iii)