Swinburne’s test cannot measure

 Q101. In a DC generator, the armature emf increases if

A) Speed increases
B) Flux increases
C) Number of conductors increases
D) All of these
Ans: D

Q102. If the speed of a DC generator is doubled, the generated emf will
A) Double
B) Remain same
C) Halve
D) Increase four times
Ans: A

Q103. The terminal voltage of a loaded DC generator is less than the no-load voltage due to
A) Armature reaction and voltage drop
B) Commutation
C) High resistance
D) Poor design
Ans: A

Q104. The slope of the open circuit characteristic (OCC) of a DC generator is proportional to
A) Speed
B) Load
C) Current
D) Power
Ans: A

Q105. The iron losses in DC machines are independent of
A) Load
B) Speed
C) Flux
D) Voltage
Ans: A

Q106. If field current in a shunt generator is increased, generated emf
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero
Ans: A

Q107. In a DC shunt motor, the torque developed is proportional to
A) IaΦ
B) Ia²
C) Φ²
D) Ia/Φ
Ans: A

Q108. The armature current in a DC motor is given by
A) Ia = (V − Eb) / Ra
B) Ia = (V + Eb) / Ra
C) Ia = Eb / Ra
D) Ia = V × Eb
Ans: A

Q109. When load increases on a DC motor, back emf
A) Decreases
B) Increases
C) Remains constant
D) Becomes zero
Ans: A

Q110. The internal characteristic of a DC generator gives relation between
A) Induced emf and armature current
B) Terminal voltage and load current
C) Flux and current
D) Speed and torque
Ans: A

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Q111. The voltage regulation of a DC generator is
A) (No-load V − Full-load V)/Full-load V × 100
B) (Full-load V − No-load V)/No-load V × 100
C) V/I
D) None
Ans: A

Q112. In DC machines, the commutation is improved by
A) Reducing brush contact resistance
B) Using high resistance brushes
C) Increasing brush width
D) Increasing current
Ans: B

Q113. For sparkless commutation, brush contact resistance should be
A) Moderate
B) Zero
C) Infinite
D) Very high
Ans: A

Q114. Armature reaction in DC motor produces
A) Cross-magnetizing effect
B) Demagnetizing effect
C) Both A and B
D) None
Ans: C

Q115. A 4-pole DC machine with lap winding has 4 parallel paths. If wave wound, the parallel paths will be
A) 2
B) 4
C) 8
D) 1
Ans: A

Q116. In differential compounding of DC machine, series field opposes
A) Shunt field
B) Armature field
C) Load current
D) None
Ans: A

Q117. A DC shunt generator fails to build up if
A) Field winding reversed
B) Residual magnetism lost
C) Field resistance too high
D) Any of the above
Ans: D

Q118. For same power output, a wave wound armature requires
A) Less current
B) More current
C) More turns
D) Fewer turns
Ans: C

Q119. In a DC machine, iron losses vary as
A) B²f
B) I²R
C) V²
D) Constant
Ans: A

Q120. The efficiency of a DC machine increases when
A) Losses decrease
B) Load increases
C) Speed increases
D) Temperature increases
Ans: A

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Q121. The commutation process can be improved by
A) Brush shift
B) Interpoles
C) Compensating winding
D) All of these
Ans: D

Q122. In a DC motor, when the load torque increases, speed
A) Decreases
B) Increases
C) Remains same
D) Reverses
Ans: A

Q123. If supply voltage is increased in a DC shunt motor, speed
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Ans: A

Q124. For constant flux, torque is proportional to
A) Armature current
B) Voltage
C) Resistance
D) Speed
Ans: A

Q125. The function of compensating winding is to
A) Neutralize armature reaction
B) Improve commutation
C) Reduce flux distortion
D) All of these
Ans: D

Q126. A DC series motor runs at dangerously high speed at no-load because
A) Flux is very low
B) Armature current is small
C) Back emf is large
D) Torque is constant
Ans: A

Q127. A cumulatively compounded DC motor has
A) High starting torque
B) Poor speed regulation
C) Constant speed
D) Low efficiency
Ans: A

Q128. In DC motors, regenerative braking is used for
A) Energy recovery
B) Reducing speed
C) Both A and B
D) None
Ans: C

Q129. In Hopkinson’s test, the machines should be
A) Identical
B) Different
C) Series connected
D) Parallel connected
Ans: A

Q130. Swinburne’s test cannot measure
A) Commutation losses
B) No-load losses
C) Iron losses
D) Armature copper losses
Ans: A

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Q131. The back emf in a DC motor acts
A) Opposite to supply voltage
B) In same direction as supply
C) Across brushes
D) Randomly
Ans: A

Q132. The direction of back emf is opposite to
A) Supply voltage
B) Armature current
C) Field flux
D) Speed
Ans: A

Q133. During dynamic braking, energy is dissipated in
A) External resistor
B) Field winding
C) Armature
D) Shaft
Ans: A

Q134. The retardation test is used to determine
A) Iron and friction losses
B) Copper losses
C) Armature resistance
D) Torque
Ans: A

Q135. The maximum efficiency of DC machine occurs when
A) Variable losses = Constant losses
B) Iron losses = Copper losses
C) Friction losses = Iron losses
D) None
Ans: A

Q136. A shunt motor runs at constant speed because
A) Flux remains nearly constant
B) Field current is small
C) Torque is small
D) Voltage drop negligible
Ans: A

Q137. In a DC generator, the terminal voltage is less than emf due to
A) Armature resistance and brush drop
B) Mechanical loss
C) Core loss
D) None
Ans: A

Q138. For a given armature current, torque in a series motor is proportional to
A) Φ²
B) Φ
C) Ia
D) Ia²
Ans: D

Q139. The most suitable speed control for DC shunt motor above base speed is
A) Flux control
B) Armature voltage control
C) Rheostatic control
D) None
Ans: A

Q140. A DC shunt generator is preferred for
A) Constant voltage applications
B) Variable load applications
C) High current loads
D) Battery charging only
Ans: A

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Q141. The compensating winding is placed in
A) Pole shoe
B) Yoke
C) Armature core
D) End cover
Ans: A

Q142. The speed-torque characteristic of DC shunt motor is
A) Nearly constant speed
B) Hyperbolic
C) Linear
D) Parabolic
Ans: A

Q143. In a 4-pole, lap wound DC machine having 500 conductors, the number of parallel paths is
A) 4
B) 2
C) 1
D) 8
Ans: A

Q144. The voltage drop per brush contact is typically
A) 0.5 to 2 V
B) 2 to 5 V
C) 5 to 10 V
D) 10 to 20 V
Ans: A

Q145. A DC motor draws 10A at 230V with back emf of 210V. Its armature resistance is
A) 2 Ω
B) 1.5 Ω
C) 2.0 Ω
D) 1.8 Ω
Ans: A
(V − Eb = IaRa → 230−210=10Ra → Ra=2Ω)

Q146. The torque of a DC motor is 40 Nm at 1000 rpm. Power developed = ?
A) 4.19 kW
B) 2.5 kW
C) 6.28 kW
D) 8.0 kW
Ans: A
(P = 2πNT/60 = 2π×1000×40/60 = 4188 W)

Q147. In a DC shunt motor, if flux decreases by 10%, speed increases by approximately
A) 11%
B) 10%
C) 9%
D) 12%
Ans: A
(Speed ∝ 1/Φ → ΔN ≈ +11%)

Q148. A DC series motor develops torque of 25 Nm at 20A. Assuming Φ ∝ Ia, the torque at 40A = ?
A) 100 Nm
B) 50 Nm
C) 75 Nm
D) 25 Nm
Ans: A
(T ∝ Ia² → (40/20)²×25 = 100 Nm)

Q149. The condition for maximum power in DC motor is
A) Eb = ½ V
B) Eb = V
C) Eb = 0
D) Eb = 2V
Ans: A

Q150. When two identical DC shunt machines are coupled for Hopkinson’s test, one acts as
A) Motor and the other as generator
B) Both motors
C) Both generators
D) Alternator and motor
Ans: A