71. In an induction motor, when the number of stator slots is equal to an integral number of rotor slots _________
A. There may be a discontinuity in torque slip characteristics
B. A high starting torque will be available
C. The maximum torque will be high
D. The machine may fail to start
Answer: D
When the number of stator slots is an integral multiple of a number of rotor slots the machine fails to start and this phenomenon is called cogging.
72. A 3-phase induction motor runs at almost 1000 rpm at no load and 950 rpm at full load when supplied with power from a 50 Hz, 3-phase supply. What is the corresponding speed of the rotor field with respect to the rotor?
A. 30 revolution per minute
B. 40 revolution per minute
C. 60 revolution per minute
D. 50 revolution per minute
Answer: D
Supply frequency=50 Hz
No-load speed of motor = 1000 rpm.
The full load speed of the motor=950 rpm.
Since the no-load speed of the motor is almost 1000 rpm, hence synchronous speed is near 1000 rpm.
Speed of rotor field=1000 rpm.
Speed of rotor field with respect to rotor = 1000-950 = 50 rpm.
73. Calculate the active power in a 487 H inductor.
A. 2482 W
B. 1545 W
C. 4565 W
D. 0 W
Answer: D
The inductor is a linear element. It only absorbs reactive power and stores it in the form of oscillating energy.
The voltage and current are 90° in phase in the case of the inductor so the angle between V & I is 90°.
P = VIcos90 = 0 W.
74. Calculate the active power in a 788 ω resistor with 178 A current flowing through it.
A. 24.96 MW
B. 24.44 MW
C. 24.12 MW
D. 26.18 MW
Answer: A
The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in the case of the resistor so the angle between V & I is 90°.
P=I2R
=178×178×788=24.96 MW.
75. The direct axis is taken along ________
A. Inter-polar axis
B. Rotor pole axis
C. In between interpolar and rotor axis
D. Parallel to the interpolar axis
Answer: B
The direct axis is oriented along the rotor pole axis and the quadrature axis is 90° electrical to the rotor pole axis. The direct axis is not oriented along the inter-polar axis.
76. A 4-pole, 3-phase, 60 Hz induction motor is operating at a speed of 1500 rpm. The frequency of the rotor current of the motor in Hz is __________
A. 5
B. 4
C. 2
D. 7
Answer: B
Given a number of poles = 4.
Supply frequency is 60 Hz.
Rotor speed is 1500 rpm.
Ns = 120×f÷P
= 120×60÷4 = 1800 rpm.
S=Ns-Nr÷Ns
= 1800-1500÷1800 = .166.
F2=sf=.166×60=4 Hz.
77. Calculate the phase angle of the sinusoidal waveform z(t)=8cos(45t+2π÷15).
A. 2π÷39
B. 2π÷15
C. π÷4
D. 2π÷44
Answer: B
The sinusoidal waveform is generally expressed in the form of V=Vmsin(ωt+α) where Vm represents peak value, ω represents angular frequency, and α represents a phase difference.
78. The leakage flux paths are ________ on the angular position of the rotor.
A. Dependent
B. Proportional
C. Independent
D. Dependent and independent
Answer: C
Leakage flux and leakage reactance are constant irrespective of rotor angular position. The armature reaction though is dependent on the angular position of the rotor in the salient pole machine but in the cylindrical rotor machine, both the quantities are independent of rotor position.
79. The hunting phenomenon in a synchronous motor is also referred to as _________
A. Surging
B. Phase swinging
C. Cogging
D. Surging and phase swinging
Answer: D
During hunting, power oscillates so power surges are observed and hence it can be called surging. Also, the rotor phase angle oscillates and it is called phase swinging.
80. Which of the following are used in preventing the hunting phenomenon in synchronous generators?
A. Damper bars
B. Short pitch chords
C. Distributed winding
D. Damper bars and short pitch chords
Answer: A
Damper bars try to maintain synchronism between the rotating magnetic field and the rotor so they help in preventing hunting. It produces surges in the machine.
