21. The slip recovery scheme is a part of the synchronous speed-changing technique.
A. True
B. False
Answer: A
The slip recovery scheme comes under the synchronous speed changing technique. It uses an external induction machine to change the frequency value. The load torque remains constant here.
22. The slope of the V-I curve is 9.1°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
A. .16 Ω
B. .26 Ω
C. .25 Ω
D. .44 Ω
Answer: A
The slope of the V-I curve is resistance.
The slope given is 9.1° so R=tan(9.1°)=.16 Ω.
The slope of the I-V curve is reciprocal to resistance.
23. If induction motor air gap power is 1.8 KW and gross developed power is .1 KW, then rotor ohmic loss will be _________ KW.
A. 1.7
B. 2.7
C. 3.7
D. 4.7
Answer: A
Rotor ohmic losses are due to the resistance of armature windings. Net input power to the rotor is equal to the sum of rotor ohmic losses and mechanically developed power.
Rotor ohmic losses=Air gap power-Mechanical developed power = 1.8-.1=1.7 KW.
24. The power factor of a squirrel cage induction motor is ___________
A. High at light load only
B. High at heavy loads only
C. Low at the light and heavy loads both
D. Low at rate load only
Answer: B
At heavy loads, the current drawn is high due to which the active power component increases. An increase in the active power component increases the power factor of the machine.
25. At low values of slip, the electromagnetic torque is directly proportional to ___________
A. s
B. s2
C. s3
D. s4
Answer: A
At low values of slip, the electromagnetic torque is directly proportional to the slip value. Due to heavy loading slip value decreases which increases the ratio of R2÷s.
26. Calculate the time period of the waveform v(t)=12sin(8πt+8π÷15)+144sin(2πt+π÷6)+ 445sin(πt+7π÷6).
A. 8 sec
B. 4 sec
C. 7 sec
D. 3 sec
Answer: B
The fundamental time period of the sine wave is 2π.
The time period of z(t) is L.C.M {4,1,2}=4 sec.
The time period is independent of phase shifting and time-shifting.
27. Calculate the total heat dissipated in a resistor of 44 Ω when 0 A current flows through it.
A. 0 W
B. 2 W
C. 1.5 W
D. .3 W
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=0×0×44=0 W.
28. The value of slip at which maximum torque occurs ________
A. R2÷X2
B. 4R2÷X2
C. 2R2÷X2
D. R2÷3X2
Answer: A
The maximum torque occurs when the slip value is equal to R2÷X2. Maximum torque is also known as breakdown torque, stalling torque, and pull-out torque.
