10. A 4-pole lap wound generator with 720 armature conductors and a flux per pole of .003 Wb has an armature current of 50 A. The developed torque is _________
A. 17.25 N-m
B. 17.19 N-m
C. 16.54 N-m
D. 16.89 N-m
Answer: B
The developed torque in the motor is Km×I. The value of the machine constant(Km) is
Eb÷ωm = Φ×Z×P÷2×3.14×A
= .003×720×4÷2×3.14×4 = .3438 Vs/rad.
The developed torque is .3438×50 = 17.19 N-m.
12. The generated e.m.f from 70-pole armature having 100 conductors driven at 10 rev/sec having flux per pole as 20 mWb, with wave winding is ___________
A. 730 V
B. 740 V
C. 700 V
D. 690 V
Answer: C
The generated e.m.f can be calculated using the formula
Eb = Φ×Z×N×P÷60×A
where
Φ represents flux per pole
Z represents the total number of conductors
P represents the number of poles
A represents the number of parallel paths
N represents speed in rpm.
In wave winding number of parallel paths is 2.
Eb = .02×70×100×600÷60×2 = 700 V.
13. The unit of current is Tesla.
A. True
B. False
Answer: B
The current is the amount of charge that can flow through an area in a given amount of time.
It is mathematically represented as d(q)/d(t). It is expressed in terms of Ampere.
14. Calculate the moment of inertia of the sphere having a mass of 12 kg and a radius of 78 cm. [/bg_collapse]
A. 7.888 kgm2
B. 7.300 kgm2
C. 7.545 kgm2
D. 7.552 kgm2
Answer: B
The moment of inertia of the egg can be calculated using the formula
I=Σmiri2.
The mass of the egg and radius is given.
I=(12)×(.78)2=7.300 kgm2.
It depends upon the orientation of the rotational axis.
15. The most suitable servo-motor application is __________
A. AC series motor
B. DC series motor
C. AC two-phase induction motor
D. DC shunt motor
Answer: D
DC shunt motor has definite no-load speed, so they don’t ‘run away’ when the load is suddenly thrown off provided the field circuit remains closed. The speed for any load within the operating range of the motor can be readily obtained.
16. In a DC series motor, the electromagnetic torque developed is proportional to ______
A. Ia
B. Ia2
C. Ia3
D. Ia5
Answer: B
In a DC series motor, the electromagnetic torque developed is equal to KmΦIa.
In a DC series, the motor field winding is connected in series with the armature so the flux in the field winding is proportional to the current.
T = KmΦIa α Ia2.
17. Calculate the value of the time period if the frequency of the signal is 70 sec.
A. 0.014 sec
B. 0.013 sec
C. 0.017 sec
D. 0.079 sec
Answer: A
The time period is defined as the time after the signal repeats itself. It is expressed in the second
T = 1÷F=1÷70=.014 sec.
18. The slope of the V-I curve is 90°. Calculate the value of resistance.
A. 1.732 Ω
B. 1.608 Ω
C. 1.543 Ω
D. 1.648 Ω
Answer: A
The slope of the V-I curve is resistance.
The slope given is 90° so R=tan(90°)=infinite Ω.
It behaves as an open circuit.
19. In a DC shunt motor, the electromagnetic torque developed is proportional to ______
A. Ia
B. Ia2
C. Ia3
D. Ia5
Answer: A
In a DC shunt motor, the electromagnetic torque developed is equal to KmΦIa.
In a DC shunt, the motor field windings are connected separately and excited by a constant DC voltage.
T = KmΦIa α Ia.
20. What is the formula for the active power in the cylindrical rotor synchronous machine? (Eb represents armature emf, Vt represents terminal voltage, δ represents rotor angle, X represents reactance)
A. Eb×Vt×sinδ÷X
B. Eb×Vt2×sinδ÷X
C. Eb2×Vt×sinδ÷X
D. Eb×Vt×sinδ÷X2
Answer: A
The real power in the cylindrical rotor machine is
Eb×Vt×sinδ÷X.
It is inversely proportional to the reactance. The stability of the machine is decided by the maximum power transfer capability.
21. Salient pole machines are more stable than cylindrical rotor machines.
A. True
B. False
Answer: A
Salient pole machines are more stable than cylindrical rotor machines because of the high short circuit ratio and more real power transfer capability. The air gap length in salient pole machines is more as compared to cylindrical rotor machines.
22. The unit of reactive power is VAR.
A. True
B. False
Answer: A
Reactive power is useless power in the case of electric circuits. It is the energy trapped that keeps on oscillating between inductive and capacitive elements.
It plays a vital role in generating flux in electrical machines. It is expressed in Volt Ampere reactive.
23. Calculate the power factor during the resonance condition.
A. .58
B. .42
C. .65
D. 1
Answer: D
During the resonance condition, the reactive power generated by the capacitor is completely absorbed by the inductor. Only active power flows in the circuit. Net reactive power is equal to zero and cosΦ=1.
