1. Calculate the voltage regulation in the synchronous machine if the no-load voltage is 12 V and the full load voltage is 15V.
A. -20%
B. -40%
C. -60%
D. -80%
Answer: A
Voltage regulation is defined as the fluctuation in the load voltage when the load is varied from no-load to full load. V.R(%) = (No-load voltage-Full load voltage) ÷ Full load voltage=12-15÷15 = -20%.
2. Calculate the condition for maximum voltage regulation in the synchronous machine.
A. Φ=ϴs
B. Φ=2ϴs
C. Φ=4ϴs
D. Φ=8ϴs
Answer: A
Voltage regulation is maximum in the case of inductive load. The condition for maximum voltage regulation is when the power factor angle of the load becomes equal to the impedance angle.
V.R=Rp.ucos(Φ)+Xp.usin(Φ).
Differentiate V.R with respect to Φ and put it equal to zero.
We will get tan(Φ) = Xp.u÷Rp.u=tan(ϴs) then Φ=ϴs.
3. Zero voltage regulation can be only achieved in leading power factor load.
A. True
B. False
Answer: A
Zero voltage regulation only occurs during a leading power factor load. Condition for zero voltage regulation occurs when Φ+ϴs > 90o. For example – Capacitive load.
4. R.M.S value of the sinusoidal waveform v(t)=211sin(7.25t+7π÷78.3).
A. 149.19 V
B. 156.23 V
C. 116.57 V
D. 178.64 V
Answer: A
R.M.S value of the sinusoidal waveform is Vm÷2½ = 211÷2½ = 149.19 V and r.m.s value of the trapezoidal waveform is Vm÷3½. The peak value of the sinusoidal waveform is Vm.
5. Calculate the peak value of the sinusoidal waveform if the r.m.s value is 21 V.
A. 29.69 V
B. 48.74 V
C. 69.23 V
D. 25.74 V
Answer: A
R.M.S value of the sinusoidal waveform is Vpeak÷2½ and r.m.s value of the trapezoidal waveform is Vm÷3½.
The peak value of the sinusoidal waveform is
Vr.m.s×2½. Vpeak
= Vr.m.s×2½=21×1.414=29.69 V.
6. If induction motor air gap power is 2 KW and mechanically developed power is 1 KW, then rotor ohmic loss will be _________ KW.
A. 1
B. 2
C. 3
D. 4
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
=2-1=1 KW.
7. Calculate the line voltage in the delta connection when phase voltage=45 V.
A. 46 V
B. 47 V
C. 45 V
D. 78 V
Answer: C
The line voltage in the case of delta connection is phase voltage. Line current leads the phase current by an angle of 30°.
VL-L=Vph = 45 V.
8. The slope of the V-I curve is 35.48°. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
A. 0.452 Ω
B. 0.462 Ω
C. 0.752 Ω
D. 0.712 Ω
Answer: D
The slope of the V-I curve is resistance.
The slope given is 35.48° so R=tan(35.48°)=.712 Ω.
The slope of the I-V curve is reciprocal to resistance.
9. If induction motor rotor power is 157.5 KW and gross developed power is 79.9 KW, then rotor ohmic loss will be _________ KW.
A. 77.5
B. 77.6
C. 76.9
D. 77.1
Answer: B
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
=157.5-79.9=77.6 KW.
10. Calculate the line current in the delta connection when phase current=17 A.
A. 29.44 A
B. 24.64 A
C. 23.48 A
D. 26.56 A
Answer: A
The line voltage in the case of delta connection is phase voltage. Line current leads the phase current by an angle of 30°.
