# Electric Drives Questions and Answers – Regenerative Braking of DC Series Motors

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This set of Basic Electric Drives Questions and Answers focuses on “Regenerative Braking of DC Series Motors”.

1. What is the condition for maximum power transfer theorem in DC circuits?
a) RL=2Rth
b) RL≠Rth
c) RL≫Rth
d) RL=Rth

Explanation: The condition for maximum power transfer theorem is load should be variable and RL=Rth. During maximum power transfer condition voltage across load becomes half of Thevenin voltage and efficiency becomes 50 %.

2. Instantaneous power in the 3-φ system is constant.
a) True
b) False

Explanation: The 3-φ system is more economical than 2-∅ system because of no vibrations in power waveform. The instantaneous power in the 3-∅ system is constant. P=3VpIpcos(φ)=√3VLILcos(∅).

3. The length of phasor is ___________
a) R.M.S
b) Average
c) Peak to Peak
d) Minimum

Explanation: Phasors are the rotating values which rotate with some angular frequency Ω. The length of the phasor value is r.m.s value.

4. Transient analysis is only applicable to bounded systems.
a) True
b) False

Explanation: Bounded systems are those systems which have some finite maximum value and are decaying functions. Transient analysis is only applicable to bounded signals as they achieve steady state after some time.

5. Calculate the velocity of the ball if the angular speed is 7 rad/s and radius is .2 m.
a) 2.5 m/s
b) 1.4 m/s
c) 4.5 m/s
d) 1.0 m/s

Explanation: The velocity of the ball can be calculated using the relation V=Ω×r. The velocity is the vector product of angular speed and radius. V = Ω×r = 7×.2 = 1.4 m/s.

6. Calculate the value of the time period if the frequency of the signal is .001 Hz.
a) 1000 sec
b) 2000 sec
c) 5000 sec
d) 1500 sec

Explanation: The time period is defined as the time after the signal repeats itself. It is expressed in second. T = 1÷F=1÷.001=1000 sec.

7. Calculate the value of the frequency of the 440 V DC supply.
a) 100 Hz
b) 0 Hz
c) 200 Hz
d) 500 Hz

Explanation: The frequency is defined as the number of oscillations per second. It is reciprocal of the time period. DC supply magnitude is constant. It does not change with time so the frequency of DC supply is 0 Hz.

8. Calculate the value of power factor if the values of R and Z are 2 Ω and 10 Ω.
a) 0.8
b) 0.5
c) 0.2
d) 0.4

Explanation: The power factor is defined as the ratio of active power to the apparent power. Cos(φ)=R÷Z=2÷10=.2. It has no unit.

9. Calculate the value of capacitor voltage during resonance condition if the value of supply voltage is 20 V and the quality factor is 3.
a) 60 V
b) 50 V
c) 10 V
d) 30 V

Explanation: During the resonance condition XL=Xc. The value of the capacitor voltage is Q×Vs. The power factor of the circuit is one. Vc=20×3=60 V.

10. When 30 A current flows into the positive terminal of current source 8 V. Calculate the power delivered by the source.
a) -240 W
b) 360 W
c) -430 W
d) 500 W

Explanation: When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power. Power delivered is -30×8=-240 W.

11. The slope of the V-I curve is 17.587o. Calculate the value of resistance. Assume the relationship between voltage and current is a straight line.
a) .322 Ω
b) .360 Ω
c) .316 Ω
d) .778 Ω

Explanation: The slope of the V-I curve is resistance. The slope given is 17.587o so R=tan(17.587o)=.316 Ω. The slope of the I-V curve is reciprocal of resistance.

12. Calculate the equivalent inductance when two inductors are connected in parallel of values 12 H and 12 H.
a) 6 H
b) 10 H
c) 12 H
d) 8 H

Explanation: When two inductors are connected in parallel their equivalent inductance is equal to the harmonic mean of the inductances. Leq=L1×L2÷(L1+L2)=6 H.

13. Calculate the active power in an 0 Ω resistor with 0 current flowing through it.
a) inf MW
b) 0 MW
c) 2 MW
d) 18 MW

Explanation: 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 case of the resistor so the angle between V & I is 0o. P=I2R=0×0×0=0 MW.

14. When 1 A current flows out of the positive terminal of voltage source 6 V. Calculate the power delivered by the source.
a) 6 W
b) 7 W
c) -9 W
d) -5 W

Explanation: When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power. Power delivered by the source is 1×6=6 W.

15. Calculate the value of inductor voltage during resonance condition if the value of supply voltage is 7 V and the quality factor is 9.
a) 59 V
b) 63 V
c) 73 V
d) 33 V

Explanation: During the resonance condition XL=Xc. The value of the inductor voltage is Q×Vs. The power factor of the circuit is one. Vc=7×9=63 V.

Sanfoundry Global Education & Learning Series – Electric Drives.

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