# Electric Drives Questions and Answers – Dynamics – Transient Stability of an Electric Drive

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This set of Tricky Electric Drives Questions and Answers focuses on “Dynamics – Transient Stability of an Electric Drive”.

1. 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

Explanation: 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.

2. Salient pole machines are more stable than cylindrical rotor machines.
a) True
b) False

Explanation: 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 compare to cylindrical rotor machines.

3. The unit of reactive power is VAR.
a) True
b) False

Explanation: The reactive power is the useless power in case of electric circuits. It is the energy trapped that keeps on oscillating between inductive and capacitive element. It plays a vital role in generating flux in electrical machines. It is expressed in Volt Ampere reactive.

4. Calculate the power factor during the resonance condition.
a) .58
b) .42
c) .65
d) 1

Explanation: 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.

5. Calculate the reactive power in a 5 Ω resistor.
a) 7 VAR
b) 0 VAR
c) 2 VAR
d) 1 VAR

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 90°. Q = VIsin0 = 0 VAR.

6. What is the unit of the apparent or complex power?
a) VAR
b) VA
c) ohm
d) Volt

Explanation: The apparent power in AC circuits is VI*. It is expressed in volt-amperes (VA). It consists of both active and reactive power. It is the vector sum of the real power and reactive power.

7. Calculate the value of the frequency of the DC supply.
a) 0 Hz
b) 50 Hz
c) 20 Hz
d) 10 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. The slope of the V-I curve is 0°. Calculate the value of resistance. The graph is parallel to the x-axis.
a) 1 Ω
b) 1.8 Ω
c) 0 Ω
d) 2.2 Ω

Explanation: The slope of the V-I curve is resistance. The slope given is 0° so R=tan(0°)=0 Ω. The slope of the V-I curve is resistance. It behaves as a short circuit.

9. Calculate the value of the duty cycle if the system is on for 5 sec and off for 10 sec.
a) .333
b) .444
c) .201
d) .642

Explanation: Duty cycle is Ton÷Ttotal. It is the ratio of time for which the system is active and the time taken by the signal to complete one cycle. D= Ton÷Ttotal=5÷15=.333.

Sanfoundry Global Education & Learning Series – Electric Drives. 