# MCQs on Electrical Machines

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01․ Two synchronous generators operating in parallel 200 MW and 400 MW respectively. The drooping characteristics of their governors are 4% and 5% from no load to full load. Consider the governors operating at 50Hz on no load. How will the machine share, if the load is 600 MW?
200 MW and 400MW
none of the above

System frequency f = f0 - (f0 - f1)/ Prated *P1 Where, f0 is rated power frequency. f = 50 - 0.04*50/200 * P1 f = 50 - 0.05*50/400 * P2 Given that, P1 + P2 = 600 MW By solving the above equations, P1 = 230.7 MW and P2 = 369.23 MW. Therefore, generator 1 gets over load.

02․ Two synchronous generators operating in parallel 200 MW and 400 MW respectively. The drooping characteristics of their governors are identical as 4% from no load to full load. Consider the governors operating at 50Hz on no load. How will the machine share, if the load is 600 MW?
200 MW and 400MW respectively
230.7 MW and 369.23 MW respectively

System frequency f = f0 - (f0 - f1)/Prated * P1 Where, f0 is rated power frequency. f1 is full load frequency. f = 50 - 0.04 * 50/200* P1 f = 50 - 0.04 * 50/400* P2 Given that, P1 + P2 = 600 MW From the above equations, P1 = 200 MW and P2 = 400MW When different rating generators are in parallel they have to share the load proportional to their ratings, this is achieved through identical governor settings across their prime mover.

03․ Two synchronous generators operating in parallel 200 MW and 400 MW respectively. The drooping characteristics of their governors are identical as 4% from no load to full load. Consider the governors operating at 50Hz on no load. What is the operating frequency, if the load is 600 MW?
50 Hz
45 HZ
48 Hz
48.5 Hz

If the governor settings are made identical as 4%, they share load exactly proportional to their ratings at a frequency f. f = 50 - 0.04*50/200 * 200 f = 48 Hz

04․ Reactive power generated or delivered significantly depends on
excitation.
both 1 and 2.
frequency.

Reactive power generated or delivered Where, E = Excitation voltage V = Terminal voltage Xs = Synchronous reactance δ = load angle From the above equation, the reactive power generated significantly depends on excitation value.
1. When excitation is rated value, i.e. E cosδ = V, which means Q = 0, the generator nether supplies nor draws reactive power and operates at unity power factor.
2. If excitation is reduced; E cosδ < V, the generator draws reactive power from the bus and operates at leading power factor.
3. If excitation is increased; E cosδ > V, the generator delivers reactive power to the load and operates at lagging power factor.

05․ When excitation is increased (over excitation), the synchronous generator operates at
unity power factor.
lagging power factor.
any of the above.

Reactive power generated or delivered Where, E = Excitation voltage V = Terminal voltage Xs = Synchronous reactance δ = load angle From the above equation, the reactive power generated significantly depends on excitation value.
1. When excitation is rated value, i.e. E cosδ = V, which means Q = 0, the generator nether supplies nor draws reactive power and operates at unity power factor.
2. If excitation is reduced; E cosδ < V, the generator draws reactive power from the bus and operates at leading power factor.
3. If excitation is increased; E cosδ > V, the generator delivers reactive power to the load and operates at lagging power factor.

06․ In a synchronous generator with constant steam input supplies power to an infinite bus at a lagging power factor. If the excitation is increased
both power angle and power factor decrease
both power angle and power factor increase
power angle decrease and power factor increase
power angle increase and power factor decrease

Power developed P = EV/Xs * sinδ Where E = Excitation voltage V = Terminal voltage Xs = Synchronous reactance δ = load angle From the above equation, when steam input is constant and excitation is increased, power angle will decrease. Power delivered P = VI cosφ From the above equation, when excitation is increased then terminal voltage will increase and to maintain power input constant, power factor will decrease.

07․ In a synchronous generator, if mechanical input is increased with constant excitation, the load angle will
increase
decrease
either increase or decrease
none of the above

Power P = EV/Xs * sinδ From the above equation, if input power is increased with constant excitation, power angle will increase. If input power is decreased withe constant excitation, then power angle will decrease.

08․ In synchronous generator V curve is drawn between
field current on X-axis and armature current on Y-axis.
field current on Y-axis and armature current on X-axis.
either of these.
field current on X-axis and power factor on Y-axis.

In synchronous generator V curve is drawn between field current on X-axis and armature current on Y-axis. For the same load and constant terminal voltage, the current supplied by the generator is minimum when it operates at unity power factor and increases correspondingly lagging or leading. At critical excitation generator operates at unity power factor and current supplied by the generator is minimum at critical excitation.

09․ In synchronous generator, inverted V curve is drawn between
field current on X-axis and armature current on Y-axis
field current on X-axis and power factor on Y-axis
field current on Y-axis and power factor on X-axis
field current on Y-axis and armature current on X-axis

In synchronous generator, inverted V curve is drawn between field current on X-axis and power factor on Y-axis. At critical excitation the power factor is unity. If excitation is decreased, the generator operates at leading power factor and if excitation is increased, the generator operates at lagging power factor. Therefore, the power factor is maximum at critical excitation and decreases correspondingly with under or over excitation.

10․ Which of the following statement is correct, if short circuit ratio is high?
Good voltage regulation
High voltage regulation
Low short circuit current
None of the above

Short circuit ratio (SCR) is the ratio of two field currents, the field current required to produce rated voltage on open circuit to the field current required to rated current on short circuit. SCR = 1/(Xs(pu) adjusted) Where, Xs = Synchronous reactance. If Xs is low, then SCR is high. The low value of Xs or high value of SCR gives,
1. Good voltage regulation
2. Power developed is high, because power developed P ∝ 1 / Xs
3. Machine is more stable but high short circuit current.

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