Assertion (A) : Synchronous machine is used as a motor and a generator also. Reason (R) : Synchronous machine can operate at constant speeds and variable frequencies under steady state.
(A) is correct but (R) is not correct
The assertion is true, but the reason contradicts the defining property of a synchronous machine — option 3.
Why the reason is false. "Synchronous" means exactly that the rotor turns in step with the rotating field, and that locks speed to frequency:
\(N_{s}=\dfrac{120f}{P}\ \text{rpm}\)
Speed and frequency are therefore rigidly proportional. Constant speed at variable frequency is impossible for a fixed number of poles — changing f necessarily changes N. If the load torque exceeds the pull-out value the rotor does not slow down gracefully; it slips a pole and falls out of synchronism altogether, and the machine stalls. The statement asserts the very independence that the word "synchronous" denies.
The true statement is the opposite pairing: constant frequency gives constant speed, whatever the load, right up to pull-out. That is the machine's chief virtue, and it is why synchronous motors drive loads needing an exactly fixed speed.
Why the assertion is nevertheless true. The same machine runs in both directions of energy flow, and which one it is depends only on the sign of the load angle \(\delta\) between the rotor and the field:
| δ | Rotor relative to field | Operating as |
|---|---|---|
| Positive | Leads — driven by a prime mover | Generator |
| Zero | Aligned | Floating, no power transfer |
| Negative | Lags — dragged by the field | Motor |
with power transfer
\(P=\dfrac{EV}{X_{s}}\sin\delta\)
Nearly all electricity is generated by synchronous alternators, and the identical machine used as a motor drives compressors, mills and pumps.
A third mode is worth knowing, since it shows how far the two roles blur: run unloaded with the field over-excited and the machine draws leading current, acting as a variable capacitor. In that role it is called a synchronous condenser, and it is used purely for power-factor correction and voltage support — neither generating nor motoring in any useful sense.
Hence, (A) is correct but (R) is not correct.
Similar Questions
Assertion (A) : The simplest method to control the speed of an A.C. motor is to vary the applied voltage by using A.C. chopper.
Reason (R) : The voltage can be varied by decreasing or increasing the firing angle.
Select your answer using the codes given below :
In induction motors, following statements are given :
(A) The MMF wave travels during one cycle of the current, a distance twice the pole pitch or wavelength
(B) For a given pole pitch and frequency, the velocity of the travelling field keep varying
(C) The MMF wave travels during one and half cycle of the current, a distance twice the pole pitch of wavelength
(D) For a given pole pitch and frequency, the velocity of the travelling field is constantChoose the most appropriate answer from the options given below :
Important Questions from Electric Drives - Teaching
For electrical drives
(A) For Tm(t) < T L(t), \(\rm\frac{d w}{d t}\) > 0
(B) For Tm(t) > T L(t), \(\rm\frac{d w}{d t}\) > 0, speed increases
(C) For Tm(t) = T L(t), \(\rm\frac{d w}{d t}\) = 0, drive attains steady state
(D) The Equation \(\rm I\frac{dw}{d t}+\frac{w^2}{2}\frac{dI(a)}{da}\) = Tm(t) − T L(t)
(E) Equation \(\rm I\frac{dw}{dt}−\frac{w^2}{2}\frac{dI(a)}{da}\) = Tm(t) − T L(t)
Choose the correct answer from the options given below:
Which of the following statements about the transformer or mechanical gear drive is INCORRECT?
Assertion (A) : The simplest method to control the speed of an A.C. motor is to vary the applied voltage by using A.C. chopper.
Reason (R) : The voltage can be varied by decreasing or increasing the firing angle.
Select your answer using the codes given below :
In induction motors, following statements are given :
(A) The MMF wave travels during one cycle of the current, a distance twice the pole pitch or wavelength
(B) For a given pole pitch and frequency, the velocity of the travelling field keep varying
(C) The MMF wave travels during one and half cycle of the current, a distance twice the pole pitch of wavelength
(D) For a given pole pitch and frequency, the velocity of the travelling field is constantChoose the most appropriate answer from the options given below :
