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 :
Both (A) and (R) are true and (R) is the correct explanation of (A)
Both statements are true, and the reason gives the mechanism by which the assertion's method works, so the code is 1.
Why voltage control is the simplest method. An induction motor's torque is proportional to the square of the applied voltage:
\(T\propto V^{2}\)
Reducing the voltage reduces the torque curve, and the motor settles at a higher slip — a lower speed — for the same load. All this needs is a pair of back-to-back thyristors (or a TRIAC) in each line, with no transformer, no DC link and no frequency conversion. That minimal hardware is exactly what makes it the simplest approach, and it is the method used in ceiling-fan regulators and small pump drives.
How the firing angle does it — statement (R). The thyristors are fired at an angle α after each zero crossing, so only the part of each half-cycle beyond α reaches the motor. The RMS voltage delivered is
\(V_{rms}=V_{s}\sqrt{\dfrac{1}{\pi}\left(\pi-\alpha+\dfrac{\sin2\alpha}{2}\right)}\)
Increasing α lowers the RMS voltage continuously from full supply at α = 0 to zero at α = π. So the firing angle is the control handle, and (R) is precisely why (A) is true.
| Method | Hardware | Range | Efficiency |
|---|---|---|---|
| Voltage control (AC chopper) | Two thyristors per phase | Narrow, near synchronous | Poor at low speed |
| V/f control (inverter) | Rectifier + DC link + inverter | Wide | Good |
| Pole changing | Special winding | Discrete steps only | Good |
The price of the simplicity is worth stating. Because the synchronous speed \(N_{s}=\dfrac{120f}{P}\) is unchanged, the motor is being slowed purely by increasing slip, and the rotor dissipates \(sP_{ag}\) as heat — efficiency falls in direct proportion to the speed reduction. The chopped waveform is also rich in harmonics, causing torque pulsation and extra heating, and a low starting torque limits the method to fan- and pump-type loads whose torque falls with speed. Wide-range, efficient control needs a variable-frequency inverter instead.
The answer is flagged for confirmation only because "A.C. chopper" is sometimes reserved for the integral-cycle or PWM AC controller rather than the phase-angle controller that (R) describes.
Hence, both (A) and (R) are true and (R) is the correct explanation of (A).
Similar Questions
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 :
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.
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?
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 :
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.
