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 Choose the most appropriate answer from the options given below :
(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 constant
(A) and (D) Only
Statements (B) and (D) are direct opposites, so exactly one is true — and it is (D), the field velocity is constant. Paired with (A), that gives option 2.
(A) — one cycle carries the wave two pole pitches. A pole pitch is the distance from one pole to the next, that is half a wavelength of the MMF distribution:
\(\lambda=2\times\text{pole pitch}\)
In one complete cycle of the supply current the travelling wave advances exactly one wavelength — which is therefore twice the pole pitch, precisely as (A) states. Statement (C) says the same distance takes one and a half cycles, which is simply wrong.
(D) — and why the velocity cannot vary. Advancing one wavelength per cycle means
\(v=f\lambda=f\times2\times\text{pole pitch}\)
Both f and the pole pitch are fixed — one by the supply, the other by how the machine was wound — so the product cannot change. Expressed as a rotational speed this is the familiar synchronous speed:
\(N_{s}=\dfrac{120f}{P}\ \text{rpm}\)
which for 50 Hz and four poles is 1500 rpm regardless of load, voltage or rotor speed. Statement (B) contradicts this and is false.
How the rotating field arises is worth recalling, since it explains why the motion is uniform. Three windings displaced 120° in space carry currents displaced 120° in time. Adding their MMF contributions gives a resultant of constant magnitude \(1.5F_{m}\) rotating at a constant angular velocity — the space and time displacements combine so that the resultant neither pulsates nor accelerates.
What does vary is the rotor. The rotor always runs slower than the field, and the difference is the slip:
\(s=\dfrac{N_{s}-N_{r}}{N_{s}}\)
Slip changes with load; synchronous speed does not. Confusing the two is exactly what statement (B) does — and the rotor must lag, since at synchronous speed there would be no relative motion, no induced EMF, no rotor current and hence no torque.
Hence, the correct statements are (A) and (D).
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 :
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?
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 :
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.
