The rotor of a three phase induction motor can never attain synchronous speed.

Question

Examoogle · Accepted Answer

Option A: True

Explanation & Concepts:
The rotor of a three-phase induction motor can never attain synchronous speed because the torque developed is proportional to the slip. If the rotor were to reach synchronous speed, the slip would become zero, resulting in zero induced EMF and zero rotor current, which causes the torque to drop to zero.

Working Principle: According to Faraday's Law, induced EMF in the rotor bars is proportional to the relative speed between the rotating magnetic field and the rotor. When the rotor speed ($N_r$) equals the synchronous speed ($N_s$), the relative speed is zero, meaning the magnetic field does not 'cut' the rotor conductors. Consequently, no current flows in the rotor, and the electromagnetic torque vanishes; thus, the rotor must always run at a speed less than $N_s$ to sustain torque.

Key Formulas: $s = \frac{N_s - N_r}{N_s}$ — Definition of slip; $T \propto \Phi I_2 \cos(\phi_2)$ — Torque production principle

Key Points: Synchronous speed is defined as $N_s = \frac{120f}{P}$; An induction motor is also known as an asynchronous motor; If $N_r = N_s$, the rotor conductors appear stationary relative to the rotating field; The difference $N_s - N_r$ is known as slip speed

Advantages: Simple and rugged construction; Self-starting capability; Low maintenance requirements

Disadvantages: Speed decreases with increase in load; Low starting torque compared to DC series motors

Applications: Industrial fans and blowers; Centrifugal pumps; Conveyor belts

Additional Info: At no-load, the rotor speed is very close to synchronous speed but never reaches it due to mechanical friction and windage losses.. Option B is false because reaching synchronous speed would violate the fundamental principle of electromagnetic induction in AC machines.

Answer: A is correct — The rotor speed must always be less than the synchronous speed to maintain relative motion between the stator magnetic field and the rotor conductors.

Exam Tip: Always remember that an induction motor is an 'asynchronous' machine; if it were 'synchronous', it would function as a synchronous motor, requiring DC excitation on the rotor.

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Option B: False

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