When load is placed on 3-phase induction motor its slip

Question

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Option A: increases

Explanation & Concepts:
In a 3-phase induction motor, the slip is defined as the relative speed difference between the synchronous speed ($N_s$) and the rotor speed ($N_r$). When load is added, the rotor speed ($N_r$) decreases, which causes the slip ($s = \frac{N_s - N_r}{N_s}$) to increase to produce the necessary electromagnetic torque required to balance the increased load torque.

Working Principle: The torque produced by an induction motor is proportional to the slip within the stable operating region. As the mechanical load increases, the motor slows down, which increases the relative motion between the stator rotating magnetic field and the rotor conductors. By Faraday's law, this higher relative speed induces greater rotor current and electromagnetic torque to sustain the new load demand, leading to an increase in slip.

Key Formulas: $s = \frac{N_s - N_r}{N_s}$ — slip of an induction motor; $T_{em} \propto s$ — torque is directly proportional to slip in the linear region

Key Points: Slip is zero at no-load (theoretically) and increases as load is added.; The rotor speed ($N_r$) is always less than synchronous speed ($N_s$) in motor mode.; A higher slip implies more rotor current and higher torque generation.; The relationship between slip and load is roughly linear in the stable operating range.

Advantages: Self-regulating nature of torque versus slip.; Maintains mechanical equilibrium under variable load conditions.

Disadvantages: Excessive slip results in increased rotor $I^2R$ losses and reduced efficiency.; High slip operation can lead to overheating if the motor is consistently overloaded.

Applications: Industrial conveyor belts; Fans and blowers; Centrifugal pumps

Additional Info: The slip of an induction motor typically ranges from 2% to 5% at full load.. Option B is incorrect because a decrease in slip would imply the motor is accelerating, which only happens when the load is removed.

Answer: A is correct — The slip of an induction motor increases as the load on the motor increases to compensate for the reduction in rotor speed and to generate higher electromagnetic torque.

Exam Tip: Always remember that in an induction motor, as $N_r$ drops due to load, the slip $s$ must rise to satisfy the torque-speed equation.

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Option B: decreases

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Option C: remains constant

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Option D: does not depend on load

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