What is common between Induction generator and induction motor?

Question

Examoogle · Accepted Answer

Option D: All of these

Explanation & Concepts:
The correct answer is Option D because induction generators and induction motors share fundamental physical characteristics and operating principles. Both machines operate as doubly fed or singly excited asynchronous machines where alternating current is supplied to only one winding (the stator), both require reactive power from the supply line to establish magnetic flux, and neither can run at exact synchronous speed under normal operation.

Working Principle: In an induction machine, torque is produced only when there is a relative difference between the stator magnetic field speed ($N_s$) and the rotor speed ($N$). If the rotor runs at $N N_s$), slip becomes negative and it converts mechanical power to electrical power as a generator. In both modes, the machine lacks an independent DC excitation field, relying entirely on lagging reactive power drawn from the grid (or capacitor banks) to establish its rotating magnetic field.

Key Formulas: $s = \frac{N_s - N}{N_s}$ — Slip equation for induction machine; $N_s = \frac{120 f}{P}$ — Synchronous speed in RPM

Key Points: An induction machine cannot develop torque at synchronous speed ($s = 0$) because relative velocity between stator flux and rotor conductors becomes zero, yielding zero induced rotor EMF.; Both modes require lagging reactive power from an external source (or shunt capacitors in isolated operation) to magnetize the core.; Induction machines are called 'singly excited' because electrical energy is supplied directly to only one winding set (usually the stator).

Advantages: Robust construction, low cost, and minimal maintenance due to absence of brush-commutator/slip-ring requirements in squirrel-cage types.; In generator mode, it automatically synchronizes with the grid frequency without requiring complex synchronizing equipment.

Disadvantages: Operates at a lagging power factor and cannot supply reactive power independently.; Requires an external AC supply or external capacitor bank for self-excitation.

Applications: Induction Generator: Wind energy conversion systems, small hydroelectric plants, and regenerative braking.; Induction Motor: Industrial drives, pumps, fans, compressors, and blowers.

Additional Info: Option A is correct because both motor and generator absorb reactive power to build up magnetizing flux.. Option B is correct because operating at synchronous speed results in zero slip, zero induced current, and zero electromagnetic torque.. Option C is correct because excitation power is fed only to the stator winding.

Answer: D is correct — Both machines require reactive power for magnetization, cannot operate at synchronous speed due to torque loss at zero slip, and are singly excited.

Exam Tip: Remember for competitive exams: Synchronous machines are doubly excited and can operate at leading/lagging power factors, whereas induction machines are singly excited and ALWAYS absorb reactive power regardless of motoring or generating mode.

Examoogle · Answer

Option A: both required reactive power

Examoogle · Answer

Option B: both can not run at synchronous speed

Examoogle · Answer

Option C: both known as singly excited machine

Feature	Induction Motor	Induction Generator
Rotor Speed ( $N$ )	Below Synchronous Speed ( $N < N_s$ )	Above Synchronous Speed ( $N > N_s$ )
Slip ( $s$ )	Positive ( $0 < s \le 1$ )	Negative ( $s < 0$ )
Active Power ( $P$ )	Absorbed from grid	Delivered to grid
Reactive Power ( $Q$ )	Absorbed from grid	Absorbed from grid

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