Slip is 1 when rotor is_____________.

Question

Examoogle · Accepted Answer

The correct answer is Option A: stationary. In an induction motor, slip is defined as the relative difference between the synchronous speed of the magnetic field and the actual rotor speed. When the rotor is stationary, its speed is $0$, resulting in a slip value of exactly $1$.

Step-by-Step Solution & Concepts:
In an induction motor, slip is defined as the relative difference between the synchronous speed of the magnetic field and the actual rotor speed. When the rotor is stationary, its speed is $0$, resulting in a slip value of exactly $1$.

Working Principle: The stator winding produces a rotating magnetic field that spins at synchronous speed $N_{s}$. When the rotor is at standstill, it does not rotate ($N_{r} = 0$), meaning the magnetic field cuts the rotor conductors at the maximum possible rate. This induces maximum EMF and current in the rotor, establishing the starting torque.

Key Formulas: $s = \frac{N_{s} - N_{r}}{N_{s}}$ — where $s$ is slip, $N_{s}$ is synchronous speed, and $N_{r}$ is rotor speed.; $s = 1$ when $N_{r} = 0$ — the condition for stationary rotor.

Key Points: Slip is a dimensionless quantity usually expressed as a per-unit value or a percentage.; At synchronous speed ($N_{r} = N_{s}$), slip is $0$.; Negative slip occurs in induction generators when $N_{r} > N_{s}$.; Slip is essential for the production of torque in induction motors.

Advantages: High starting torque capability when slip is 1.; Self-starting mechanism due to high relative speed between stator field and rotor.

Disadvantages: High starting current at $s = 1$ often necessitates starters.; Increased rotor heating due to heavy current induction at stand-still.

Applications: Industrial induction motor drives.; Variable frequency drives (VFD) controlling motor slip.

Additional Info: Option B is incorrect because when $N_{r} = N_{s}$, the slip is $0$.. Option C is incorrect as the slip for a running motor is always between $0$ and $1$.. Option D is incorrect as this represents the induction generator region where slip is negative.

Answer: A is correct — The slip $s$ equals $1$ when the rotor speed $N_{r}$ is $0$, which corresponds to a stationary rotor.

Exam Tip: Always remember that an induction motor cannot reach synchronous speed; if it did, relative motion would cease, no current would be induced, and torque would vanish.

Examoogle · Answer

Option B: rotating with syncronous speed

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Option C: rotating at a speed lower than synchronous speed

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Option D: rotating at a speed higher than synchronous speed

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