Question

The rotor of a single-phase induction motor is running at N rpm, what is the slip with respect to forward rotating flux?

1. \( {N \over N_s}\)
2. (NS - N) / NS
3. (N + NS) / NS
4. (N - NS) / NS

Answer 1

Correct Answer - Option 2 : (NS - N) / NS

Slip:

The difference between the rotor speed (N) and the rotating magnetic flux speed (Ns) is called slip.

The induction motor slip is usually expressed as a percentage of synchronous speed (NS) and is represented by symbol s.

Mathematically, Percentage slip, \(\% \;s = \left[ {\frac{{{N_s}\;-\;N}}{{{N_s}}}} \right] \times 100\)

Or fractional slip, \(s = \frac{{{N_s}\;-\;N}}{{{N_s}}}\) 

At start, N = 0, therefore, Slip, \(s = \frac{{{N_s}\;-\;0}}{{{N_s}}} = 1\)

At synchronous speed, Slip, \(s = \frac{{{N_s}\;-\;{N_s}}}{{{N_s}}} = 0\)