Which of the following is the correct expression for slip (S) in terms of synchronous speed Ns and rotor speed Nr?

• A. Nr - Ns
• B. Ns - Nr
• C. Nr / Ns
• D. (Ns - Nr) / Nr

Answer: (B)

Slip is the difference between the speed of the rotating magnetic field (synchronous speed) and the rotor speed, showing how far the rotor lags behind the field. In this form, slip is simply the speed difference: S = Ns − Nr. This is positive when the rotor falls behind the field, which is the normal operating condition for induction machines. For example, if Ns is 1800 rpm and Nr is 1720 rpm, the slip is 80 rpm. The fractional slip, S/Ns, exists as another way to express slip, but the question asks for the direct speed difference in terms of Ns and Nr, so the appropriate expression is Ns − Nr. The other options either give a negative value, a ratio, or mix speeds in a way that isn’t the standard speed difference.