Question

A four-pole, three-phase Induction Motor operates from a supply with 50 Hz. What is the frequency of the rotor currents when the slip is 0.06?
1. 50 Hz
2. 48 Hz
3. 3 Hz
4. 47 Hz

Answer 1

Correct Answer - Option 3 : 3 Hz

Concept:

A synchronous motor rotates due to the interlocking of stator R.M.F. poles and rotor poles. Due to this Interlocking, the rotor always rotates at the synchronous speed.

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

The 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, \(s = \frac{{{N_s}\;-\;N}}{{{N_s}}}\) 

The frequency of rotor current = sf

Where s is the slip

f is the supply frequency

Calculation:

Frequency (f) = 50 Hz

Slip, s = 0.06

Frequency of rotor current = 0.06 × 50 = 3 Hz