Question

An 8 pole, 3 phase, 50 Hz induction motor is operating at speed of 700 rpm The frequency of the rotor current of the motor in Hz is _______

Answer 1

Concept:

The frequency of rotor current or rotor induced EMF in an induction motor is,

                   f_r = sf

where,

          s is slip 

          f is supply frequency

Explanation:

For the given induction motor, we have \(P\; = \;8\), \(f\; = \;50Hz\), \(N\; = \;700\) rpm

So, we obtain \(Ns = \frac{{120f}}{P} = \frac{{120 \times 50}}{8} = 750rpm\).

Therefore,

                 the slip \(s = \frac{{{N_s} - {N_r}}}{{{N_s}}} = \frac{{750 - 700}}{{750}} = \frac{1}{{15}}\)

Thus, the frequency of rotor current of the motor is obtained as

Frequency of rotor current = s × f \(= \frac{1}{{15}} \times 50 = 3.33Hz\)