Determine the rotor frequency of a two-pole, 50 Hz
induction motor if the rotor speed is 2850 r/min.
First find the slip speed
!
Ns = 120 x 50 = 3000 r/min
2

Next find the slip
!
Slip = 3000 – 2850 x 100 = 5 %
3000

Then find the frequency
fr = fs x % Slip=
Frequency = 50 x 5% = 50 x 5 = 2.5 Hz
100
Another way to write the formula for rotor
efficiency is:
fs x %slip
fr =
100