I am given the rated values of a squirrel cage induction motor:
A 300hp, 2300V, 3‐phase, 60Hz squirrel‐cage induction motor turns at a full‐load speed of 590r/min. What is the approximate value of the rotor I2R losses?
However, the synchronous speed is not given, nor are the amount of poles in the motor. How would one solve this problem without knowing these values?
The rated speed is usually about 90-95% of synchronous speed. You can guess the synchronous speed and number of poles by looking for a number of poles that gives a rated speed slightly less than synchronous speed.
In this case, I would guess you are dealing with a 60Hz 12-pole motor, which would have a synchronous speed of 600 rpm.
I've done a quick lookup table below for reference.
Poles 50hz 60hz
2 3000 3600
4 1500 1800
6 1000 1200
8 750 900
10 600 720
12 500 600
16 375 450
20 300 360
24 250 300
30 200 240
40 150 180
It will be a 12 pole motor with a sync speed of 600 RPM.
Speed = 2 · 60 · f / n
where f = frequency (Hz, cycles/sec) and n = number of poles
