Machine Riddle No.40 - No Load current of Induction motor
The no load current of the induction motor should be less than 30% of rated load current. Is this a standard? I am confused with the different no load amps of different manufacturer. Could anyone please explain me more in detail? please send me any link of explanation if possible.

#1
Thu, February 2nd, 2012 - 11:39
I didn't see any standard amount. For 50 (60) Hz motors, I0/In (no-load current/rated current) decreases with motor power from 0.5 to 0.8 (in sub kW power range) to 0.2 to 0.3 in the high power range, but it increases with the number of pole pairs. For low airgap flux densities, the no-load current tends to be smaller. A typical magnetization curve is shown in following picture for motors in the kW power range at 50 (60) Hz. The I0 and In are no load and full load stator phase currents and F1m0 is

The no load (zero rotor current) design airgap flux density is Bg1 = 0.6 – 0.8T for 50 (60) Hz induction motors and goes down to 0.4 to 0.6 T for (400 to 1000) Hz high speed induction motors, to keep core loss within limits.
Based on metioned formula, the no-load current (I0) may be calculated with the number of turns/phase W1 and the stator winding factor Kw1 already known. Varying as the value of Bgm desired the magnetization curve–Bgm(F1m)–is obtained.

#2
Thu, February 2nd, 2012 - 14:26
Dear Mr. Hamid

Many thanks for the explanation. I would like to discuss one point to you

We have one motor and its specification is given as below
Make- HOYER MOTORS, kW- 11, Amps- 23, Volts- 400, RPM- 970, PF-0.78

This motor takes around 17 amps at no load. Is it acceptable value?

#3
Thu, February 2nd, 2012 - 15:11
Generally it is possible, of course I must to bring to your attention to this point which the pf=0.78 is rated power factor in full load current and in no load current, the power factor is smaller than it.

#4
Sun, February 5th, 2012 - 17:07
Thanks Mr. Hamid.

Yes sir, the PF at no load will be small than full load, due to inductive circuit. Once again many thanks for this useful information regarding no load current of the motors.