Induction Motor Slip decrease with frequency decrease for same load.

[Physicist3]

Hi,

Does anyone know or could help me understand the reasoning for the amount of slip within an induction motor decreasing with frequency under the same load conditions? I.e. For no external load, the slip is higher at 50Hz than at 20Hz etc? Thanks

 

[jim hardy]

What about internal load, ie bearing friction and windage loss?

Is windage proportional to speed² or to speed³ ?

 

[Babadag]

“No-load” it does not mean “no load at all”. There are no-load losses –iron losses, ventilation and friction-bearing losses and copper losses. The no-load current-mainly reactive-is about 30% of rated and copper [winding] losses could be elevated. As jim hardy already said, ventilation and friction are proportional with square of rpm, so less frequency less rpm [rpm=60*freq/p where p= no.of pole pairs] less slip. The iron losses decrease with the frequency and so the no-load current and in turn no-load copper losses. The mechanical power requested in order to maintain the rotor in rotation will decrease even if the torque remains constant P=T*(2*PI()*Freq:
$P=T\omega=2\pi_fT$
Less power less slip.