Induction motor rotor and stator fluxes

[Physicist3]

Hi,

When a load is applied to the rotor of an induction motor, the rotor speed decreases and as a result, the induced currents in the rotor and the torque produced increases due to the relative speed difference between the rotor and the rotating field within the stator. My question is, should the magnitude of the flux remain constant for both the rotor and the stator when load is applied, or should it increase/decrease?

Many thanks.

 

[cabraham]

Your question is ambiguous because "the flux" consists of more than 1 quantity. The stator flux is determined by the input power source, typically constant voltage. Some but not all of it links the rotor. The currents in the rotor generate their own magnetic flux opposing the stator linked flux per law of Lenz. Then some of that rotor flux couples the stator. So it isn't as simple as 1 flux value. When the load is connected, currents in both stator and rotor increase, IR drops increase as well. Overall the motor is modeled like a transformer. The stator flux decreases a little with loading. Any rotor current mmf (amp-turns) is balanced by the stator mmf per Ampere's Law. Stator flux is nearly constant since any tendency for stator flux reduction by rotor currents is balanced by increased stator current. But IR drops increase and flux in stator is reduced since the net emf across the stator winding is reduced due to IR drop.

Did I help or make things worse?

Claude