Induction Motor Stator Balance Current Generation Due To Transformer Action

[shahvir]

Hi Guys,

As is aware, an induction motor acts as a generalized transformer. So whenever rotor MMF is created on load condition, immediately a balancing current flows in the stator windings to neutralize the rotor MMF by transformer action. However, if we assume the rotor conductors to be purely resistive, the rotor MMF is in perfect quadrature with the stator flux. Due to the interaction of both these fluxes, main air gap flux becomes cross-magnetizing or distorting in order to generate torque on the rotor conductors.

So my query is; if the main air gap flux is only distorting in nature and not de-magnetizing, how is a reflected balance current generated in the stator windings by transformer action in the absence of de-magnetizing Amp-Turns? For eg. In case of single winding 2-pole shaded pole motors this phenomenon seems all the more complicated.

Thanks.

 

[waross]

Very low slip values are the result of very light loading and very light currents. At light or no load, the air gap distortion is at a minimum. However, with light or no load, the current is predominantly magnetizing current, hence the quadrature effect.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[shahvir]

Thanks for reply. I had referred to low slip values in order to simulate ideal conditions of a purely resistive rotor.

The premise of the question is that how a distorting air gap flux elicits a MMF balancing current in the stator windings if the flux density is not reduced in the absence of de-magnetizing amp-turns.

 

[7anoter4]

The stator magnetic flux behaves as a crown of poles rotating with synchronous speed 2*π*frq rad/sec [or frq*60/p rot/min p=number of pole pairs] all the time as the stator stays. If stator rotates then the poles rotate - with respect of a static point-with an increased speed. The same with the rotor: the rotor magnetic flux "slips" on the rotor surface with the s*frq*60/p rot/min, but with respect stator with 1-s+s=1 synchronous speed. That means both flux rotate together with the same speed.

 

[electricpete]

I'm not sure I understand your question.

> As is aware, an induction motor acts as a generalized transformer. So whenever rotor MMF is created on load condition, immediately a balancing current flows in the stator windings to neutralize the rotor MMF by transformer action. However, if we assume the rotor conductors to be purely resistive, the rotor MMF is in perfect quadrature with the stator flux.

So far, so good.

> Due to the interaction of both these fluxes, main air gap flux becomes cross-magnetizing or distorting in order to generate torque on the rotor conductors.

Due to interaction of rotor current and stator load component of current, the main air gap flux stays almost the same as it was in no-load (it would be identical if we neglect R1, L1, X2 in the equivalent circuit). In what sense is this "distorting"?

> how is a reflected balance current generated in the stator windings by transformer action in the absence of de-magnetizing Amp-Turns?

I'm going to assume "reflected balance current" means the load component of stator current.
I'm going to assume "de-magnetizing Amp-Turns" means rotor current-turns.
The answer to your "how" question is then... there is no explanation needed. The stator load component of current (reflected balance current) is not generated in absence of de-magnetizing amp-turns (rotor current).
If you have other definitions of these terms in mind please clarify

I assume you are familiar with the induction motor equivalent circuit? Can you rephrase your question in terms of that? Is there are part of the circuit that you can't explain by EM theory? ... or some aspect of induction motor action that you don't think is properly captured by the equivalent circuit?







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(2B)+(2B)' ?

 

[shahvir]

Due to interaction of rotor current and stator load component of current, the main air gap flux stays almost the same as it was in no-load (it would be identical if we neglect R1, L1, X2 in the equivalent circuit). In what sense is this "distorting"? 

I'm going to assume "reflected balance current" means the load component of stator current. 
I'm going to assume "de-magnetizing Amp-Turns" means rotor current-turns. 

Thanks for the prompt reply. Your assumptions are correct, and I do agree with those concepts in theory. But in order for the stator component of load current to develop and hence neutralize the rotor MMF, there should be de-magnetizing or weakening effect of the air gap flux on the stator windings.

If we assume the rotor conductors to be purely resistive (analogous to a transformer with unity pf load) the stator and rotor fluxes will be in perfect quadrature with each other. The air gap flux will stretch and distort around the rotor conductors to generate mechanical torque.

As we know, distortion of the flux lines do not reduce or weaken the average air gap flux density in order to demand a neutralising stator component of load current. So how is this reflected stator current generated in the stator windings in the first place! I am unable to grasp this concept under practical conditions.

 

[waross]

The magnetizing VARs will increase under load due to the distortion of the air gap.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

If I well understood the o.p. question then my explanation it could be as following:
If we shall interpret the Steinmetz induction motor diagram the results it would be the current which produces the air gap magnetic field is constant what ever is rotor current. Io=Istator-I'rotor[rotor current referred to stator].Now, if power factor changes with the load the field rotor could be split in two one in d-axe-at the same line with stator field-and one in q-axe at 90 [electric] degrees. The d-axe rotor field will reduce the stator field up to no-load field ,but the second -from q-axe-will produce some harmonics by deforming the field sinusoidal form.