Asynchronous Motor Starting: Understanding the Initial Current Flow

[fonz]

One of the characteristics of full voltage starting (DOL) of an asynchronous induction motor is that full load current is exceeded quite significantly when the motor is started in this way.

So what is happening is that when line voltage is applied, the difference in synchronous and rotor speed (slip) is maximum causing the motor to produce torque equal to the voltage * current (assuming no losses).

If you take away the rotor and consider just one of the motors windings as a pure inductor. Applying line voltage to the inductor will cause no current to flow initially because it is behaving as a pure inductor i.e. resisting the change in current and shifting the current and voltage out of phase.

So in the motor application, something is happening that is causing a current to flow initially. Can somebody explain this?

Thanks

 

[FOIWATER]

if you take away the rotor, it will draw starting current.

It is true that it will behave as an inductor, but it will still draw that current.

The reason the current is large on starting is because only winding impedance prevents the flow of current.

When the rotor rotates, its magnetic field cuts that of the stator (in the very same way the stator initially cut the rotor) now current flows out of the motor.

With current being generated from the machine, and current being applied to the machine, basic subtraction will allow you to see how the motor current is limited when the rotor is moving.

This phenomenon is apparent in all electric motors with slip

 

[Bob S]

If you take away the rotor and consider just one of the motors windings as a pure inductor. Applying line voltage to the inductor will cause no current to flow initially because it is behaving as a pure inductor i.e. resisting the change in current and shifting the current and voltage out of phase.

Not quite true. If you take the rotor out, cut all the conductors in the squirrel cage, and then replace the rotor, the measured inductance will be the same as the rotor running synchronously. You must have the laminated iron of the rotor in place to measure the inductance. If the motor is a single phase induction motor, the measured inductance will include the starting windings as well as the running windings.

 

[fonz]

if you take away the rotor, it will draw starting current.

It is true that it will behave as an inductor, but it will still draw that current.

The reason the current is large on starting is because only winding impedance prevents the flow of current.

When the rotor rotates, its magnetic field cuts that of the stator (in the very same way the stator initially cut the rotor) now current flows out of the motor.

With current being generated from the machine, and current being applied to the machine, basic subtraction will allow you to see how the motor current is limited when the rotor is moving.

This phenomenon is apparent in all electric motors with slip

I understand this concept of slip and I also understand why only the winding impedance prevents the flow of current while the rotor is not moving (i.e. not cutting the windings)

What I don't understand is why the windings do not behave as inductors, they look like inductors to me?

 

[Bob S]

What I don't understand is why the windings do not behave as inductors, they look like inductors to me?

The iron in the rotor is part of the magnetic circuit. The high inductance is due to the fact that most of the field is in the iron, where H = B/μμ_o. In the air gap, the H is much higher: H = B/μ_o. A major part of the inductance of an induction motor is the air gap between the stator and rotor. Typically, 1" of iron in a magnetic circuit is equivalent to 0.001" of air. If you widen the air gap, or remove the rotor (without squirrel cage) entirely, the inductance drops very significantly and the reactive current soars.

 

[FOIWATER]

they do behave as inductors for an AC motor, the total current drawn is a function of the windings resistance (do to it being a resistor) and its reactance (due to it also being an inductor)

 

[Averagesupernova]

Fonz are you wondering why the motor draws excessive current at startup? Not sure I fully understand what you are getting at. You are saying that since the motor windings are inductors that they should behave more like inductors and not draw excessive current at startup? If so, remember that at locked rotor the motor is behaving like a transformer with a shorted secondary.

 

[fonz]

Fonz are you wondering why the motor draws excessive current at startup? Not sure I fully understand what you are getting at. You are saying that since the motor windings are inductors that they should behave more like inductors and not draw excessive current at startup? If so, remember that at locked rotor the motor is behaving like a transformer with a shorted secondary.

You see this was what i was thinking.

So if this is the case, as the rotor starts to speed up the current flowing in the rotor must create a magnetic field that opposes that created by the stator windings. I'm sure somebody could be more technical in their explanation but what I am saying is that the flux from the rotor is cancelling the flux from the stator as the speed builds up is this correct?

 

[Averagesupernova]

You see this was what i was thinking.

So if this is the case, as the rotor starts to speed up the current flowing in the rotor must create a magnetic field that opposes that created by the stator windings.

Yes, you are on the right track.

I'm sure somebody could be more technical in their explanation but what I am saying is that the flux from the rotor is cancelling the flux from the stator as the speed builds up is this correct?

I would not say it is cancelling the flux. It acts like a generator in that it creates a back EMF the same way any other motor would. I would say it is a case of choosing the right word.

 

[jim hardy]

FOIWATER wrote :

remember that at locked rotor the motor is behaving like a transformer with a shorted secondary.

that was the answer. When stopped, the squirrel cage windings are almost a dead shorted secondary and their current cancels primary flux just as in a transformer.
Recall that cancellation of flux reduces counter-emf thereby allowing more primary current to flow to re-establish voltage balance..

you said you understand slip - well, imagine yourself small enough to fit in the airgap and sit on the rotor with a magnetometer in hand.
As the rotor speeds up it approaches speed of the rotating magnetic field caused by primary , so the squirrel cage sees less dphi/dt, hence there's less voltage induced in the squirrel cage and rotor current goes down.
Less rotor current cancels less of the primary flux... so primary current goes down too.

Oversimplification, but sometimes that's a necessary step toward understanding something well enough to remember it.

Check your textbook for the slip term in rotor current formula.

hope this helps,
old jim

 

[fonz]

I'd say that's just about cleared things up. Thanks a lot

 

[FOIWATER]

that was averagesupernova Jim, but I would agree with him for sure!

 

[jim hardy]

oops sorry for the mistake.

fellows, this getting old ain't for sissies i tell you !

old jm