Variable frequency drive and induction motor?

Question

I've got a 0.5hp rated ac motor controller, and a 0.5hp rated motor. 208V 3ph. I want to power up the motor's stator coils without the rotor inside of it (magnetic field experiment). When I try to run it without the rotor, the controller quickly goes into overload mode. Why? Does the rotor spinning actually decrease the stator current?
(note: I am able to run the stator from 0-10Hz, any higher and it overloads or faults)

I have hooked it right to the wall source and the current I measured was within the AC drive's capabilities.
Don't really have access to a synchronous motor... a bit hard to come by, but the stator windings are identical.

Answer 1

This is electricity 101: why doesn't a motor with negligible winding resistance not cause a short when connected to a power source? Answer: because the rotor produces counter electro-motive force (EMF), which prevents a rise in current and acts like internal resistance to the windings. If you remove the rotor, you can't have a spinning magnetic field to induce the counter EMF. You can't run the stator over 10Hz because that is apparently the threshold where the current flowing through your stator reaches the breaker's tripping point. If you want to run your stator without a rotor, install some resistance in series, in each leg/phase, in the circuit (even light bulbs will do). This will prevent the high current situation from tripping your breaker.

Answer 2

I'll start with what I know for sure.

An induction motor will consume more real power if it has a rotor that is not turning compared to having a rotor that is turning. The stator produces a rotating magnetic field. Normally eddy currents are produced in the rotor. As the rotor speeds up it starts to rotate at a speed closer and closer to the speed at which the magnetic field rotates at. The difference is called the slip. The greater the slip the grater the real power consumed by the motor. The smaller the slip the more inductive the motor appears (less real power consumption).

When a variable frequency drive reduces the frequency it also typically reduces the voltage (using PWM). The motor is somewhat like an inductor. Lower the frequency and the current is going to go up unless you also reduce the voltage.

The rest is what I think

If you remove the rotor you are going to mess up the magnetic circuit since much of the iron is gone and the air gap has gone from being tiny to being very large. The inductance will drop to a tiny fraction of what it was (perhaps by a order of magnitude). It is the inductance that is providing some impedance to current flow so the current will jump.

You may be able to reconfigure the drive to change the slope of the voltage frequency curve but you would need to look at the manual to see.

Answer 3

Insufficient counter EMF is produced without the rotor. The rotor is part of the magnetic coupling, and without this, the stator's impedance appears closer to resistance of the windings. As far as the ability to run from 0-10Hz, I am not sure why. Try searching the web.

Answer 4

have you tried a synchronous motor ?
it might give better magnetic fields anyway.

my guess is that the Variable frequency drive tries to figure out how fast the rotor is spinning and it gets confused.

sorry I know nothing about stator current, but I do not think that it would go up without a rotor.

did you try plugging it in to the outlet without the Variable frequency drive and measure the current with and without the rotor ?

Answer 5

It depends on whether the motor is synchronous or not. If it is, it will synchronize to the inverter's frequency, if not it will vary with the power (voltage) applied, though not in a linear relationship because of magnetic slippage due to load of the pump.