Induction Motor Query: Current, Power, Torque & Speed

[Physicist3]

Hi, I am currently simulating a 3 phase induction motor system. One of the tests that I have run is to keep the motor parameters (reactance, resistance etc.) constant, and then change the frequency of the supply (50, 40, 30 Hz etc.) before measuring input parameters (current and power drawn from supply), along with output parameters such as torque and rotational speed. Throughout, I have kept the value of the voltage supply constant. Having completed the tests, I have found that when the frequency is dropped from 50 to 40, and from 40 to 30 Hz, both the current and power drawn from the supply decrease. However, when dropping the frequency below 30Hz, I have found that the current drawn from the supply increases again, yet the power (watts) continues to decrease with a constant voltage. I don't quite understand how this is possible, would it be the result of a really poor power factor at low frequency as the motor begins to stall and inductive parameters become more influential on the system?

 

[jim hardy]

It's properties of inductors allright.

You need to try this for real with a small transformer and Variac adjustable transformer.

Remember that flux is integral of voltage
so,
as you lower frequency with voltage held constant,
the period of each half cycle grows longer,
which means the integration has a longer interval,
which gives it a larger result, it is after all a definite integral;

which means flux will reach a level that's unsustainable in iron;
which means magnetizing current will go up to force the extra flux that the iron can't accept out through the air ;
which makes the motor hum and vibrate and get hot;

and your simulation will not emit the sounds and smoke that a real motor would.

That's why you need to take a small transformer, ammeter, and adjustable voltage source
and drive the little transformer into saturation while you watch current increase sharply at saturation point,
hear it hum and feel it vibrate and get hot, maybe even smell the hot insulation.

Look up term "Volts per Hertz" for a better explanation.

I encourage that experiment with real parts. It'll make your mind intuitively aware of what the math is telling you.

I appreciate the power of simulation, but it's a poor substitute for the visceral reality of a proper electric machinery lab.