I was musing about electric motors after seeing one randomly on TV, and there's something I'm must be remembering incorrectly.
It's just a thought experiment, so I'm imagining some theoretical DC electric motor, with a constant RMS current/voltage feeding in, and some constant load on it causing it to spin at some constant rpm and produce some constant torque.
Since voltage x current = electrical power = motor's mechanical power, that's the cap on how much you can get out of the machine.
But I remember that the greater the difference in rotational speeds between the rotor and stator fields (slip?), the more force/torque will be exerted on the rotor.
So in the hypothetical, if the motor is doing its thing and is stable with the load, if you suddenly increased the frequency of the input voltage/current, that in my mind would result in suddenly increasing the difference in relative rotational speeds of the stator/rotor, meaning the stator would feel more force/torque on it, which would make it want to speed up and catch up to the stator field, speeding up the load.
But only frequency changed and not how much electrical energy is entering the device, so there's no way that's possible! So I MUST be wrong, I just don't see where the error in my thinking is…
What's wrong with my thinking, and what would actually happen if you increased the frequency like that?
(I'm not an electrical person by trade, so there's a limit to my knowledge of concepts/terminology.)
[EDIT: I failed to mention that by "frequency", I'm referring to how DC motors can be controlled with pulse width modulation, and those pulses go in at some frequency. Otherwise it wouldn't make sense to talk about "frequency" with direct motors]
Best Answer
The type of problem you are talking about might be be explained by an induction motor.
In a dc motor increasing the no of pulses would only increase the average voltage which would then lead to motor entering a transition state till the new steady state is achieved(if achievable). If a steady state is achieved in accordance to both load and motor torque the motor might work at a new speed and hence the frequency(actual not the pwm one) of the internal ac currents and voltages might increase.
As for an induction motor when you increase the frequency there are multiple factors(load characteristics and motor characteristics at that frequency) that govern what the new steady state might be and all are in agreement with the conservation of energy. If you keep the voltage constant, the current is governed by the motor load requirement. Hence it will under normal operation, increase the current intake if load speed is increased at same torque. Well you see things start getting a little tricky here, in case you are changing parameters such that input power remains constant( and trust me that is not done by just changing the frequency) Then to increase speed the torque would have to decrease and this new steady state(if the load allows it) will have same product of torque speed as before.
It's really good to see people trying to think deeply about motors.Normally people are just interested in the applications. You should do some serious study if you like the topic, it's really indulging.
Thank you for taking time to read the answer.