Induction motor At constant RMS current and different frequencies.

[James20]

What will happen with Power and Torque if I supply simple induction motor with constant RMS current at different frequencies.

Let's consider "Ideal" Motor. No or very low losses (Eddy current, friction , stray , hysteresis and so on).

Let's say range 50Hz to 500Hz , interval 10Hz.

Power at constant RMS AC Sinewave current ranging 50Hz ... 500 Hz
Increasing, Stays the same, Decreasing

Torque at constant RMS AC Sinewave current ranging 50Hz ... 500Hz.
Increasing, Stays the same, Decreasing

Thank you.

 

[crutschow]

For an ideal motor:
The torque is determined by the load.
So, since motor speed is proportional to frequency, for a constant torque load, the power output and the power (current) input to the motor will also increase.

 

[rjenkinsgb]

AC Induction motors are usually rated for maximum load with a certain "slip"; eg. A common 50 Hz speed is 1440 RPM. That's 4% slip. (4% less than synchronous speed).

Assuming you keep the load adjusted to maintain 4% slip over the speed range, I'd expect the torque to initially increase as the frequency increases then drop off as eddy current and inductive losses reduce efficiency.

(Mechanically, any large motor is likely to disintegrate before 10x rated speed! They do have some safety margin, but nowhere near that, more like 1.5x to 2x speed at most).

 

[James20]

Also The rms of supply current is kept constant at 50...500Hz frequencies.

 

[crutschow]

Also The rms of supply current is kept constant at 50...500Hz frequencies.

Do you mean the voltage is kept constant?

 

[ChrisP58]

Generally, the supply voltage is constant and the load draws whatever current it needs to do the work at hand.

You can supply a constant current, but then the supply voltage must vary to match the required power.

Generally, constant current supplies are really voltage supplies whose output voltage is varied to keep the current constant.

 

[James20]

I supply constant rms sinewave current , but increase value from 50 to 500Hz.
Same rms .

 

[schmitt trigger]

No,
To a motor you supply a voltage, and the motor will draw the necessary current which is, to a first degree, only dependent on the mechanical load. Plus the current requirements of the internal mechanical and electrical losses.
If you change the frequency, the voltage should change in the same ratio to maintain a constant V/F ratio.
EDIT: I see you are talking about an ideal lossless motor. Therefore I have removed my comment about losses at higher than the base frequency.

 

[James20]

If I change/adjust voltages quickly at specific frequency, depending on loads in order to maintain constant rms supply current.
Increasing frequency. Const. rms supply current. Torque = Increasing/Decreasing
Increasing frequency. Const. rms supply current. Power= Increasing/Decreasing

 

[schmitt trigger]

I have the feeling that your post is a classic example of the XY Problem. It doesn’t make sense. Please instead explain exactly what you are attempting to do or accomplish.

The XY problem is asking about your attempted solution rather than your actual problem. This leads to enormous amounts of wasted time and energy, both on the part of people asking for help, and on the part of those providing help.

• User wants to do X.
• User doesn't know how to do X, but thinks they can fumble their way to a solution if they can just manage to do Y.
• User doesn't know how to do Y either.
• User asks for help with Y.
• Others try to help user with Y, but are confused because Y seems like a strange problem to want to solve.
• After much interaction and wasted time, it finally becomes clear that the user really wants help with X, and that Y wasn't even a suitable solution for X.

The problem occurs when people get stuck on what they believe is the solution and are unable step back and explain the issue in full.