# How can I calculate the input power at a given frequency for a PWM motor?

Is it possible to calculate what input power (watts) is needed to run a given PWM at a certain line frequency (hertz)?

I have all the motor parameters, I know:

• P-nom, nominal power, kW
• U-nom, nominal voltage, V
• f-nom, nominal (line) frequency, Hz
• N-nom, nominal speed, RPM
• I-nom, nominal current, A
• cosPhi, power factor
• pole count

I need to make a function that takes in a line frequency and outputs power used by the motor at that frequency.

EDIT Will give some more information on what i'm actually trying to achieve.

I work at a company that makes inverters that has true sinusoidal line frequencies. PWM chops up the volt into pulses with different widths that makes it so the flow resembles a sinus wave. When doing this energy is lost into uninteresting forms of energy for the system (sound, heat, etc) that we can disregard. Especially at lower frequencies. At 50Hz our inverter is not that much more energy saving than a PWM, our big strength is that at lower hertz (like 10-30Hz) there is a big waste in PWM motors.

My boss asked me to make a program that can show 2 line graphs, one for our inverter and one for a standard/average PWM inverter. Where the Y-axis of the graph is power consumed (watts) and the X-axis is the line frequency from the inverter to the motor (hertz).

• If your PWM works properly, its frequency should not influence the motor power. Only the duty cycle is supposed to control the power. Do you mean variable-frequency drive instead of PWM, maybe? Commented Nov 10, 2022 at 13:36
• But i thought PWM motors were less effiecient at lower speeds? Thats the reason im tasked with making this function. To calculate the waste of energy at different hertz. Commented Nov 10, 2022 at 13:38
• PWM = pulse width modulation. The frequency doesn't change. PWM controlled motors might be less efficient at low speed (I don't know,) but it won't have to do with the frequency. You might be trying to figure out the power for a particular pulse width (duty cycle.) That would be a reasonable thing to think about.
– JRE
Commented Nov 10, 2022 at 13:55
• Depends on the motor type. Which is yours? Commented Nov 10, 2022 at 13:57
• This question requires many questions to answer. different motors give different torque vs speed profiles. The motor controller will have different losses based on the frequency. You will have to do bench testing to achieve what you want, or if you can find the data from the motor supplier and the motor controller. Commented Nov 10, 2022 at 14:05