Question about VFD (variable frequency drive)

torque."

torque?

(I am not sure that there is the precise statement, but I think theoretically it can change the on/off time to change RMS voltage at the motor). That VFD introduction does not include voltage variation?

Normal mode for a basic VFD is volts to hz... Which means by the time the drive reaches max output, it is generating max voltage and frequency the motor is designated for.

At minimum output (zero speed), it will generate no voltage and no hz..

At 50% output, it is generating half the motor rated voltage and half the motor base frequency..

It is a proportional operation and since this is true, the torque capabilities for the motor equal.. In other words, we can generate max current for that motor at any speed except 0 of course.

Lets say you have a motor that handles 10 amps max, if you were to lower the frequency to 10 hz for example, you also need to lower the voltage to the same proportion. Reason for this is induction of the motor coils. Lower frequencies will generate higher currents and currents are what drives the machine (magnetically). If you don't lower the voltage to match the ratio of frequency, you'll exceed the motor's current rating. Of course, higher Hz requires higher voltages to maintain that max current due induction.

If you wanted to flux the motor with constant current then look at operations like vector control. Vector systems attempt to maintain a near constant current in the motor, even when it's not spinning (0 hz).

Basically, it would appear like having pure DC current being applied to the coils, which basically it is. This will give you a stiff holding drive shaft. THe control electronics will modulate this constant current (Mag current) to form the AC frequency and voltage required to move the shaft. It's almost like having a servo motor as a results in effective output.

When things are operating smoothly in vector mode the motor current should be reading the same through out the power band. If the current starts to increase in vector mode it means that you don't have the mag current up to the 50% mark or, you have exceeded the motor and drive's ability and most likely will respond to things like speed and over voltage errors.

Jamie.