Protecting you from your deaths? You must realize you cannot run a motor at any speed you feel like? Asking it to go considerably faster than twice its rated speed is asking for it to grenade on you.


Safety aside a motor is designed to a specific Volts/Hertz ratio of magnetic excitation. As you overspeed the motor supplying it higher and higher Hertz without doing anything about the Volts you are driving the motor farther and farther from its designed region of operation until it becomes unstable and loses sync with the magnetic field its chasing which causes it to simply stop.

You also are reducing the motor's torque from its rated torque for every Hertz you go above the frequency listed on its plate.

Keith Cress
kcress - http://www.flaminsystems.com

Thanks so much for your valuable feedback Sir! We actually thought this occurrence was due to the type of application macro we selected in the ABB -ACS550 drive, but we were wrong. We have now taken safety limits into account and set the maximum frequency of the drive back to 50 Hz and are operating the motor below this range

Good to hear. Glad you came by for some advice!

Carry on.

Keith Cress
kcress - http://www.flaminsystems.com

A VFD allows an AC induction motor to change speed and manitain rated torque by maintaining the ratio of voltage and frequency that it was designed for, aka the "V/Hz ratio". When you get to full rated frequency, in your case 50Hz, the VFD is at full voltage, probably 400V, so a V/Hz ratio of 8:1. But the VFD cannot create voltage that is above what you feed it, so 400V is the maximum. When you continue to increase the Hz without increasing the volts, you are operating the motor at a constant POWER, not a constant torque because you are weakening the fields in the motor. So as that V/Hz ratio changes, the torque changes at the SQUARE of the ratio change. Assuming then that you are 400V 150Hz, your ratio went from 8:1 down to 2.67:1, so 33.3%, meaning your shaft torque became just 33.3%² or 11% of the motor rated torque. At that point you enter the region of the motor not having enough torque to move it's own rotor mass, let alone a load, so it slows down.

VFDs have the ability to run at higher frequencies because there are MOTORS designed to run at higher frequencies. These are often called "Spindle Motors" for high speed machine tools and you will see on the nameplate that it is designed for 400V 400Hz or something like that. THAT is when you need that feature in a VFD.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

Thanks Sir for taking the time to explain so clearly, It all makes sense now!
The place where we went wrong was to assume that the motor would run at any RPM as long as the VFD allows for it (clearly that isn't the case). We understand now why the motor was behaving in this way and have taken precautions to run the motor at it's base frequency