Freqency Drives / Running Motors Over Rated Speed

[rpdodd]

If you have a 3 phase motor rated at 3500 rpm is it possible to run it through a freq. drive at more than rated value, i.e. 4000 rpm? What would be the consequences or damage to the motor if any? This is a 3 phase , 60 hz. 480 volt motor.

[jraef]

If you have a 3 phase motor rated at 3500 rpm is it possible to run it through a freq. drive at more than rated value, i.e. 4000 rpm? What would be the consequences or damage to the motor if any? This is a 3 phase , 60 hz. 480 volt motor.

 

It depends upon the connected load.

 

In general, a motor is rated for a specific torque at a specific speed, we call that the HP (or kW) rating. That is based upon a relationship between the applied voltage and frequency. So a motor power rating can be expressed in terms of a V/Hz ratio. In your case, you have a 480V 60Hz motor, so the V/Hz ration is 8:1 (480/60). When you over speed the motor with a VFD you have no more voltage to give it so you will be lowering the V/Hz ratio. So to go from 3500RPM to 4000RPM, you are going to increase the Hz as follows: 4000/3500 = 114.3%, 60 x 1.143 = 68.58Hz. So if you still only have 480V, the V/Hz ratio will drop to 7:1. So your motor will put out about 13% less torque at that speed.

 

if you have a centrifugal load, such as a pump or fan, your load power requirement will INCREASE with speed, but your motor power will remain flat and torque will DECREASE, so you run the risk of overloading the motor. If your load is mainly inertial, you may be able to get away with it. Keep in mind that most off-the-shelf NEMA design motors are rated for a 1.15 "Service Factor" which means they can sustain brief overloads of up to 115% of rating. So you may never know the difference. In addition, many times a motor is slightly over sized because they only come in specific ratings, so there is often a overshoot rather than an undershoot when designed into a machine.

 

Bottom line, you are probably OK with that small of a speed increase, but watch that motor carefully.