I had a question about motor speeds on VFD equipment. I think I mentioned before that we are just getting involved with these here at our plant.

Is it possible for a motor to run faster than the designed RPM? Some of these are set to operate at a maximum of 70htz.

Just wondering. I've been running the formula by a couple of the fans we have here on VFD systems.

You know the 120 X HTZ divided by # poles...

Thanks,

Rod,

We have some newly installed case conveyors running 4 pole (plate data 1725rpm) motors at 80hz. I tached them at 2379rpm. It kind of blew my mind at first but the electrician that is in our CBM group said it is getting more common with the technology advances. We monitor quite a number of VFD driven assets.

Regards, Dave Dane

Rod

You can overspeed a motor up to the limits of the components of that machine (check with the supplier). However, once you get above about 70 Hz, in a 60 Hz environment, you will begin to see that the torque will begin to drop off by the square of the speed.

The 'designed RPM' is the speed set by the supply frequency. The synchronous speed is the (120*f)/poles that you outlined the output speed is that value less the slip, in an induction motor. The ways of changing that speed include changing the number of poles or changing the supply frequency. However, if you were to maintain the supply voltage when you adjust the supply frequency, you will cause over-excitation of the stator and rotor. So, you have to adjust the voltage along with the frequency.

Howard