QES
Senior Member
- Location
- California
What is the current/voltage characteristics when you go over 60Hz or "overspeed" a 3phase induction motor?
The speed increases, the available torque reduces, and the available output power stays the same.What is the current/voltage characteristics when you go over 60Hz or "overspeed" a 3phase induction motor?
The speed increases, the available torque reduces, and the available output power stays the same.
Are there any problems to watch out for when overspeeding a motor with a VFD? We are in the design phase for a compressor installation that requires 325 HP at 720 RPM and can operate at a max speed of 760 RPM, where it would require 375 HP. The VFD and motor application engineers both say I would be best served by a 10-pole (720 RPM) 400 HP motor on a VFD rather than an 8 pole (900 RPM) motor on a VFD. I don't have a full torque curve for the compressor, but it supposedly requires much less torque than the rated curve of either 400 HP motor. I couldn't get a good answer out of anybody exactly why I should overspeed up to 63.3 Hz rather than run a 900 RPM machine at 51 Hz, assuming that the speed/torque curves of both machines are sufficient for the load.
Besoecker has a good answer. Let me add that there will be SOME slip at full load, so figure on something more like 64 Hz. Unloaded speed may hit 768 in case that is a problem.up to 63.3 Hz ...
Simple quick answer is that, at 51Hz, your 400HP motor will give only 340HP at rated current.
The 8-pole machine at 51Hz gives the right speed but less power thus less torque.Would HP really matter as long as the motor could supply the required torque at this particular speed? I know HP is a function of torque and speed.
The 8-pole machine at 51Hz gives the right speed but less power thus less torque.
What you need to consider is that the motors will produce 400 hp at base speed and rated voltage. The 8-pole machine doesn't get to base speed so won't produce 400 hp.Seeing that these are both the same power motor (400 hp) I tend to look at things from a torque perspective? Is this an incorrect approach for such problems?