Hello
I have a Brook Crompton 3ph/1.26kw/2 pole motor that would normally do 3450rpm at 60Hz. The VSD I have can drive the speed up to about 80Hz.
How fast can a motor be driven above 60Hz, or is that the safe limit?
There was a thread a few months ago regarding this very same question.
http://www.practicalmachinist.com/vb/showthread.php?t=101940
In it Forrest Addy said....
"In years past I looked into this. I experimentally ran an unloaded 5 HP 3450 RPM motor to 400 Hz for a few minutes until something inductive happened and the motor fell out of sync. That's - um - 23,000 RPM. I recall when I dismantled the motor that there were no indications of failure, there were no cracks or yielding of the cast-in armature fan, radial distortion of the assembly, the sqirrel cage passed the growler, etc.
Remember that induction motor armatures are of nearly solid steel construction, its squirrel cage is high pressure cast aluminum, and the laminations are in the plane of rotaton. Thus the typical squirrel cage armature structure is quite robust especially when compared to a wound DC armature or an internal combustion engine crankshaft and flywheel with all the associated reciprocating mechanism.
The steel itself isn't high strength by any means but I think it's 35,000 PSI yield or there abouts. I failed to note the armature diameter when I ran the test and I since reassembled it and put it back on the shelf. Without taking the motor apart and by peeking in the ODP vents I would guess the armature is about 4 1/2" is dia.
Maybe someone with an engineering bent would shed a little analysis on the topic of the centrifugal strength of induction motor armatures.
If we over-speed the motor we have to consider what happens if something fails at high RPM. Given the fully housed construction of a typical motor and the strangth of the stator I would think that even an utter and explosive failure of the armature would be fully contained. The motor itself might jump if not restrained but there would be practically zero chance of flying debris. Consequent electrical casualty in the motor would be mitigated by the VFD's fault protection. There's an excellent chance that a motor subject to such a failure would be a complete write-off.
All is subject to experimental verification, of course. On the basis of my crude experiment I'd suggest that induction motor armatures may be presumed safe to 10,000 peripheral feet per minute which is about 1/2 the speed I took the 5 HP armature to.
Since radial stress goes up as the square of the RPM increase the 10,000 ft per minute figure would suggest an interim safety factor of roughly 4 to 1 better than the crude experiment I ran."
Motion Guru also said "I have operated a 50HP Reliance motor on the 230 volt windings on a 200HP Vector drive that operated at 480V and run the motor at 120Hz at 100HP continuously and 240Hz (7200 rpm) at up to 150HP for minutes at a time and 10,000 rpm for extended periods at 50HP - 75HP. This was a dynomometer application that I did for Borg Warner about 15 years ago - to my knowledge the dyne is still operating.
The only modification to the motor was the removal of the fan and the installation of an incremental encoder. Bearing grease was flushed from the bearings and a lighter grease was used."