Suppose a squirrel-cage induction motor is driven by a VFD (variable frequency drive), does that mean that the main line frequency may not be 60 Hz anymore but the frequency what the VFD provides (which is varying). If this is true, then how should we determine the sideband frequencies and the new main line frequency in order to look for broken rotor bar or eccentricity faults in the motor current spectrum? Does the formula (1-2s)f still apply but with a varying f?
Thanks a lot for your help.

Are you interested in energy efficiency efforts in motors coz your name SaveNature? What is the size of this motor?

Hello Josh, I am interested in the condition monitoring part. The size of this motor is 1HP, but my point is in the VFD part that drives this motor and whether VFD changes the main line freqency (60 Hz), thus the sidebands which eccentricity and broken bar faults show themselves? And if this is true (that is main line frequency varies according to VFD), how to determine the frequency location of the new main line and the sidebands from the current spectrum? Thanks.

One way I have used is to temporary convince the drive that it should not interfere by driving it to the line freq. 50 or 60Hz pending where you are. Another complicating factor is that the modern thingy also fiddle with the cos fi and by that the lag so VFD drives run close to synchronous to the freq. the driver supply and the wanted sidebands is hard do find so close to the "line" freq. So you also temporarily have to turn of that in the drive. On the other hand, most you would find is rotor bar problems and they mostly come from bang on starts and is almost totally avoided by the VFD having a very smooth startup ramp..... So VFD it makes it hard, on the other hand you may not need to bother? Still also VFD driven motors do go bad from time to time so it is good to have the possibility to check them. Olov

Gentlemen:

Yes, you consider the output frequency of the variable frequency drive as your line frequency and everything changes. The best way to determine that line frequency is to use the display on the VFD or, to look at your running speed. The speed and line frequency are linear as ((LF * 120)/#poles) * slip. If you are looking at a four-pole motor at 60 Hz (excluding the slip), the result would be the synchronous speed (1800 RPM). If the speed is changed to 30 Hz, the synchronous speed would be 900 RPM.

Please find a short tutorial on motors and vfd's from my days at the University of Illinois at Chicago's Energy Resources Center attached.

Sincerely,
Howard
howard@motordoc.net for any additional questions.

drives.pdf (267 Kb, 47 downloads) Induction Motors and VFD's