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09-27-2008, 08:56 AM #1
Inverter duty motor/regular motor?
What are the drawbacks to using a regular motor with a VFD? What is the difference between an inverter duty and a regular motor?
Thnx - John C
09-27-2008, 11:02 AM #2
if you listen to marketing hype, there is a huge difference, with special super duty wire that makes all the difference.
In reality, inverter duty motors are generally sorted out from normal production, as the ones that stood up to somewhat higher voltage surges.........
There is a special testing system for determining motors which are likely to fail the "hipot" test, and find it out before final assembly. I have designed testing equipment of that type. It will find mis-placed insulation, too-close clearances, etc. It will also "grade" stator assemblies for voltage withstand, so that higher grades can be marked as inverter duty.
So in general, there is little difference, except that you generally have more assurance that the insulation sheets between phases etc are all actually in their correct places, and so forth. The hyped-up wire has very little to do with the voltage withstand.
Of course, nothing says that a "standard" motor won't have just as good an insulation withstand.... and you KNOW it has to pass a minimum "hipot" which is considerably above regular mains voltage. The inverter duty ones are just capable of passing a higher hipot than others.
When the motor and VFD are close together, your chances of serious voltage surges is much lower than if they are at a distance of several meters.
09-27-2008, 12:09 PM #3
JST is correct.
Assuming you are referring to a 230 volt motor, there is no drawback. There is a lot of misinformation getting spread around that basically says that you need inverter rated motors, but to date I have not found one study that shows that non-inverter rated motors powered from a VFD failed at a higher rate when operated at 230 volts. One study actually showed that that the rate of failure actually went down.
When I researched this topic last year, I was amazed to find the number of bogus "studies" published. I suspect that these were not studies but the result of poorly formed and executed surveys.
A recent random field test of premium legacy motors (motors made prior to 1960) showed that they performed as good, and in some cases, better than a modern inverter rated motor in insulation breakdown tests. The high number of motors that exceeded modern inverter ratings was surprising to me.
When you get into the higher operating voltages like 660 volts, and to a certain extent 440 volts, is when failure rates rise. Bearing failures are the most common failure and some extra steps need to be taken to prevent such failures. Insulation breakdown can and does occur at these higher voltages but the rate is not as high as you might think. Mitigating insulation breakdowns at 660 volts is something that could be the subject of a Phd dissertation.
09-28-2008, 01:15 PM #4
Inverter duty motors
Thanks JST and Pierce, you have saved me some money and that's very important these days.