# Funny voltage on load side of VFD



## Sparkee (Sep 22, 2009)

I need some assistance diagnosing a problem on a Delta drive on our saw.

We were getting an over load fault on our drive and we found some bad bearings on the shaft. But even after we disconnected the belt and there is no load we were still getting the same fault.

It is a 6 lead part winding motor and I rang it out and all the windings had continuity were it should and none were going to ground so I think the motor is good

I pulled the motor leads off the drive and checked voltage and I am getting 119 volts to ground on my A and C phase but only 63 volts to ground on the B phase. Phase to phase I get 198 volts from A to C but only 165 volts from my B phase. This is at 58 hertz. Could I have a problem with the drive?

The incoming voltage is 218 volts three phase. And the drive is rated at 230 volts three phase.


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## Gravy (Feb 19, 2009)

Maybe a stupid question but did you try spinning the motor by hand once uncoupled?


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## Sparkee (Sep 22, 2009)

Gravy said:


> Maybe a stupid question but did you try spinning the motor by hand once uncoupled?


Yes. The motor spins fine.


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## Gravy (Feb 19, 2009)

Reading output voltage on a VFD with a rms meter is often misleading.

You can check the SCR's in the drive by powering down and doing a continuity check across them. 

Some good info here:
http://www.brosea.com/faq_023.html

Sorry I can't be more help than that. You could try contacting the manufacturer of the drive.


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## JRaef (Mar 23, 2009)

Gravy said:


> Reading output voltage on a VFD with a rms meter is often misleading.
> 
> You can check the SCR's in the drive by powering down and doing a continuity check across them.
> 
> ...


Yes, it takes a very specialized meter to read anything accurate on the output of a VFD, and even if you have it, NEVER NEVER measure the output to ground!!!! The output of the VFD is not the same as the power source, it is no longer a grounded Y source. So the output is floating with reference to ground, meaning any capacitive issues will screw up your meter readings at the very best, and might cause the transistors to misfire, toasting the VFD or if you are lucky, just trip the VFD on Ground Fault.

Back to your issue of the drive tripping:
There are two types of "Overload" protection in the drive now: Motor overload, and DRIVE overload. Motor overload is the same as any motor overload, it is because the motor is drawing too much current. That's where the bearing issue, jammed load, something like that comes to play. And by the way, just because you can spin it easily when un-powered, does NOT always mean there isn't a bearing related problem. Axial thrust (meaning in the same direction as the shaft) as a result of torque can cause a failing bearing to sharply increase load on the motor, but ONLY when you get enough torque to make it move. If you are getting motor overload trips, read the motor current in the VFD display, that is the most accurate way to see what's going on. But fair warning, the Delta VFD is not an expensive one, so it most likely will only display AVERAGE current. See if there is a parameter that tells you the motor thermal overload percentage.

But now you have the addition of DRIVE overload, which means the DRIVE is using more current than it is rated for handling safely. Usually as you might imagine, the Motor overload would always trip before the drive overloads, as long as the drive is properly sized, and by that I mean that *the drive is sized for the motor FLA*, not "HP", because HP is not a valid measurement of electrical power, it's a mechanical rating. So since that is easy, always check that first.

But in addition, Drive overload can mean that the drive is over heating (in some mfrs designs). That can be from a number of factors, but a common one that is not obvious is that someone, in an effort to get rid of the high pitched "whine" in the motor made by running from a VFD, has turned up the "Carrier Frequency" on the drive. It doesn't really get rid of the whine, it just increases the pitch to where we humans can't hear it. But the down side of doing that, often missed, its that it ALSO increase the switching losses inside of the VFD. So even though the current going to the motor is lower than the motor OL setting, the EFFECT that the current has on the VFD is higher because of the higher losses inside of it, and it trips on Drive Overload. Delta drives require de-rating if the carrier frequency is increased.


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## Big John (May 23, 2010)

JRaef said:


> Yes, it takes a very specialized meter to read anything accurate on the output of a VFD, and even if you have it, NEVER NEVER measure the output to ground...!


 I've never heard that before. Is that true even with a 1MΩ impedance? I would think the capacitive coupling on a long load-side run would often be indistinguishable from that, no?


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