Basic VFD/motor troubleshooting: no torque

I'm trying to troubleshoot a machine tonight and am running into the limitations of my VFD and 3-phase motor knowledge, and hope you good folks can help.

The machine has an Emerson Commander SK AC variable frequency drive connected to a 2 HP, 460V AC 3-phase motor. (The actual nameplate was removed but that's a different story).

The machine has run fine for several years but this evening stopped moving its load, a large turntable frame connected via a stout 225:1 gearbox and an ordinary Lovejoy jaw coupling. The motor is equipped with a brake on the back, which we can disengage by hand.

I should say "stopped moving its load reliably". Usually when this turntable runs you could chain a bull to it and not slow it down. Tonight I can stop it with my hand. So it's got juuust enough torque to move the load.

I unbolted the motor from the gearbox. The drive will start it turning but it has so little torque I can stop it with my hand on the output shaft.

Incoming voltage is 450 VAC. The drive does not fault and just displays the output frequency and either "Rd" or "Fr" for "Ready" and "Output Frequency".

I measured 7 ohms across each phase of the motor (I disconnected the wires at the motor junction box to verify they were tight) with my ordinary handheld digital meter. No shorts to ground. No high temperature or funny noises.

The output current display shows about 1.5 to 2 A while it's turning the load, which makes sense for the 2 HP motor.

Everything looks right to me... just very little output torque.

Any suggestions on what I can troubleshoot next ?

What mode is the drive in? (Don't know much about Emerson drives)...
If it is in a vector control mode, you can try autotuning the drive and motor again. Is there an encoder? AC Drives that lose encoder feedback often act, well, stupid.

The SK is open loop only
are you sure it is not mechanical

Great point !

There's no encoder and I think the Commander SK is a simple open-loop V/Hz drive.

This is a very inexact application where they used a VFD to get the output speed down, then the gearbox to trade torque for speed. It just turns this carousel 180 degrees one way, hits a hardstop, then turns 180 degrees the other way. Each 180 degree turn takes about 10 seconds.

I uncoupled the motor from the gearbox and the rest of the system to narrow down the mechanical aspect of it.

It's a Marathon motor, with a tailshaft mounted electromechanical brake with an override lever.

With the override engaged, the motor shaft spins freely, so I know the brake isn't damaged or dragging.

I don't hear any scraping or dragging noises when the VFD starts the motor, so I think the electric brake is working correctly. I can hear it make a typical "thunk" as it engages and disengages.

I know for certain nobody here could have changed the drive's parameters; they don't know how and never open the cabinet.

I'm puzzled !

That is what I would do, of course this could not be the best option as I'm a programmer and I've got always people that has made the electrical job (even parameterizing drives...).

Try to connect the motor directly to the electricity without the drive... if you still have the same behaviour then it's the motor, if it works well, then the drive has lost the parameters or it has died.

If it is the second case, then you should get the parameters into a backup and make a factory settings, adjust it again and try, if it keeps failing get a new one and send that one to repair...

I like the test method. Eliminate as big a chunk of the system as you can first, then focus your attention to the locus of the issue.

jraef said:

I like the test method. Eliminate as big a chunk of the system as you can first, then focus your attention to the locus of the issue.

Click to expand...

Yep, do this. Then stick an amp clamp on each leg of the motor to make sure they are equal and roughly 25% of the motor nameplate with the motor unloaded. You shouldn't be able to stop the motor by hand (in fact you shouldn't even try!).

I'm not familiar with this type of drive but try and find somewhere where it displays output torque, not current. The current reading can be misleading.

Check to make sure the drive is not in sensorless vector mode. If someone changed it from V/HZ to sensorless vector and didn't tune it I would expect very anemic response from the motor.

As you say, there is electrical brake on the motor, right?
I would check it. on most of the motors, it is set to 1.25-1.5 of the motor nameplate torque. If the brake doesn't open, it would be not problem for the VFD to reach supply higher torque and move the motor. I have a couple of brake rectifiers blown out, and the result is same + higher temperature of the motors, regarding the friction, of course

Not mentioned and probably does not apply in this case, but, long wiring runs more than a couple hundred feet between the VFD and the motor can also cause this problem. Normally this problem shows up on new installations.

If the motor runs good across the 460v supply, dump the VFD and replace it with a new one. A 2hp 460V drive is not worth monkeying with or surely repairing.

I have seen similar circumstances when one of the phases (in the VFD) is not operating properly or totally failed. It acts like a single phasing motor. The clamp meter "might" help you identify this, but be aware it may "lie" to you because of the noisy output caused by the carrier frequency of the VFD.

I have also seen this happen when a motor is miswired, (wye when should be delta). Only usually see that on larger motors, but I did see a 5HP that was supposed to be delta, wired for wye and got similar behavior.

Another thought is that if the system is designed to run up against a hard stop, there may be a current limiting, or 2nd current limit type of input on the drive to reduce the torque for that part of the cycle. If that (speculative) input to the drive is "stuck on" or the drive "thinks" it is always being told to run in a lower current limit mode due to a faulty input point, then that could be an explanation as well.

This 2 current modes idea is all just my speculation, but if I designed a system that "torquey" I would want:
a) not to break the positive stop
b) not fault the vfd when it hits the end limit

So, I would use that feature if available.

+1 for the bypass the VFD test.
+1 for what Dick said if the VFD is bad, just swap in a new one.

Thanks for all the input, guys.

It's about ten feet from the cabinet to the motor, so I'm confident I don't have a damaged cable or changed wiring betwen the motor and VFD.

There are some, er, political problems with maintenance at this facility. Run-to-failure is the guiding principle.

I'm going to push hard to get a temporary starter and 460V AC hookup to test the motor, and order a new drive in the meantime, while backing up the drive parameters by hand.

The Commander SK has a neat "Smart Stick" parameter backup device and probably a straightforward serial connection, but I don't have the software or experience with it.

Eddie Willers said:

The Commander SK has a neat "Smart Stick" parameter backup device and probably a straightforward serial connection, but I don't have the software or experience with it.

Click to expand...

I think the RJ45 connector on the SK is RS485. There are a lot of parameters in the SK, so I think it's probably worth getting the SmartStick. I don't know what your local Emerson dealer gets for it, but Galco has it for $23.



-Eric

DickDV said:

If the motor runs good across the 460v supply, dump the VFD and replace it with a new one. A 2hp 460V drive is not worth monkeying with or surely repairing.

Click to expand...

Dick,
You have obviously never had to buy a replacement Emerson / Control Techniques drive...