DC motors causing line voltage to increase

[CalRred]

Hello,
I have an issue where I have a set of DC motors that are causing my line voltage to increase. I don't have a meter that can test it, but two of my AB PF 700 show DC bus voltage climbing and steadying at 780 V. One of them, a frame 2 trips on over voltage when I run this set.

I have 4 of these setup identically with one of them causing this issue. They are 4 Emerson Mentor MPs with two(8 total motors) 240VDC motors in series.

I was thinking one of the motors is causing a bunch of back EMF, but it's a little above me. Does anyone have an idea?

Edited to change drives to motors.

 

[itsmoked]

Yes! Motors being driven do indeed 'put-back' into the drive capacitor(s). Most drives have a system that watches the voltage on the cap-bank and if it climbs above a certain point starts dumping power back out of the cap-bamk into a resistor that consumes the excess via heat. Large drives sometimes don't have this and it needs to be added as an option. Also while most drives have this feature built in it is only capable of handling the typical motor being used by that size drive. And, by that, I mean handling that motor decelerating over a couple of seconds. If the decel gets to quick or the motor is driving something with inertia (like a saw blade) then the decel has to be exaggerated to perhaps 10+ seconds or more. Or, you need to add an external resistor to carry the excess energy out of and physically away from the drive.

Commonly, if you have more than one drive in close proximity they can have their cap-banks hooked together in parallel. Then any excess generated in one can be directly spent by the others that are still consuming power to drive their respective motors. This reuses the excess regenerated electrical power instead of wasting it as heat.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[CalRred]

This is interesting, but why would it only happen on one motor set? If I power the other three of the four it doesn't happen. Only when I power this one. If I switch the leads and power this motor set with the other drives it happens on that drive, too.

A friend of mine said I might have my motor windings backwards so it is generating?

 

[davidbeach]

Winding backward just make it turn the other way, if anything. A motor is an electric machine where electrical energy creates torque that becomes mechanical energy while a generator is an electrical machine where mechanical energy creates torque that becomes electrical energy.

What's different about this motor in the installation? Does it have a "load" that can actually drive the motor? What do the motors do? If for example the four motors all drive the same conveyor belt and the problem motor wants to drive it slower than the rest it will overload the other motors and become a generator resulting in your overvoltage.

Details, we need details.

 

[Compositepro]

Is it possible that the motors are supposed to be synchronized but the controls are not tuned correctly. This could result in one motor producing braking torque to resist the driving torque of the other motors. What does your machine or process do?

 

[CalRred]

The machine is a rail crane with 4 legs moving a crane on rails. It has two motor per leg.

I thought that they were moving slower and we're generating on that leg, so I uncoupled the motors from the wheels and the reduction gears are free Wheeling.

There are 4 drives, 1 per leg. Each drive has 2 240vdc motors in series for a total of 8 motors. The line voltage spike only happens when this drive is on and running with or without the other drives being on or running. The problem also moves to the other drives if I wire the motors up to the other drives, (These are normally on drive 1, I move the motors to drive 2, and drive 2 behaves this way.)

 

[itsmoked]

OK. So the problem follows the motor when you move the motor.

A motor with a fundamental internal failure.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[CalRred]

This is what I was thinking, or the wiring to the motor. This leg of the drive system has not worked since I started with the company. First the Drive would fault on Armature Over current. Then they removed the motors to replace two failed motors on the other three legs. We just got those motors that were rebuilt back, and put them on.

I did not program these drives, and I do not have a lot of experience with DC motors. In the past when one motor fails in the set, we have tried to run a single motor on the drives and get a Field Winiding Overcurrent fault. What would I need to do in order to run one motor. They are set to with 500VDC to the armature and 120VDC to the field winidngs. I thought it was normal for the fields to be half the armature voltage. I assumed the drives are programmed to send 500VDC to the 240VDC motors because they are wired in series. Would I need to step it down when I run one motor on the leg?

 

[waross]

If the motors are not matched then one may overdrive the other.
If a motor on one leg is not doing its full share, the other leg (on the same track) may over-drive it and produce an over voltage.
Check your field winding. If the field has been rewound with a heavier gauge wire, it will be too strong. If the motor has a field stronger than similar motors working together, it will run slower. The other motors will then over-drive it and push the bus voltage up.


Bill
--------------------
"Why not the best?"
Jimmy Carter