Drive/Servo Multi-Motor Application

[JRoss]

This is a question for anybody out there who has an in depth understanding of AC motors and how they work with VFDs, particularly in a multi-motor application. I'm putting this in the General Topics forum even though we're using Mitsubishi equipment because the problem is related more to the motors than anything.

I'm working on a job upgrading the drives on a massive pavement testing rig. The rig weighs in the realm of 1.4 million pounds, and rides on two parallel railroad tracks that straddle the testing area. The rig moves up and down the tracks on 16 sets of "bogeys" (8 per track) that are each driven by a 50HP constant torque motor. The motors are connected in groups of four to 350HP VFDs (total of four VFDs). Originally, There was an encoder on one of the motors that was split out to all four drives (for speed feedback) and the PLC (for position feedback). Moving the rig involved turning on all four drives with the same speed and direction; stopping involved pneumatic disk brakes on each bogey. The system is 12-15 years old.

The upgrade replaced the original Mitsubishi drives with Mitsubishi A700 VFDs (the most advanced VFD line they make) of the same size, as well as adding braking resistors to use "dynamic braking" and save the disk brakes for emergencies. In addition, we sought to improve the system by adding three encoders, for a total of four. Now each group of motors has its own encoder going back to its own VFD for speed regulation. For a variety of reasons, we took this a step further by adding motion option cards to the drives so that we could also close the position loop and control the system similar to servo. We had to send three motors back to the motor OEM to add extended shafts for the encoders. The customer also chose to have their spare motor modified in the same way (total of four motors modified).

After some initial difficulty tuning the system due to mechanical backlash, we had the system running fairly well. We took several weeks off while some other work was being done in the area, and then the fun began. After sitting idle for a couple of weeks, the customer powered up the rig to move it to a different section of the track. It got about 10 feet at a fairly slow speed, when one of the encoder motors failed catastrophically. Sparks, smoke, the whole nine yards. We came in and helped them mount the spare, then moved the rig about 4 feet to test motion. All was well, so we started to move again, and got a Ground Fault error on one of the drives. Turns out that a second encoder motor (NOT the spare) had failed, though very quietly. At this point, we megged all the motors, and found that most of them had more than adequate insulation (including the spare). However, a third encoder motor was nearly ready to fail, measuring at about 0.05 megaohms. I'm told readings should be in the 4-5 megaohm range. The three failed (or almost) motors were the ones that had been modified by the OEM.

So now we are facing the unenviable task of trying to figure out why the motors failed. One theory is that something happened during the modification to damage the insulation, and the OEM has all three motors and is trying to determine that.

A second theory is that something about the design of the upgrade is damaging the motors, and this is the one I'd like input on. Other than that, I'm not entirely sure what questions to ask. Any help would be appreciated!

I will also be posting this on the MrPLC forums.

 

[bce123]

The three failed (or almost) motors were the ones that had been modified by the OEM.
and the OEM has all three motors and is trying to determine that.

We had to send three motors back to the motor OEM to add extended shafts

How was the shaft extended? was the motor completely re wound or was a shaft welded to the end? If welded get a new OEM

 

[JRoss]

As I understand it, the shaft was replaced without needing to rewind the rotor, then the motor was cleaned and reassembled.

 

[bce123]

when you say OEM do you mean the motor mfg.?

 

[JRoss]

Yes, the company that custom-built the motors 12-15 years ago did the modifications.

 

[dmargineau]

Replacing the original VFDs with a newer generation in a multi-motor application while keeping the old motors could cause major issues especially in a torque application, however, this does not explain why only the modified motors have failed within the new controls configuration.
Obviously, the shaft modifications had damaged the windings' insulation to a lesser or higher degree (Ground Fault is a motor windings internal short to the ground).

 

[leitmotif]

I wonder if somehow the OEM fouled up the rotor construction which resulted in overheating the windings. The other thing that comes to mind is somehow they fouled up and a mechanical failure locked the shaft = again overheating windings.

I think I would have OEM test run the motors ideally with design load for a time duration double that of what your normal cycle time is OR have them come out and start the motor themselves.

THe other thing that comes to mind with so many encoders is could one or more have been incorrectly connected thus telling VFD the motor is runnng in reverse when it is actually going foward?

Dan Bentler

 

[JRW]

I'm sure Dick will chime in ,
but I would like more data on the motors
Also what type of drive was originally on this?

I've seen some special matched motor and drive sets

 

[DickDV]

Interesting situation! Do I understand correctly that four motors are powered by one VFD with one of the four having the encoder? And, are the four motors mechanically independent of each other except for the connection thru the wheels to the rails?

My first reaction to reading this is that the new much-higher-carrier-frequency Mitsi drives are simply destroying the older motor insulation and the encoder motors failures are simply a co-incidence. Mitsi has one of the highest carrier frequency carriers around (I believe its 18khz) and that mandates motor lead reactors or dv/dt filters pretty much every time. It would be useful to know what length the motor leads total up to. That would be the leads for all four motors added together.

What can you tell us about the motor insulation ratings?

And, do the motors have thermostats and P1/P2 leads, or is the thermal protection done thru motor OL blocks?

 

[Peter Nachtwey]

I'm working on a job upgrading the drives on a massive pavement testing rig. The rig weighs in the realm of 1.4 million pounds, and rides on two parallel railroad tracks that straddle the testing area. The rig moves up and down the tracks on 16 sets of "bogeys" (8 per track) that are each driven by a 50HP constant torque motor. The motors are connected in groups of four to 350HP VFDs (total of four VFDs). Originally, There was an encoder on one of the motors that was split out to all four drives (for speed feedback) and the PLC (for position feedback). Moving the rig involved turning on all four drives with the same speed and direction; stopping involved pneumatic disk brakes on each bogey. The system is 12-15 years old.

Your description sound familiar.
Are you talking about this beasty?
http://www.airporttech.tc.faa.gov/naptf/

 

[leitmotif]

Your description sound familiar.
Are you talking about this beasty?
http://www.airporttech.tc.faa.gov/naptf/

Peter that referral is worth its weight in gold. Saved a million words discussing it. Thank you.

Dan Bentler

 

[Highland Controls]

I think that the basic design is off, though I can't say that is burning up the motors. From what I understand, you are trying to control the position of 4 mechanically connected(through the wheels and rails) systems. I would stick with one encoder to one positioning drive, then torque share the other drives. It seems that the 4 systems would be fighting each other much of the time. Also be sure you are running in V/Hz mode, and not vector mode, and be sure to have an overload for each motor that is monitored.

 

[iant]

How much testing did you do before going away for while
12-15 years ago the idea of vector drives and using an encoder to pace the remaining drives was not uncommon
If this machine only went 10 feet then how far had you driven it before
It sounds like there has been a mechanical change that has caused the four motors to fight each other. If it was an encoder failure than how did it fail
These are extra low voltage devices. They must ave seen a fault.
How is your screening connected to the encoder. Not grounded at both ends I hope

 

[hoy7t]

1. How are the four motors connected to the same load? Is this a 'rigid' connection"?
2. Are the VFDs in CLV mode?

IMHO it is not possible to operate multiple independent position controllers on the same (rigidly connected) load. This can only work if all VFDS and motors stay within +/1 encoeder tick of each other. Not gonna happen. Very quickly, the VFDs will no longer be anywhere close to synchronized, and will begin to fight each other to control position. They will pull all the amps they can until something breaks or trips.

You could use VFDs in CLV connected to the same load as long as the load is adequately 'elastic'. meaning: each VFD/motor has some leeway to measure and correct error without fighting another drive. still not preferred.

You could creative and use some form of clutch to provide elasticity. expensive.

I use VFDs and induction motors to do semi-precise positioning. There is one master - either one of the drives, or motion control logic separate from the VFDs. The other drives can operate in V/F or torque follower. Some VFDs can be the master to control other drives as slaves (in V/F or torque follower).

With that very large load, you may need CLV performance within each drive to get maximum torque at take-off speed (near zero). If so, the architecture above will work best.

 

[rdrast]

Since all three of the failed motors are ones that were removed, sent out, modified, and reinstalled, my first thought is that somebody reconnected them wrong, or the OEM mislabeled the leads.

It really sounds like they were connected for low voltage in a high voltage application.

@hoy7t:
There is no problem connecting multiple motors on a single drive to a rigid load if done properly and the motors are similar. I've done it on conveyers, surface winders, and machines with long line shafts. Never had an issue.