AB VFD Switching polarity

I would recommend at this point the restart the system, once running measure the curent through each lead. The current through all six leads should be within 5% -10% of each other. If one lead has a significant difference Ie. higher amp draw it can indicate a failing winding. I would also totalize the current reading and compare them to the current ratings of your VFD.

I'm also assuming that your motors are 3 phase being fed from a 3 phase source. If one your feed lines into the VFD is loose, your VFD maybe simulating the third phase (Robicons will do this) and your VFD may be failling.

But first check your current balance then your current load and compare it to the VFD ratings and settings. You may find that the current overload setting was set to low, which would be an easy fix.

I am having a hard time understanding how this is possible. Tell me... why do you think the outer drive motor switches its direction of rotation?

We (well I am) are going on the assumption that he is talking about properly wired 3 phase motors, in which case your confusion is understandable because we know that a AC motor changes polarity a few time ever second. We also know that unless someone rewires the motor swapping two phases it won't change direction on it's own. North, I'm not jumping your case just clarifying assumptions.

I think PTI mis-spoke when trying to describe what he was seeing.

PTI I think the the general agreement is one of your motors is nearing end of life and will fail completely soon (soon may be next year or tonight). You may want to take advantage at this time to re-evaluate your drive system. Why are there two motors? Can one larger motor do the job? If two are needed look at adding a second VFD, and depending on your current control scheme keeping the two motors running at the same speed should be easy to do.

...same speed should be easy to do...

With a feedback rheostat governor that is

The PowerFlex 700 is a sweet little VFD and has quite a bit of flexibilty when it comes to being configured so yes a rheostat could work.

I would guess you are experiencing an intermittent failure on that motor and that it won't be long before it fails completely.

When using multiple motors from a VFD or Soft Start, its good design practice that each motor get individual overcurrent protection. Generally I never run multiple motors off a VFD because: VFDs are relatively cheap, our customers generally don't allow it, and a VFD output is not sinusoidal so general purpose overload relays don't work. Allen Bradley does state that you can use bimetallic overloads (bulletin 193-T or 193-M for class 10) or bulletin 592 eutectic relays (class 10,20,or 30) for individual protection of multiple motors on their VFDS if the frequency stays above 25%. Your overloads are checking out fine, so this was just some reference on the overcurrent protection differences of VFDs and contactors.

Electrically, it's a possibility that if both motors are tied together on one overload, you could be back-feeding one of the windings on one of the motors if the resistance through that winding in less than specs for that particular motor.......just speculating here......I've never seen this happen before.

If this event has always occurred since "day one" of the installation, then you will probably need two independent drives (VFDs), one for each motor, both drives "synced" together. If this event just began, I would guess that resistance has changed in one of the windings of one of the motors and a "back feed" condition is occurring. The conditions for this to take place would have to be greater than perfect. When replacing either of these motors, use two motors of the same manufacturer in the event that Brand X motor and Brand Y motor might be marginally different in resistance through the windings.

Mechanically, it's possible that a key or key way is close to being totally sheared away somewhere along the drive train. Check the drive motor output shaft where it connects to the input of the gear reducer. Check any sheaves, sprockets, couplers where there is a keyway. It's always possible that on "start up", the key will hold for a short period of time but once the conveyor takes on a load, then the sprocket, sheave, coupler, etc., will start to slip. If the conveyor is loaded at that point and without a brake on the end of the motor, the conveyor will spiral down with gravity until the load is off of the conveyor, appearing to be running in reverse.

Good luck with this anomaly!

Jeff

Please check parameter settings for multiple motors application.It should be from there only.

Is the change of direction of rotation happened at start or when running? If at start -electrical problem when running, mostly mechanical/could be in conveyor assembly.

One phase could have bad connection/contact/ too high phase resistance.

This case I had with a small motor getting very unsymmetrical 3-phase supply voltage.

Without knowing the details of the mechanical system of the conveyor, it is hard to diagnose this condition via this forum.

I am willing to bet that one of the two motors is being over driven by the mechanical system.

When an induction motor is driven faster then its base speed, in RPM, as determined by the formula... [ (frequency of the line voltage x 60 seconds in a minute x 2 (for the negative and positive pulses of the cycle)) / number of poles in the motor ], it turns into an induction generator and dumps energey back to the VFD bus. This is commonly refered to regenerative energy.

This condition can cause all sorts of problems on the bus of the VFD when there is another motor in parallel.

What mode is the AB drive running in, vector or V/Hz?

Please describe the mechanical system as much as possible.

Rick...

Both motors are wired to individual overloads and each are properly wired for three phases. There is no splice or local disconnect between the motors and the VFD. We checked all of the connections in the cabinet and the potheads, all are tight and corrosion free. My electricians had megged and ohmed the motors, from the control cabinet, and both checked okay when megged with 1000V and they were reading some ohms so nothing is shorted. But I assume it could be possible to be experiencing an intermittent problem with one of the windings insulation. The problem has only happened after the drive has been running. So far we have only experienced it twice, 6 months apart. After it happend the first time we changed the motor during the next outage we had. I haven't contacted AB yet, but I will on Monday. I have gone through the parameters and they are correct, nothing had changed since intial install 3 yaers ago. I guess the next step would be to change the VFD, but I am just not convinced that it is the problem. Usually when the PowerFlex's fail they usually blow a capacitor or fry a board and become completely unuseable.

It was the "mechanical system" not the eletrcial one that I wanted more details on.

What do you meany by changing polarity?

By switching polarity I mean the motor was switching direction. Without someone changing the wiring. As for the mechanical system ther is an inner drive that turns clockwise and an outer drive that turns counterclockwise. It drives the spiral conveyor which rides on rails with steel ball bearings between the rail and the conveyor. The lubrication system is working correctly, it applies lube to the rail and the flange bearings at the head & tail rolls. This is in a freezer and we are still using the manufacturer recomended lube oil. The gear boxes are in working order with plenty gear oil. Thats all that I can think of now, if I come up with more tommorow I will post it.

I'm still curious about the amp draw on each leg.

Are the two motors located on a curved or straight section of the conveyor? Can you be seeing the effect of the greater circumference of the outside rail making that motor run faster?

If not... I have no other avenues for you to look at.

Two thoughts here.

1. The outside motor is most likely going single phase, if one phase fails the motor will run in whichever direction it is initially spun. Check all the connections to the motor and monitor phase currents. It is like the direction was set while single phasing was occurring and then it comes good occasionally.
2. I've seen motors that do weird things. Years ago I came across a Chinese injection moulder on which the motor would not reverse when the phases were reversed. At first I thought I'd put the phases back in the same order, but not so. I eventually got it go right by continually swapping phases. The phase currents were not equal, but not hugely different.

I confirm your observation and finding about single-phasing (1 & 2).

3. One of my other finding was a brief (short time) unbalanced 3-phase supply voltages that create so called reverse magnetic field inside the motor. Sometime this reverse moment (driving force) becomes effectively stronger than forward (normal, working) moment because of differences in loads in both direction of rotation (subtraction!).

And it is hard to identify. I found this event when a huge single-phase heater had connected to the 3-phase system. If I recall measured voltages were something like: 240-240-160 Volts. Checking megaohms (isolation) and ohms/milliohms of phase resistances could not indicate this condition.

Another effect of unbalanced 3-phase voltages is shorted life expectancy of the motor.