Another (different) drive issue-

katratzi · May 11, 2006

Here's one thats new to me: customer sent out a 460 volt 3 phase
motor because of a bad bearing. Motor shop replaced both bearings, tested motor, and shipped it back. It was re-installed and connected back exactly as before to the same drive, a Magnetek GPD 505. However, now when the motor is connected to the fan by the belts (which are also brand new) the entire blower enclosure just vibrates like crazy. When belts are removed, motor and blower turn very easily by hand. I tried running the motor while it was disconnected from the blower (removed belts) and here is what happens:

- with a 10 second accel time (original parameter setting) the motor shaft jerks clockwise and counterclockwise rapidly at low hertz before getting up to set speed. (I have never seen this before)

-I tried a 1.0 second accel time, and the motor shaft cw and ccw movement was barely perceptible (less time spent at low hertz?)

- I then observed the motor as the drive ramped up slowly to 45 hertz. there was significant shaft chatter at 5,6, and 20 hz. Other than those frequencies, it seemed to run fine.

Now when the belts were put back on, at any accel time the vibration and belt shredding that goes on is so awful you have to stop the drive pretty quickly, but the output has been left on for enough time to notice that the motor shaft never rotates, it just oscillates back and forth. Like I stated before, the motor shaft turns freely, as does the blower. A light push on the blower shaft is all it takes to get it rotating. The motor FLA is 12, but when it is trying to turn the blower and starts vibrating, the drive reports the output amps are 20.

I would suspect the blower, but the motor does that weird oscillation dance even when the belts are off. Sorry about the length. but I wanted to provide all the details I could. Any ideas as to what the heck is causing this?

JesperMP · May 11, 2006

Hi katratzi.

"A light push on the blower shaft is all it takes to get it rotating."
Does that mean that the motor doesnt start rotating by itself ?

A quick stab at it would be to check for an eventual lost phase.
Use a clamp ammeter to see if all phases draws current.

katratzi · May 11, 2006

Hi Jesper. No, by "light push" I meant that the blower itself did not seem to be in any sort of a bind. I will check for current draw
on all three phases. Current loss/imbalance seems a likely source of this problem. I do not know if this drive can detect phase imbalance, and indicate and/or respond to this condition or not.
Thanks for the reply.

rdrast · May 11, 2006

Dangerous, but see if you can give it a 'light push' in the direction opposite normal rotation. If it starts to spin up, then your motor is missing a phase.

JesperMP · May 11, 2006

Hi katratzi.

Ok, the other thing I can think of is that maybe the drive is wrongly configured.
Either it is not set to the correct FLA, or it does not allow enough overcurrent during the startup phase.
Or it may be some kind of dynamic parameter that causes it to react to the load in an instable manner.
I would recheck all the parameters (you have them written down right ?).
That, or I would start all over with setting up the drive.

20A when FLA is 12A is not alarming.
It sounds like the max current is set to approx 150% which is quite normal.

katratzi · May 11, 2006

Hi rdrast. I asked the maintenace mechanic if I could try to

hold back the motor shaft as the drive was programmed for a long accel, but for plant safety reasons I was told no way. When a 3 phase motor is missing a phase and is allowed to run anyway, this will damage the motor, but I am not sure why and exactly what on the motor is damaged.

cntrlfrk · May 11, 2006

I'd have to lean toward JesperMP's response. I would go through the motor setting in the drive, possibly restore to defaults and reconfigure, an incorrect kW setting can cause odd behavior as well. Either that or try a different drive if you have one available and see what kind of response you get.

Thomas Sullens · May 11, 2006

If it is where you can bypass the drive and start your blower with the disconnect. If it runs like this you know your motor is not the problem.

bill4261 · May 11, 2006

Did you try auto tuning the drive?

katratzi · May 11, 2006

I checked the drive parameters and they are correct for the motor.
I will try connecting the motor directly to supply voltage to see if the problem goes away. This drive is an older Magnetek GPD505 and does not have an autotune feature, but after obtaining a manual I see it does have an output phase loss detection parameter setting. When I get back to the plant, I will check to see if it is enabled.

jraef · May 11, 2006

Assuming you don't have a lost phase or a bad transistor in the drive (both of which the drive would tell you about), there are 2 other possibilities that I can think of right now.

1) You may just have a bad connection at the motor terminal box, high resistance that only shows up when current is flowing. You can usually pick that up with a thermal scanner on the open terminal box, or look for damaged insulation etc.
Start with the simplest things first!

2) You may have a broken rotor bar in the motor. That creates discontinuous torque in each revolution, which gets amplified at certain speeds and load conditions. This may not show up at all on a simple winding test, the motor really needs to be put on a dynomometer test stand and examined for vibration under stress to find it. It's a real bugaboo, but I have seen in all to often in the last few years, probably because low cost motors and mass production techniques are reducing QC at the factories. VFDs, because they can operate the motor longer at these critical frequencies and current limit, can make problems such as this more evident than if it were run across-the-line.

russrmartin · May 11, 2006

I am extremely curious about this one. Everyone has mentioned great ideas. First, I'd do as you said, connect the motor directly to a line voltage and see how it acts. This will let you know whether the problem is in the drive or the motor. Once that is determined, you can further find the problem. I am really curious about the broken rotor bar. I have never seen this before. Is it really all that common?

Thomas Sullens · May 11, 2006

Russ i`ve only seen bad rotor bars twice in 30 some odd years. One had aluminum rotor bars it was on a hoist. The other one was on an A frame motor and had the old copper bars and where bars where soldered on the ends to short circuit them had broken. It was much more common when the bars where copper i guess the copper bars where not as tight in the rotor slots and had a lot more vibration?? Saw several with copper bars that had been repaired very old motors don`t believe it`s really a problem anymore, but could be wrong??

504bloke · May 11, 2006

russrmartin said:
I am really curious about the broken rotor bar. I have never seen this before. Is it really all that common?

Its not that common over here in the UK. Our company also does rewinds and of the 20 years i have been with them we have only seen two motors with faulty rotor bars, and that was in the last 5 years.

katratzi said:
It is not that common over here in the uk. Part of our firm does rewinds and of the 20 years I have been there i have seen only two motor faults with broken rotor bars. One tested as a faulty rotor on our scope tester but looked immaculate, the other had one bar melted (spash marks like aluminium welding) this also tested bad but could be visibly seen.

Here's one thats new to me: customer sent out a 460 volt 3 phase
motor because of a bad bearing. Motor shop replaced both bearings, tested motor, and shipped it back. It was re-installed and connected back exactly as before to the same drive, a Magnetek GPD 505. However, now when the motor is connected to the fan by the belts (which are also brand new) the entire blower enclosure just vibrates like crazy. When belts are removed, motor and blower turn very easily by hand. I tried running the motor while it was disconnected from the blower (removed belts) and here is what happens:

- with a 10 second accel time (original parameter setting) the motor shaft jerks clockwise and counterclockwise rapidly at low hertz before getting up to set speed. (I have never seen this before)

-I tried a 1.0 second accel time, and the motor shaft cw and ccw movement was barely perceptible (less time spent at low hertz?)

- I then observed the motor as the drive ramped up slowly to 45 hertz. there was significant shaft chatter at 5,6, and 20 hz. Other than those frequencies, it seemed to run fine.

Now when the belts were put back on, at any accel time the vibration and belt shredding that goes on is so awful you have to stop the drive pretty quickly, but the output has been left on for enough time to notice that the motor shaft never rotates, it just oscillates back and forth. Like I stated before, the motor shaft turns freely, as does the blower. A light push on the blower shaft is all it takes to get it rotating. The motor FLA is 12, but when it is trying to turn the blower and starts vibrating, the drive reports the output amps are 20.

I would suspect the blower, but the motor does that weird oscillation dance even when the belts are off. Sorry about the length. but I wanted to provide all the details I could. Any ideas as to what the heck is causing this?

katratzi - a question for you ?

Was the motor and drive working ok (apart from the bearings) before it went away for overhaul ?

One thing could be that when the bearings were changed and the rotor went back in the windings were caught and you now have a shorted turn in one or more stator windings ? ( I know that you said the firm checked the motor before sending it back, but i have seen this before!)

DickDV · May 11, 2006

For me, the most likely problem here is that the motor leads were not connected correctly. Twenty amps on a twelve amp motor that cannot accelerate is typical of that kind of wiring error. And, keep in mind that the motor leads might be mislabeled.

The second most likely source of this behavior will only occur if the drive is operating in vector mode, either sensorless or flux vector. There was no mention of an encoder on the motor so it would have to be sensorless vector. On a fan, there is very little benefit on most brands of drives to run in vector mode. Volts/Hz or scalar operation has no tuning requirements and, as long as the motor nameplate data has been entered correctly, the shaft should turn and accelerate smoothly. On the other hand, if the drive is in vector mode, you may have to do a new ID run on the motor or at least check the P and I gains in the speed regulator with possible adjustment there.

It still sounds like a miswired motor to me.

Another (different) drive issue-

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