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Old 03-09-2013, 12:47 PM   #1
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Default PF 700 failure

Have a customer that has a piece of equipment that uses a 100hp PF700 to run two 40hp motors in parallel.
In the past 6 months, the drive has failed 3 times.
We have had Rockwell specialists in to check system and installation and they have not been able to make any suggestions or recommendations.
When they test the failed drives, they say the IGBTs have become saturated but can't say why.
Any insight would be welcome
Thanks
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Old 03-09-2013, 12:50 PM   #2
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What do the motors do?
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Old 03-09-2013, 12:54 PM   #3
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Each motor drives a roller (one top and one bottom) that pushes rubber into an extruder.
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Old 03-09-2013, 02:02 PM   #4
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Quote:
Originally Posted by Tsmil View Post
Each motor drives a roller (one top and one bottom) that pushes rubber into an extruder.
Banbury mixer?

A slight difference in the speed of the two motors can cause one to regenerate into the other. Generally that doesn't happen when the two motors are mechanically linked with gears or good belts. But on a Banbury mixer, the motors are NOT mechanically linked, they turn independently, it's the nature of the machine. Think of it as a giant "Indian Burn" machine, like the thing your older brother gave you on your arm when he twisted the skin one way with one hand and the other way with the other. Each roller is geared to turn at slightly different speeds, so as the rubber block goes between them, it stretches and shreds (explodes really) as it is fed unto the extruder. In the process the roller motors can end up being driven at slightly different speeds that are changing very rapidly because of the elastic nature of the rubber. That isn't really a problem if running across the line, but with a VFD, can cause very short duration spikes in the outputs that could be taking out the transistors over time, they don't like spikes and the damage it causes is very slight, but incremental. It builds up until the transistor fails.

Is there a load reactor between the drive and where the two motors are split off? I would most definitely have a reactor there, it will slow down the rise time of those spikes and likely absorb them or allow the VFD to catch them and put them into the DC bus, which if they then buid up will cause the drive to shut down, which would let you know what is happening.
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Old 03-09-2013, 02:11 PM   #5
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I am not sure what saturation of the IGBTs means but I know disconnecting the motors from the an energized drive can take out the IGBTs.

Is there a disconnect or contactor between the powerflex and the motors.

When the 2 motors are parrallelled then it seems likely there would be a protection device for each motor. When the drive fails is it after an event where one or two overload devices have tripped?

Just putting in my two cents worth.
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Old 03-09-2013, 03:08 PM   #6
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The machine is actually a rubber strainer. The rollers are somewhat like the rollers on an old wringer washer washing machine. The rollers are not mechanically linked. They take the rubber and push it into the rotors ( which are driven by a 250hp motor and another pf700 ) which push the rubber through a screen to remove lumps.
There is no reactor between the vfd and the motors. There are overloads and contractors for each of the motors but none of these have tripped. The contractors are controlled by a a safety circuit. If either of these contractors is open, the enable signal to the vfd is removed.

You have me thinking that if the safety circuit is tripped while the drive is running, that the contractors are opening while the vfd is still powering the motors which might be causing a problem. It is something that I will discuss with the plant engineer. I am sure that this scenario must have been looked at when the machine was designed but still worth the look.

The other good point was the difference in speeds due to the elastic nature of rubber makes a lot of sense to me and will also discuss this with the engineer.

Thanks guys, you have given me some validation to the thoughts I have already had.

Last edited by Tsmil; 03-09-2013 at 03:12 PM.
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