VSD's and Chokes

[pirates]

I have a VSD that has suffered many problems basically it cannot handle the sometimes large upset currents from the load (rapid changes in load conditions) Recently I had a very knowledgeable gentlemen visit me. He says that perhaps we can reduce the effect of some of the problems through the use of Chokes. I thought we already had chokes stupid me but these it seems are input current filters (Why I do not know). This has left me with a few questions. I did a bit of reading and find that chokes should be used on long cable runs. The cable runs on this drive are short as the VSD is mounted next to the motor. Will chokes have any effect on the problem with the drive? How do I size the chokes (I have asked him to do it but after 4 weeks cannot wait any longer). Where can I get chokes?

Any help greatly appreciated.

 

[Marke]

Hello pirates

Output chokes are used to reduce the charging current into the capacitance of the cables and motor. As such they reduce the rate of rise of current at the output of the drive.
If you have load transients, then this is a different problem.
Many drives offer a current and/or torque limiting function and this is what you use to reduce the effects of transient torques on the output. If the motor is subjected to a transient torque, the torque limiter will cause the drive to slow down to reduce the overload effect.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Yes, as Mark said.

You can find out if there is any "spare torque" in the drive by checking the setting of torque and current limits. Most drives can deliver 50 extra percent torque for short periods (tens of seconds, usually) and if the limits are set to 100 %, it may be an easy solution just to increase the limits. But, do not change the thermal protection. It is there to protect the motor in overload situations. It usually has a time constant between minutes and an hour, depending on the motor size, and does not stop the motor from delivering peak torque if it doesn't happen too often.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[DickDV]

pirates, you first need to ask yourself "what do I want to happen when this torque surge occurs? Do I want the speed to hold constant thru the surge or is a corresponding sag in speed acceptable?"

If you want the speed to hold constant, you will need to determine if the motor has enough overload torque capacity to at least match the requirement. Then, you need to determine how much current the motor requires to deliver this peak torque and check the sizing of your drive to see if it can supply this peak torque. If the drive or the motor cannot supply the requirement, you need larger components.

On the other hand, if a reduction in speed is acceptable when this peak loading occurs, the motor sizing is still the same but the drive limits can be set to "yield" to the peak load reducing the speed and hopefully the current requirement of the peak until the overload condition passes. This only works for short term overloads, say 1 to 10 seconds. If much longer than that, you will find the drive slowing down all the way to stall. That likely would be unacceptable.

Hope that clarifies things a bit.