VSD output damage

[THD]

We have the output damaged on a VSD driving a 1200kW cyclone pump on a mine. After three weeks of operation the drive did not restart after it was switched off. The power unit on the white phase had burn marks on and the fuses driving the power unit from the dc bus were open. Can this of been caused by back emf from the motor when stopped? The motor turns backward when stopped from the water header pressure that comes back down after the motor stopped. PS: There is no contactor between the output of the drive and the motor - only at the input.
Please help!!

 

[Skogsgurra]

Very probable. The SC current of a 1200 kW motor is substantial and the inverter usually has free-wheeling diodes that will conduct when a voltage is impressed from the motor side. Cannot the control be arranged so that the motor slows down under power until all water has left the tube? There is a check valve, I hope.

I am assuming an induction motor and a standard PWM VFD. If you have a synchronous motor and some other VFD, there are other options to get the VFD damaged. A WRIM motor with a cascade control adds even more possibilities. Which type of motor and drive do you have?

Gunnar Englund

http://www.gke.org

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

 

[THD]

Skogsgurra

There is no check valve :/
Its a vacon drive (PWM) with teco motor (standard induction motor - 4pole)
The drive stop has been set to coasting stop - not ramped.

Kevin Bosch
Rainbow Technologies

 

[itsmoked]

Yes the motor was generating and once the DC capacitors were fully charged there was nowhere to to go but up-voltage.

So the pump is going to force the motor to turn continuously after the drive stops or just long enough to drain the pipe back down to zero head?

The DC bus fuses blew too.<sigh>

Can you slow the motor to the point that the flow thru it is not supported but the pump is still being driven. At some point the flow thru the pump would reverse and drain the pipe while still controlling the motor. Once there's no column left you should be able to zero the drive and shut it off.

Alternatively use a valve and bring the flow way down then manually turn off the valve as fast as you can and then zero the VFD.

You should really be using a check valve that you slow the pump down until the flow just stops and the check seats. Then you zero out.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[controlsdude]

I am curious why a contactor was not put between the vfd and the motor.

Contactor on output of VFD would have this functionality:
1 On Estop would open up after ramp down of VFD.
2 During normal stop the contactor would open up after a predetermined amount of time after ramp down.
3 When the start button was pushed the contactor would be pulled in, then the enable to the drive would come on.

This assumes that the VFD is in ramp mode on stop. If doing this make sure that the ramp is long enough that there is no overvoltage on the VFD.

 

[itsmoked]

The manual probably screams, "Do not interrupt the output!", is probably one reason none was included.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

At 1.2MW it will either be an MV contactor or an LV breaker. Both are expensive items of hardware if they are not needed, and as Keith says most manufacturers recommend a continuous connection between the drive and motor.

No ramp-to-stop either, the OP said it coasts down.


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If we learn from our mistakes I'm getting a great education!

 

[THD]

Thanks for the input gents. Pity my output is fried.
Would a ramp down help the situation? We are getting new parts airfreighted and we would hate to fry them (and our jobs!!)

My understanding was that it could be the rapidly collapsing field on a coasted stop, or the "generating" action when the motor spins backwards. We cannot put a check valve on a slurry line.

Sigh.....

Kevin Bosch
Rainbow Technologies

 

[Skogsgurra]

You have to do something about the generating situation. Either use a fully regenerative (four quadrant) drive or add a brake resistor that takes care of the energy. Or ramp down slowly so that the liquid leaks back without turning the motor backwards until pipe is empty. Then switch off.

I know a Vacon chap in Europe (if that is where you are, time zone indicates EU or Africa) that can help you out with the details. He is with Vacon Stockholm. I also have a very good contact in Vaasa, Finland (Vacon HQ).

But do not just fix it and blow it again. That would definitely be bad.

Gunnar Englund

http://www.gke.org

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

 

[controlsdude]

http://www.postglover.com/literatureresources.html

I would recommend what gunnar is saying braking resistors. It would dump all that excess energy during gen on the motor. Above is a manufacturer of braking resistors, excellent literature to know what you need, in the USA, but not available in europe. Might get you started in right direction. I would think ABB, Siemens, or some other manufacturer of VFD that you have can recommend a local source.

That way you get the right sizing on the braking resistor with the right resistance for the load that you want.

 

[ScottyUK]

Cressall Resistors have pretty much cornered the high power resistor market over here in the UK.

Slight aside: I picked up some info from a US resistor manufacturer called Mosebach a little while ago - any comments on their products anybody?


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If we learn from our mistakes I'm getting a great education!

 

[ozmosis]

Controlsdude
" It would dump all that excess energy during gen on the motor"
Not sure if it is just your terminology but you don't dump it to the motor if brake resistors are connected to the brake chopper, the energy is 'dumped' into the resistors and dissipated in heat. If it is an active front end then the energy is returned back into the AC supply.

 

[controlsdude]

http://www.mosebachresistors.com/affiliations.html

Looks like both sources given are in bed with each, claiming they are sister companies.

Cressall Power Resistors is out of the UK and is on this site, might get support out of them for braking resisters.

 

[jraef]

The problem with slurry applications is that once you turn off the drive, the slurry keeps going and pulls the pump with it, creating that potential regen situation. The same thing happens on oil well screw pumps. Turning off the output of the VFD should be OK even though, because without excitation the motor field will collapse in a very short time, maybe a second or 2 at the most. The caveat to that is if there are capacitors on the load side. For example if, for some reason, someone has connected surge capacitors to that motor. It used to be a very common practice on large MV motors, but you must NEVER have capacitors on the down stream side of a VFD unless they are part of a very specifically engineered filter system. Then of course there is the possibility that there ARE filter caps on the output side, which can unintentionally allow regen in your application. I once heard of an installation where capacitance on long output cables created enough energy to damage components as well, so that's something else to investigate if none of the other issues apply.

But leaving the VFD output on to decel (ramp down) and then providing someplace for the regen energy to go should solve the problem regardless.


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