Powering Soft-Start with a VFD

[luktek]

I am wondering if anyone knows of any issues that could arise by powering a soft-start with a VFD.

Basically, we need to use the VFD as a phase converter to supply a 3 phase actuator that has SCRs.

Has anyone run into anything like this before?

 

[djs]

The output of most VFDs is PWM: Pulse Width Modulation. PWM and SCRs will not get along. You might try using a rotary converter to go from single phase to three pahse.

 

[ScottyUK]

Bad plan. Can you bypass the SCRs and wire direct?


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

 

[luktek]

I think we are going with djs suggestion in this case. The SCRs are internal to the actuator and we would prefer not to bypass them.

 

[jraef]

It's done all the time. Most portable demo units that soft starter manufacturers send around with their salesmen are doing just that very thing.

The three tricky parts however, are;
1)You need a good sine wave filter on the output side of the VFD, because the Soft Starters have a line sensing circuit that determines the zero-cross points of the 3 phases in order to set up the gate firing sequence. Non-sinusoidal outputs can confuse them.

2) You need to have a contactor on the output of the VFD that only closes when the VFD is at full frequency, otherwise the Soft starter is getting low frequency to the gate firing control board and that can mess you up as well (but it depends on the soft starter).

3) You need to power up the soft starter control circuitry from the LINE SIDE of the AC supply, before the VFD. But on some soft starters, this may interfere with proper operation because they may have a system that detects line loss if there is control power but no line power. If that's the case, then the control circuit power needs to be interlocked through an aux of the same contactor mentioned above.


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[itsmoked]

jraef; Can you turn ON the VFD and once it's ON close a switch between the VFD and the SS? Another words not have to somehow monitor the VFD output?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Yes, in fact it's usually automatic by using the output of the VFD programmed for "At Frequency" to close the contactor only after it is at 60Hz (or 50 as the case may be).


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[itsmoked]

Ah yeah, A cool "freebie".

Keith Cress
kcress -

http://www.flaminsystems.com

 

[controlsdude]

I think you would cause the vfd to trip out and might just cause it to fail if you have a contactor close during the vfd running. Best practice is to close the contactor before the VFD is in run mode.

 

[itsmoked]

Hi dude. This would be a scalar drive that just puts out a frequency and has no knowledge nor cares what the motor might be doing. AND it's just feeding the SS to start with.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Right. Normally you would NOT want to do that, but in this particular case, the soft starter will not conduct until AFTER the VFD output reaches it, and then it will be reduced current. So in this specific instance, it's OK to close that downstream contactor.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376