Soft Starter design 1

[brock1155]

I have a dilemma. Basically, Im implementing a soft starter to replace a old reduced voltage starter circuit. I found several potential good soft starters. However the problem was I overlooked the fact that the power system that the soft starters will be connected to is corner grounded delta system. This is an old power system arrangement, that is rare today,and mainly in the US. Many soft starters are not compatible with this type of system. I came across some soft starter that "claimed" they were compatible with delta grounded. However, whenever I asked the vendor's tech support person, why they are compatible.Most did not know why, and some didn't even know that their soft starter was listed as compatible. The tech support individuals could never fully explain what makes a soft starter compatible compared to one that is not. I would like to know, from anyone with experience, what make these soft starters compatible to a delta grounded power system? What specific properties on the soft starter makes it compatible? Is there a unique way that I must connect the soft starter? Thanks for any help

 

[itsmoked]

Typically a SS is just a bunch of SCRs that are turned ON in a controlled manner to result in a reduced voltage profile when coupled with a motor's inductance and resistance. The SCRs can easily be arranged so as to not be grounded in any way. Acting essentially like any regular contactor which doesn't care if the any one phase is grounded or not. The SCRs are simply replacing the mechanical contacts in the conventional contactor albeit electronic contacts that can happily operate every power line cycle.

That's how they can support a corner grounded delta system.

However. There are, of course, a few details that if ignored or cheaped-out on then one or more of the SCRs can find themselves referenced to... something, ground or the neutral. Usually this all relates to the SCR triggering part of the circuitry. The non-isolated method saves a few dollars on trigger transformers and the like. I suspect that's the difference between SSers that can and cannot tolerate corner grounding. Also, when you corner ground the other two phases are elevated above ground farther than in the non-corner grounded situation which can stress a design's mechanical voltage isolation more, possibly causing some SS suppliers to avoid supporting corner grounding situations.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

One more point. Some SSers (jraef's favorites!) only actually switch two legs of the 3 phase. (seriously cheaped-out) I would imagine they might take issue with corner grounding as there is likely some serious impedance differences involved in only switching two phases and on top of that maybe having one tied to earth.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

It's typically the electronics that can make it compatible. We measure and phase angle reference phase to phase and design the board to also work on a floating or resistance grounded system which can be corner grounded by a ground fault. If the circuit does any phase to ground measuring or phase angle referencing then it may not work on a corner grounded system.

 

[brock1155]

The problem is I keep hearing different ideas as to why a soft starter is compatible OR not compatible. One tech support specialist says because their(soft starter) rated insulation voltage-to-gnd is 600VAC. Another person says if a soft starter use MOVS on the front end to protect SCR's ARE NOT compatible with delta grounded systems. I don't even know what MOV is. Then on here it deals with if the board has to work with a floating or resistance grounded system. I wish I had some documentation that stated the what was right. Or all these things related?

 

[LionelHutz]

They are possible reasons. BUT,

Any 600V equipment built should be capable of withstanding 600V to ground.

A MOV is a voltage suppressor. I haven't seen any manufacturer using MOV's to ground in a starter for a while, unless you're talking about the cute little small HP din rail mounted starters.

Every make and model is built differently so you're not going to get any specific answers unless you start being specific. Also, if you don't know what a MOV is then you're really not going to be able to figure out if a starter really is or isn't compatible despite what the manufacturers tech tells you. Keith and I both basically posted the same thing, it's usually the controls that determine compatibility.

 

[itsmoked]

As LionelHutz and I have both stated it's in small details deemed important to some manufactures and not to others. It's usually the controls or as pointed out spike suppression circuitry (MOVS) that can prevent use in the application you seek.

Whatever the reason, YOU knowing the precise reason will in no way allow YOU to assess the fitness of any particular SS to work in your specific application. You do not possess the schematics or detailed knowledge to understand the minutia presented in them to assess the suitability nor will your assessment matter from a UL listing point of view. I posit that while you learning the typical reasons for non-suitability/suitability is nice it is not going to solve your problem.

If the unit you accidentally purchased does not allow corner grounding then return it or sell it on ebay and move on with purchasing one that states in writing somewhere that it will handle corner grounding and be done with it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Marke]

The soft starters comprise reverse parallel connected SCRs in series with one or more phases of the supply. (Don't even consider a single phase control if you respect your motor).
The SCRs can be controlled tow ways, either by phase delayed firing relative to the phase to neutral voltage waveform, or by phase delayed firing relative to the phase current commutation.
If there is an internal reference to neutral where the neutral reference point is connected directly or indirectly to ground, you will have problems with corner grounded. If the neutral reference is simulated by a floating star point from the three incoming phase voltage waveforms, then the SCR firing will not be an issue.

The only other problem that I would be concerned about, is circuitry from phase to ground for EMC and spike protection which will be effectively connected phase to phase by a corner grounded system. If the voltage rating of these components is limited to phase to neutral voltage, then you may have an issue.
If you use a 480 volt rated starter on a 480 volt corner grounded system, you could stress any EMC capacitors, but if it is a 240volt corner grounded system the EMC capacitors will be fine.
Any SS designed to be used on an IT supply should either have phase to phase rated EMC components, or more commonly, have them ungrounded.

Mark Empson
Advanced Motor Control Ltd

 

[HCBFlash]

You may well wish to consider installing a transformer to isolate this motor and starter, if not more of your equipment as well.

.


Me wrong? I'm just fine-tuning my sarcasm!