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Is it possible to build short-circuit proof softstarter ?


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#1 frs

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Posted 09 July 2006 - 08:51 AM

I want to configure short-circuit proof softstarter (protected by motor circuit breaker) from standard components offered by manufacturer.
I think this way:

1. Oversize the softstarter by 2 - 2.5 times, because manufacturers ratings are overly optimistic.

2. Place a 1.5% reactor upstream of softstarter. This reactor will limit current in case of short-circuit.

Is this idea good or wrong? Any comments are welcome.

#2 mariomaggi

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Posted 09 July 2006 - 09:02 AM

Dear frs,
plese give more data.
Are you thinking to a 1 kW or 1 MW soft starter? Voltage? Solutions are different!

Regards
Mario

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#3 frs

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Posted 09 July 2006 - 09:58 AM

Voltage is 400V. Motor power can be in 1 to 400kW range.

To be more specific, lets start with 11kW motor, driving centrifugal pump smile.gif

#4 jraef

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Posted 09 July 2006 - 07:19 PM

I'm not sure what you mean by "short circuit proof". It's a concept that is an impossibility for anything, not just soft starters.

If what you mean is that in the case of a short on the load side of the soft starter that might damage the SCRs, then there really is only one valid protection scheme, high speed fuses designed to protect semiconductors. Even they will not protect you 100% of the time, there are a lot of subtle issues that can make the fault cause damage even before they clear as well. The reasons why manufacturers don't automatically include semiconductor fuses in soft starters are:
1) Fuses and fuse holders are expensive (although always cheaper than SCR replacement) and take up real estate. If a competitor doesn't include them, you may not get the sale based on price and/or size. Even though the user SHOULD consider that issue, many (if not most) will not, they simply look at the price on the quotation and/or the whether or not it will fit in the box they want to put it in. I know of one major player in the SS business who didn't include an overload relay in their standard package for the same reason. Users often ASSumed that it was in the electronics and lost their motors as a result!

2) The mere mentioning of this issue can have a cooling effect on the buyer's decision to switch from an electro-mechanical reduced voltage starter to a soft starter, and that challenge still exists in the marketplace. So what they (the SS mfrs) do is to mention it only in the installation manual, because by then the user already owns it so they got past that hurdle in the sales cycle.


Increasing the SCR size is not really going to help with a shorted load because the damage to the SCRs takes place as a result of dI/dt, a rapid rate of change in current. The dI/dt ratings of SCRs don't vary enough from size to size to make much of a difference. In fact, depending on the brand it may even be lower on a larger size. If you look at the chart on the linked page below, you will see that the di/dt rating of a 900A SCR is 1/2 that of a 300A SCR. Oversizing does help with applications that cause the SCRs to overheat and short out from junction thermal breakdown however, but that's different from short circuit survival.

A reactor might help IF the soft starter has an electronic trip feature that is fast enough to clamp the gating circuit in time to prevent damage, but that is rare. Adding a reactor to a soft starter introduces power losses in the system that will likely end up costing you more in the long run than an entire crate of semiconductor fuses.

Amps Abundant SCR data link
"He's not dead, he's just pinin' for the fjords!"

#5 marke

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Posted 09 July 2006 - 07:33 PM

Hello frs

I do not believe that it is possible to design a soft starter that will be short circuit proof under all load conditions.

The problem is that once an SCR is triggered ON, it will remain ON until the current has passed through zero. That means that the current could continue to flow fo one half cycle which is 10mS at 50 Hz and 8mS for 60Hz.
If the current during this period is less than the half cycle maximum current of the device, then there should not be an instant failure, however, if the current is too high, the SCR will fail.
So the two major factors are the fult current available from the supply and the short term rating of the SCR.

If the SCR survives the current, and is not retriggered, then we have a situation of a high current interuption. This will result in a voltage transient due to the collapsing of the magnetic field in the supply components. The stored energy in the supply components could result in a voltage surge high enough to damage the SCRs.

The best means of protecting the SCRs against a short circuit, is the use of semiconductor fuses.

You can minimise the failure rate by ensuring that the surge rating of the SCRs is higher than the fault current rating of the supply plus over voltage diverters to catch the surges.

Best regards,

#6 frs

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Posted 28 July 2006 - 09:38 PM

Thanks to all !

Topic subject is not a real actual task for me, just a mental exercise. Targeted not to price cutting, but to make system not dependent on human factor. Sometimes those maintenance bad guys do replace blown semiconductor fuse with "general purpose" one, even without motor/cabling check...


And question about manufacturer ratings. For example, within "frame size 0" they have three devices: 5.5; 7.5; and 11kW. For all of them the same semiconductor protection fuse is recommended. Permissible starts per hour count is 30; 24; and 12 respectively.
Similar situation within "frame size 1" and so on.

I think that all three devices of "frame size 0" contain the very same SCR's (consequently, with same I^2t). Is that possible? Or, at least, can I suspect this type of things to happen? smile.gif

#7 marke

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Posted 28 July 2006 - 09:52 PM

Hello frs

Yes, it is possible to use the same SCRs in a number of different soft starter sizes.
The ratings on the soft starter are a function of the junction temperature of the SCR at the end of the start. This temperature is a function of the SCR and the heatsink and the starting characteristics.
If you use a large SCR on a small heatsink, you will get a low rating.

Some maunfacturers even use the exact same unit with a different label on it just to get an economy of scale. Do not however jump to conclusions and use a unit above its ratings with out the belssing of the supplier as there may be many subtle differences that you are not aware of.

Best regards,




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