Soft start problem

[panelman]

Gents

I’ve just had a thyristor in a 250A soft start go short circuit, is the conventional wisdom to just replace the failed one or replace all three?

Cheers

 

[burnt2x]

panelman,
Try to know what caused the thyristor to go busted. Most thyristors that I have tinkered with went busted due to overheating, a case of loose mounting nut. When you're done replacing, try monitoring the load amps if they are within the ratings of the thyristors.

hope this helps.

 

[jraef]

I agree, you need to determine the cause of the failure if at all possible. If it was a voltage spike, the damage would likely be to at least two of the SCRs, but you will not know which two (other than the one that has already shorted). You can test them all to see if another one has failed, but unfortunately the only test available may not detect an imminent failure of the silicon.

If the damage was due to a shorted load or come other dI/dt spike (instantaneous or very rapid current rise above rated), again at least 2 SCRs were involved so it is likely that the stress occurred on both.

If the damage was due to overall thermal buildup in the enclosure, you have likely stressed all six SCRs because it would have happened under load.

If the damage was localized thermal buildup, as mentioned above with a lose connection, you will likely be better off replacing both SCRs on that phase.

And last but not least, if the damage was due to a misfire, you will likely see it happen again as soon as you energize the starter. Use a small test load when you fix it.

 

[panelman]

Thanks for the responses, the thyristors are packaged as an anti-parallel pair per phase so changing both on a phase is the only way to go.

My thoughts were that as one has gone short then the resulting fault current would have passed through one or more of the non failed ones admittedly only for a short time until the electronics picked up the fault and shut down.

Will this have stressed them such that they should be replaced too?

The unit is 10 years old but the duty is fairly light, thyristors are rated at 250A, pump current is 170A and is started maybe five times a day and bypassed once it’s up to speed.

No obvious fault on the pump or drive board and the unit is happily running again with the failed thyristor having been temporarily replaced with a salvaged unit from the famous “will come in useful one day” box that all engineers have tucked away somewhere.

 

[burnt2x]

Good luck then!

 

[jraef]

Problem #1:
250A on a potted anti-parallel pair? (We call them "bricks" in the industry). Most conservative manufacturers stop at about 156A continuous on bricks, roughly 125HP at 480V. Above that we tend to go with "hockey puck" style SCRs in clamped heatsinks. It can be done, but it is done by "creative math" in the overload capacity of the entire unit. In other words, the soft starter you have is definitely NOT a heavy duty device by any stretch. So your starter needs to have one heck of a cooling system or your load must be able to start with very little current (and maybe only once or twice per hour). Sounds like you are skirting the very edge here.

If you want to keep using that starter, I think you are wise to keep replacement SCRs at the ready. You are correct that it is most likely that one of the other pairs was severely stressed, it's just that the one failed first.

One thing I would do if you can't afford to buy a better starter, is to implement a Starts-per-Hour limit on it. Some of the more advanced soft starters are capable of that, but being 10 years old, yours probably isn't. There are Solid State Overload Relays out there now which have that kind of capability built-in, I would swap out your Overload (assuming it is separate) for one of those. If not, you can do it with a timer. That will prevent some other relatively insignificant control problem from cascading into another SCR failure.

 

[panelman]

Thanks Jraef

The one that failed is one of three starters which have all worked faultlessly for the last 10 years so the client is unlikely to spring for new starters. The soft-starts are bypassed when up to speed and the cooling is just a simple fan blowing across the heat sink.

Looking at the RS book the brick type are now available up to 500A

Think I’ll go with my initial gut feel and recommend changing all three and maybe up the rating slightly if I can get a bigger one in the same package size

 

[LionelHutz]

You can get a 330A brick in the same size. I'm not sure how much it will help though because you're still trying to get the same amount of heat (caused by the starting current) out of the package within the same time through the same path for heat conduction.

Rating the 250A bricks for around 125A motor current can get a starting current rating of 500% for 30 seconds. This is a typical starter design. However, your pump will take much less current and time to start so it's likely still within the rating. For example, if you use 250% current during starting the brick would be good to start a 250A motor (possibly not to run it on the SCR's though).

You should check that there isn't a problem with the control boards causing the start to be different. Either the current is higher or the SCR's have some level of misfiring. You may want to get with the manufacturer to check this because 3 x 330A bricks will run you a lot of coin to only find there is a control board problem too and you should have changed the whole unit for a new warrantied one.

Of course, sometimes SCR's just fail with no real explanation. If you find no reason then just replace the one device and carry on.

The fan on the heatsink really does nothing to help during a start. Well, you might get an extra 5% if you're lucky. It will only help to cool the starter down so you can start the motor again in a shorter period of time.