Soft Start Overcurrent Protection

[tsgelectric]

Is anybody familiar with fusing Soft Start controllers? I ususally fuse my motors at 125-150% of the motor's nameplate FLA. Due to the high cost of semiconductor fuses, most people fuse soft starts with regular fuses. What are the best semiconductor fuses to use with soft starts and how should they be sized?

 

[LionelHutz]

Depending on where you are in the world, semiconductor fuses may not be recognized as a motor protection fuse. So, they may not be suitable for use as the only protection means in a motor circuit.

You have to balance the I^2t rating of the fuse vs the starting current to find a fuse that protects the semiconductor and yet does not blow due to the starting current of the motor. There are lots of fuses out there that are suitable so picking one is not really possible. Finding a type that is available which covers the right voltage desired current range is a good start.

I personally would not bother with semiconductor fuses on a soft-starter.

Typically, you will find semiconductor fuses need to be around 3 times the FLA of the motor. This goes over the typical motor circuit protection requirement of the fuse or breaker being less than 2 times or 2.5 times the motor current rating, adding yet another reason they aren't suitable by themselves.

 

[tsgelectric]

That's excatly what I was expecting to hear. It seems that there is no convenient way to protect these.

However, I did find out that Ferraz Shawmut came out with a High Speed Class J fuse (HSJ). They claim that it combines the low I^2t of a semiconductor fuse and branch circuit performance of a Class J. It was specifically designed for solid state motor controllers. It almost sounds to good to be true. I still have to check the sizing to see if it will be excessively high or not.

Thanks for your reply