SMC-3 Allen Bradly Soft Starter Problem

Sounds like your windings are shorted to me....

I'm not sure if it would apply here, but when I've worked on smaller motors I would disconnect the windings and measure the resistance between of the windings and windings to ground. There should be a know resistance value for these measurements to cross reference. If not the manufacturer should know.

Thanks a lot. Here, I would like to mention that, when the same motor is operated with VFD, it works very well. There is no MCCB trip or VFD trip at high current.

I think you need to reference the manual and reset your dip switches...

http://www.electriciantalk.com/f28/smc-3-soft-stater-ab-72200/

This is most likely a misapplication of a soft starter, or a problem with your compressor unloader. In any positive displacement compressor (piston or lobe), the chamber must be open to atmosphere in order to accelerate with any kind of reduced voltage start. If yours has that, it is not working correctly and if it does not, then you cannot use any sort of reduced voltage / reduced torque starting method, unless maybe if it is grossly over sized to handle an extended time at a high current limit. So lets start there. Do you have an unloader and is it working properly?

There is no unloader in the compressor. This compressor is for areation at ETP plant. The same compressor is working well on VFD.

This soft starter was working well before on the same compressor. It worked for 2 years and now this problem created. All 3 internal contactors were welded. So we just replaced them. Now after this, it behaves like what i have briefed.

Sorry, I just notice that you said there is NO unloader. However I find that difficult to believe. It may not be a separate valve specifically for that purpose, but something in your system must be designed to allow the compressor to get to speed before compressing, otherwise it will not start in enough time to use a reduced voltage starter.

The VFD working is not a good indicator, because a VFD is not a reduced TORQUE device, it provides CONSTANT torque from the very beginning, so it can likely accelerate a loaded compressor eventually. A soft starter works to reduce current, which reduces torque, so if that reduced torque is insufficient to accelerate the load, it doesn't work. Compressors that have no unloading system almost NEVER work with soft starters, so if it EVER worked, it must have had something to unload it, and that is no longer working correctly. Bottom line, SOMETHING has changed, you need to find that. You cannot keep throwing the same parts at it and expect different results.

The OP says that the contacts on the bypass are rated for 50 amps, whereas motor FLA is 135. Unless the 50 is a typo, the contacts are way too small.

In AB soft starters, there are 3 bypass contactors used. These contactors are inside the soft starter used for each phase. Contactor rating is 50amps, 48v ac coil. AB has made these special compact design. Each Contactor's 3 poles are conncted with each line and the other 3 poles are connected with the output. This way it is making 50*3= 150amps contact for each phase.

What are your scr's rated at?

My case is probably not the same but the symptoms certainly reminded me of the time I was called to troubleshoot an SMC soft-starter. It was tripping during start up so I measured the current and got 300+ amps for a 100A motor. I also tried current limiting but the thing would trip once the ramp up finished.

I had just come from a vacation and it was my first day back on the job. I asked the operator about the history. The original motor had failed (burned windings) and they had replaced it. The problem started after the replacement.

You get a case of "aha" when you hear that something had been changed so I went up to the motor to check it. The FLA was correct but the rpm, instead of 1800 rpm was 3600 rpm.

I relaxed at the canteen while the crew who did the first replacement did it a second time. (",)