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SMC-3 Allen Bradly Soft Starter Problem

02/24/2017 11:40 PM

I have an problem with SMC-3 Soft starter (150-c-135) 135 Amps. It is used to run a motor of 75 kw, 130 amps full load amps of a aeration compressor at ETP. The problem was with the Internal bypass contactors. There were 3 nos. of contactors of 50 Amps rating and 48V, 50 Hz coil voltage which were welded and due to that the main MCCB of 150 Amps was tripping. I replaced all 3 contactors with new once of 50amps, 48V, 50Hz coils. Now, there is another problem.

Now, when i start the soft starter with 10 second, soft start mode, initial torque 15% and kick start off - the motor starts, takes current of 190 amps and after 10 second current goes up to 550-600 amp and the main MCCB (150 amp) trips.

i tired with different settings in soft start mode by increasing the time, torque but the result is the same. The main MCCB trips after the time limit.

I also tried with current limit start, 10s, current limit %FLA 350 & 450 Amps, but it behaves the same. At start up current remains at 350-400 amps and after the time delay instead of dropping, the current goes upto 500-600 amps and the main MCCB trips.

what could be the reason?

1. Is it because the SCRs are not able to give initial start up torque to the motor and after that, when bypass contactors are on, due to high current the motor trips?

2. we did the DIP switch changes several times within a minute time after tripping of MCCB. Can due to that, the SCRs got heated and are not functioning well at start up?

Please note that, when I was doing the changes of the DIP switch (for time, torque, current limit) in current limit start mode, 10 second delay, 350% current limit once the soft starter worked well. But all the three internal bypass contactors where starting & stopping frequently at 130 amps so we got affraid and stopped it.

3. Shall the internal bypass contactos becomes ON permanently after the set time delay? Or they will start stop to do what?

Please guide...

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#1

Re: SMC-3 Allen Bradly Soft starter problem

02/25/2017 3:30 AM

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

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#2
In reply to #1

Re: SMC-3 Allen Bradly Soft starter problem

02/25/2017 9:29 AM

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.

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#3
In reply to #2

Re: SMC-3 Allen Bradly Soft starter problem

02/25/2017 10:00 AM

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.

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#4

Re: SMC-3 Allen Bradly Soft Starter Problem

02/25/2017 12:03 PM

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

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

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#5

Re: SMC-3 Allen Bradly Soft starter problem

02/25/2017 12:15 PM

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?

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#6
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Re: SMC-3 Allen Bradly Soft starter problem

02/25/2017 1:10 PM

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

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#7
In reply to #6

Re: SMC-3 Allen Bradly Soft starter problem

02/25/2017 1:14 PM

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.

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#12
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Re: SMC-3 Allen Bradly Soft starter problem

02/26/2017 12:04 PM

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.

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#8

Re: SMC-3 Allen Bradly Soft Starter Problem

02/25/2017 3:29 PM

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.

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#9
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Re: SMC-3 Allen Bradly Soft Starter Problem

02/25/2017 10:45 PM

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.

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#10
In reply to #9

Re: SMC-3 Allen Bradly Soft Starter Problem

02/26/2017 11:10 AM

What are your scr's rated at?

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#11
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Re: SMC-3 Allen Bradly Soft Starter Problem

02/26/2017 11:40 AM

You don't measure current in a 3phase circuit as divided by the number of poles, current should be the same on all 3 legs. So if something is rate for 150A, that is 150A per phase.

In this case, I fear that the OP made this same mistake in ASSUMING that each pole was rated for 50A, the replaced them with 50A contacts, which would have been a mistake. Each of the contacts in an SMC are rated at the rating of the starter, in this case 135A. But they are rated for what is considered "shunt duty", meaning they are NEVER intended to start or stop the motor directly, the SCRs are ALWAYS used for that, so long as the soft starter is being properly used. If the bypass contacts were replaced with the proper part numbers from A-B, then no worries but if the OP replaced them ASSuming they were 50A, then that needs to be rectified and the exact part numbers from A-B must be reinstalled. There is more to this than just the rating and coil voltage, you could end up over drawing the coil current.

But that's where the problem here likely lies. If the motor started by the soft starter with reduced torque cannot get the load to about 80% speed before the ramp time expires, the current surge will still take place when the bypass contacts close. But if that current surge still exceeds the capacity of the bypass contacts and they over heat, the SMC will switch back to the SCRs to attempt to finish accelerating the load. But if THAT doesn't work, it will close the bypass contacts again and keep cycling between the two until the load accelerates. That however should not happen repeatedly for the very reason that is apparently happening here, you can weld the bypass contacts. They are NOT designed to be abused, their only role is to remove the heat losses through the SCRs when ramping is complete.

So the core of this problem is that the ramping is not getting the motor to >80% eyed in the allotted time, plain and simple. If, as reported, it was working before and only now started not working, that points to a change. In a compressor that must be started unloaded, that USUALLY means the unload system is malfunctioning. Just because it has one does NOT mean it is working. And just because the solenoid is being powered does NOT mean the valve is opening. And just because the valve is opening does NOT mean the chamber is open to atmosphere, it could be completely clogged with rust or other debris.

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#13

Re: SMC-3 Allen Bradly Soft Starter Problem

02/27/2017 3:24 AM

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. (",)

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