Failing soft starter?

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Electron_Sam78

Senior Member
Location
Palm Bay, FL
Hey all I came across a strange situation where a motor panel with soft starters was supplying power to a motor with no indication in the controls that anything was on. In fact the motor HOA switches were turned OFF and there was no RUN or RAMP UP/DWN lights on the soft starter. The motor in question has had problems with getting air-locked which shouldn't happen with normal control operation. Could there be a sticking contactor within the soft starter that is keeping power supplied to the motor after the controls disengage? I don't have a whole lot of experience with soft starters...

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kwired

Electron manager
Location
NE Nebraska
Hey all I came across a strange situation where a motor panel with soft starters was supplying power to a motor with no indication in the controls that anything was on. In fact the motor HOA switches were turned OFF and there was no RUN or RAMP UP/DWN lights on the soft starter. The motor in question has had problems with getting air-locked which shouldn't happen with normal control operation. Could there be a sticking contactor within the soft starter that is keeping power supplied to the motor after the controls disengage? I don't have a whole lot of experience with soft starters...

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They only have contacts in the lines if they have a "bypass" mode. If that is sticking you could turn off input power, let motor stop, turn it back on- if motor starts but it doesn't "soft start" then it is likely the bypass contacts are stuck closed for some reason.

Otherwise the device has solid state devices that control the output to the motor - some maybe only on two of the three lines, but regardless if those devices fail in a shorted condition - you still have a "bypass" condition. I don't know enough about them to know how likely that is to happen though, something tells me they are more likely to fail in an open condition.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Hey all I came across a strange situation where a motor panel with soft starters was supplying power to a motor with no indication in the controls that anything was on. In fact the motor HOA switches were turned OFF and there was no RUN or RAMP UP/DWN lights on the soft starter. The motor in question has had problems with getting air-locked which shouldn't happen with normal control operation. Could there be a sticking contactor within the soft starter that is keeping power supplied to the motor after the controls disengage? I don't have a whole lot of experience with soft starters...

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what makes you think there is power being supplied to the motor?

if you measure the voltage on the output of a solid state device like that with a high impedance meter you will probably see whatever the line voltage is because the meter impedance is much higher than the solid state device.

many soft starts only have solid state devices on 2 of the 3 legs anyway.

most soft starts have a bypass contactor that turns on once the motor is up to speed. it is far more efficient that way and allows for much less expensive solid state components.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
what makes you think there is power being supplied to the motor?

if you measure the voltage on the output of a solid state device like that with a high impedance meter you will probably see whatever the line voltage is because the meter impedance is much higher than the solid state device.

many soft starts only have solid state devices on 2 of the 3 legs anyway.

most soft starts have a bypass contactor that turns on once the motor is up to speed. it is far more efficient that way and allows for much less expensive solid state components.
I agree. If you use a digital multi-meter, it will read the leakage current through the RC Snubbers that protect the SCRs. It's only going to be 1-2mA of current and if you measured it with an old analog meter the meter burden drives it down to zero V, but the DMM presents almost no load so it reads close to the line voltage potential right through it. The snubbers are just a small resistor and a capacitor in series, connected in parallel to the pair of SCRs in each pole. This is what it looks like (except without the inductor on the output side).
unpolarized-snubber-circuit.jpg

There are (unfortunately) cheap soft starters out there that only put SCRs on 2 of the 3 phases, which makes this even worse in that if any ONE SCR shorts, there is no way to stop current flowing to the motor. In a full 6 SCR design, it takes at least two shorted SCRs in separate phases for that to happen. That unit from SAF is not one of those, it has all 6 SCRs. But if you are using an analog meter and still reading a voltage across from L1 to T2 for example, then you may have multiple shorted SCRs. It can happen.

That particular soft starter DOES have integral bypass contacts. It's not a "contactor" in the conventional sense, each individual pole has its own separate 1 pole contactor that shorts out the SCRs when it is done ramping. It could therefor be that one of those contacts is welded closed, you would not be able to tell the difference between that or a shorted SCR. BUT, the soft starter has a detection circuit inside that would give you a fault light warning you that you have an SCR shorted. That light only comes on when you turn the motor OFF, and the voltage drop that should normally be there across the SCRs is not there. If you don't have that light, I doubt there is anything wrong.

If you really want to know if the SCRs are shorted, disconnect power and the load, then read the resistance across each pole. Less than 10ohms of resistance and there is a shorted SCR in that pole. Doesn't matter which one, you will need to replace them all if even one is shorted (in my professional opinion) because whatever took out one also came close to taking out the rest, but once the first one (or two) failed, that prevented the rest of them from shorting. They will however "catch up" in short order.
 
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kwired

Electron manager
Location
NE Nebraska
what makes you think there is power being supplied to the motor?

Kind of a good point/good question.

I guess there is a difference between whether there is voltage present and whether or not there is current flowing. Technically you need both to have power, but OP may not have been all that clear on what he has. I figured "supplying power to the motor" meant motor was running, but maybe he just meant there is voltage present but no current - which you would have with a unit that only controls two of the three lines.
 

Electron_Sam78

Senior Member
Location
Palm Bay, FL
Kind of a good point/good question.

I guess there is a difference between whether there is voltage present and whether or not there is current flowing. Technically you need both to have power, but OP may not have been all that clear on what he has. I figured "supplying power to the motor" meant motor was running, but maybe he just meant there is voltage present but no current - which you would have with a unit that only controls two of the three lines.
Sorry if unclear. The motor was fully powered and pumping water. No run, ramp, or fail light illuminated on SS. No low voltage control activation. Very strange...

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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
Sorry if unclear. The motor was fully powered and pumping water. No run, ramp, or fail light illuminated on SS. No low voltage control activation. Very strange...

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Also this problem is intermittent. Sometimes the motor stays running aND other times it turns off just fine

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Sounds like the bypass contacts, or at least two of them, are sticking. I would imagine that soft starter should detect that as a shorted SCR, maybe not.

I'm afraid I may have led you astray earlier. I downloaded a copy of the manual for that starter, it does NOT have Shorted SCR detection! I've never heard of that, it's a critical need in my opinion. I can't believe anyone would build a soft starter without that, it's super simple to implement and costs pennies (or in this case, agoras, because they are made in Israel). The negative consequence of that is exactly what you are experiencing, uncontrolled operation of the motor. Very dangerous.

I would not use that starter, replace it with a better one.
 

Electron_Sam78

Senior Member
Location
Palm Bay, FL
Is there a way to set up external shorted scr detection?

Also can anyone recommend a better replacement with dry contact single circuit start/stop?

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meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
It may depend on size, but most of our soft starts are built with with a bypass contactor AND an isolation contactor (wired ahead of the feed to the soft start/bypass wiring) with a short time delay before the start command to the soft-start. For the obvious reason that SCR's can short. If they do, there's no way the motor will stop until the breaker is turned off. But be aware the there is also a safety issue, because even when off, a soft start can still supply line voltage to the load, just not much current. That's why they come with stickers warning about load voltage even when stopped. That's also why I trained the electricians to ALWAYS turn off the breaker and test for dead before you start working on the starters. If energized testing is required, appropriate PPE is a must. You said the motor kept on running. Was the load current normal on all three phases? I don't think I've seen a soft start that had more than one or two SCR's shorted. Sounds more like a control problem. You'd need to see what circuitry supplies the ramp/up to speed indications to see why they are off with the motor still running. Ain't it fun being a detective? Being retired is nice, but the brain gets stale pretty fast.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Is there a way to set up external shorted scr detection?

Also can anyone recommend a better replacement with dry contact single circuit start/stop?

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Is there a reason it's a soft start? Pretty expensive if across the line would work just as well. Usually the soft start is to limit inrush or reduce the effect of high torque or water hammer if a pump motor starts at full throttle. Just curious. We only use soft starts on 75HP and over. Our biggest is 500HP. Square D/Schneider/Telemechanique or whoever they are now. Older starters are part winding WYE. As far as SCR detection, you could always just put in a simple series relay circuit that operates if there is no start command and the output is energized. Maybe that suggestion should be in the Engineering forum. Pretty high tech.:)
 
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Electron_Sam78

Senior Member
Location
Palm Bay, FL
No idea why. That is an engineering issue. I'm just troubleshooting and replacing bad parts. It is a large vault sewage lift station if that helps...

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