Contactor rapidly pulling in and dropping out.

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richard.jackson

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Location
Friars Point, Mississippi
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Electrician
I have a contactor that pulls in a drops out rapidly and continuously when started. I know it's from a chattering relay that supplies the 120VAC control power. My question is would a solenoid type tester such as an Ideal Vol-Con detect this? My digital multimeter doesn't respond fast enough to detect the change. any and all suggestions on a certain Brand or model of tester would be greatly appreciated>
 
Any chance you have an NC contact in series with the coil, like an old-fashioned doorbell buzzer?
 
Solenoid type tester possibly just sort of "buzzes" as well depending on it's response time to the voltage fluctuation.

A true mechanical analog meter might show a fairly steady but lower than actual voltage if the cycling is within a certain frequency range. If cycling is slow enough it may actually move some with each cycle.

Not sure why it would be important to get any accurate reading here, presuming you know there is a controlling device that is actually chattering.
 
Isn't the key to the problem to find out why is it chattering? Motor load drawing voltage down too much? Of course that is part of the reasons to use a soft starter, but still a possibility, particularly if the source is undersized for the load.
 
kwired said:
Solenoid type tester possibly just sort of "buzzes" as well depending on it's response time to the voltage fluctuation.

A true mechanical analog meter might show a fairly steady but lower than actual voltage if the cycling is within a certain frequency range. If cycling is slow enough it may actually move some with each cycle.

Not sure why it would be important to get any accurate reading here, presuming you know there is a controlling device that is actually chattering.
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Not really trying to get an accurate reading, was really just curious about a tester other than an oscilloscope that would detect the rapid change.
 
richard.jackson said:
Not really trying to get an accurate reading, was really just curious about a tester other than an oscilloscope that would detect the rapid change.
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I think an oscilloscope is about the only thing that will give some sort of definite indication of what is going on, other things may have a noticeable reaction but if you didn't know what was causing the abnormal reading you still don't know what that reading is about.
 
kwired said:
Isn't the key to the problem to find out why is it chattering? Motor load drawing voltage down too much? Of course that is part of the reasons to use a soft starter, but still a possibility, particularly if the source is undersized for the load.
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I absolutely agree that finding the cause is the important issue. The soft start actually chatters when you signal it to start, I'm pretty sure that is causing the control relay and main contactor to chatter to the point that the load on the main contactor causes the contactor to fail. I definitely appreciate your help.
 
kwired said:
I think an oscilloscope is about the only thing that will give some sort of definite indication of what is going on, other things may have a noticeable reaction but if you didn't know what was causing the abnormal reading you still don't know what that reading is about.
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I appreciate the help, will have to look into an oscilloscope for future use.
 
richard.jackson said:
I absolutely agree that finding the cause is the important issue. The soft start actually chatters when you signal it to start, I'm pretty sure that is causing the control relay and main contactor to chatter to the point that the load on the main contactor causes the contactor to fail. I definitely appreciate your help.
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What does supply voltage look like at that time? If dropping too much it can make contactors and/or control relays chatter which make whatever they are controlling chatter which ultimately makes the whole situation worse.

I've also had situations where a control contact happened to have weak performance for various reasons and caused chattering of what it was controlling. Specifically remember replacing a contactor that controlled a heating element in a wash tank type machine that melted down one time. Got called back like the next day and my new contactor was also melted down. Turned out the control thermostat was bad and causing the contactor to chatter. It just couldn't take operating several times a second for a rather indefinite time period.
 
kwired said:
What does supply voltage look like at that time? If dropping too much it can make contactors and/or control relays chatter which make whatever they are controlling chatter which ultimately makes the whole situation worse.
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What is it about lower than expected voltage that causes chatter?
 
Pinnie said:
What is it about lower than expected voltage that causes chatter?
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Where I've seen this the most is with starting a motor, current is much higher than normal running voltage which can bring supply voltage down and cause control contactors or other control relays to drop out momentarily, then voltage picks back up and the process starts all over again. Presuming the motor is successfully accelerating it gets a little better every cycle as the motor current would be a little less every cycle, but is still hard on motor contacts and contactor coils as they are also drawing more current when they pull in than they draw when they are "sealed in".

Or my mentioned contactor controlling the heater element that had bad thermostat - it melted down the contactor coil because it was nearly constantly drawing "pull in current" while it was cycling so fast" and that current is a lot more than "sealed in current" they are only designed to handle that draw in current for brief period of time.

If motor contactor is chattering it is hard on the motor as well, but chances are the contactor coil fails before the motor winding fails in that situation.
 
richard.jackson said:
I have a contactor that pulls in a drops out rapidly and continuously when started. I know it's from a chattering relay that supplies the 120VAC control power. My question is would a solenoid type tester such as an Ideal Vol-Con detect this? My digital multimeter doesn't respond fast enough to detect the change. any and all suggestions on a certain Brand or model of tester would be greatly appreciated>
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For things like that I prefer an analog meter with a needle. Even if it is too fast to read the voltages, you see the meter movement. You might be able to feel it with a solenoid tester, but if the voltage change is too fast, you might not feel it.
 
kwired said:
Where I've seen this the most is with starting a motor, current is much higher than normal running voltage which can bring supply voltage down and cause control contactors or other control relays to drop out momentarily, then voltage picks back up and the process starts all over again. Presuming the motor is successfully accelerating it gets a little better every cycle as the motor current would be a little less every cycle, but is still hard on motor contacts and contactor coils as they are also drawing more current when they pull in than they draw when they are "sealed in".

Or my mentioned contactor controlling the heater element that had bad thermostat - it melted down the contactor coil because it was nearly constantly drawing "pull in current" while it was cycling so fast" and that current is a lot more than "sealed in current" they are only designed to handle that draw in current for brief period of time.

If motor contactor is chattering it is hard on the motor as well, but chances are the contactor coil fails before the motor winding fails in that situation.
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That makes perfect sense. I feel like my issue is within the soft start itself. Upon startup (even with motor leads disconnected) it immediately starts chattering it then ramps up all the while chattering to go to full power and sends signal to pull the control relay and main contactor in they in turn chatter in synch with the soft start. I have no issues with the contactor if I bypass the soft start and just bring the motor up at full speed. Most definitely open to suggestions. If I could show the facility manager the voltage fluctuation on the coils it would help him to understand my point. You are absolutely correct it definitely let the magic smoke out of the contactor itself, blew it up, that's how the issue came to light.
 
richard.jackson said:
That makes perfect sense. I feel like my issue is within the soft start itself. Upon startup (even with motor leads disconnected) it immediately starts chattering it then ramps up all the while chattering to go to full power and sends signal to pull the control relay and main contactor in they in turn chatter in synch with the soft start. I have no issues with the contactor if I bypass the soft start and just bring the motor up at full speed. Most definitely open to suggestions. If I could show the facility manager the voltage fluctuation on the coils it would help him to understand my point. You are absolutely correct it definitely let the magic smoke out of the contactor itself, blew it up, that's how the issue came to light.
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The soft starter electronically varies output of minimum two legs, more expensive units may vary all three. I could see it maybe acting rather strange if one the outputs is failed (partially or completely) and may be similar effect to contact chatter.

Do you have a main contactor in series with soft starter? And if so is any of the control to this contactor through any control components of the soft starter? Even if just overload or fault relay? If so maybe temporary bypass it just to see if it makes the so called chatter go away? Doesn't solve the problem but maybe helps narrow it down to where the problem is. Also can try temporarily bypassing as many normal controls as possible and give a rather direct run input to the soft starter to possibly confirm/eliminate any aux controls as being the cause of the chatter.
 
kwired said:
The soft starter electronically varies output of minimum two legs, more expensive units may vary all three. I could see it maybe acting rather strange if one the outputs is failed (partially or completely) and may be similar effect to contact chatter.

Do you have a main contactor in series with soft starter? And if so is any of the control to this contactor through any control components of the soft starter? Even if just overload or fault relay? If so maybe temporary bypass it just to see if it makes the so called chatter go away? Doesn't solve the problem but maybe helps narrow it down to where the problem is. Also can try temporarily bypassing as many normal controls as possible and give a rather direct run input to the soft starter to possibly confirm/eliminate any aux controls as being the cause of the chatter.
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The control signal that pulls in the control relay and the contactor that I am referencing do come from the soft start in auto. In bypass which is working fine it comes straight from the start button. I really appreciate the in depth advice and information.
 
A Fluke 43B hand held power quality analyzer will do the job.
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Soft starters shouldn’t be chattering, at all. Since we have to guess, my guess would be that a gate firing circuit for one of the SCRs of the soft starter is dead or dying, likely the pulse transformer. If the gate doesn’t fire one of the SCRs, the result is a DC pulse going out to the motor in each cycle, which makes the motor fight itself due to negative sequence current, pulling high current and in all likelihood, causing a severe voltage drop that is pulsating in rhythm with the failed SCR, causing the contactor coil to chatter.

Bottom line, replace the control board of your soft starter. Check the SCRs too, one shorted SCR can have a similar effect, but most soft starters have shorted SCR detection that would throw a fault.
 
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