Engaging a Starter

B

Billy H.

Member
Location
CA, USA
Occupation
Asphalt plant electrical technician
I have a Nema size 3 starter (AB 509-DOD) which operates a conveyor. Suddenly, the coil no longer stays engaged when the start button is released. The plant operator just sticks in a paperclip to hold down the start button.

So far I have:
- Changed the start/stop buttons.
- Changed the aux contact.
- Bypassed the aux contact with a separate relay.

None of this has worked. Then today, I noticed that it will operate correctly if I have the breaker off (no current through the starter). Does this make any sense?
 
What is the power source for the control circuit? Is it possible that the voltage drop when the motor starts is dropping the voltage at the coil below the hold-in voltage? I would check the coil voltage under both conditions...the motor power off and the motor power on.

However is this is the case and it had worked before, you need to find out what else has changed.
 
Measure the voltage at the coil with the breaker off and the breaker on to see if the control voltage is correct as Don suggested. The fact that a paper clip allows the coil to stay closed would lead me to believe that the voltage isn't the problem. Sounds like an issue with the holding contact.
 
don_resqcapt19 said:
What is the power source for the control circuit? Is it possible that the voltage drop when the motor starts is dropping the voltage at the coil below the hold-in voltage? I would check the coil voltage under both conditions...the motor power off and the motor power on.

However is this is the case and it had worked before, you need to find out what else has changed.
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If voltage dropping below hold in voltage during motor acceleration, one may be able to hold the start button until motor has accelerated and voltage recovers to some degree. You likely get some chatter out of the contactor during this though as it will still be attempting to drop out.
 
If the starter picks up, it is not the Start button/contact.
If the starter will not stay "held in", it is the holding circuit. On size 3 starters it is not unusual to find some type of mechanism involved with the holding contact.
 
What I am calling an aux contact is what you are, more correctly, calling a holding contact. Therefore, checking voltage as suggested, would this be done at the line terminal of the holding contact? Does not the fact that the paperclip works, tell me that the actual coil voltage is fine?

There is no chatter. When the start button is pushed, the coil clunks in; When released, it drops out.

One interesting point from experimentation today, when I found that it did fine without power the breaker bucket: When the breaker was off, the start button only needed a brief push to engage the coil. When the breaker is on, the plant operator has to hold the start button for a second or so before the coil engages (before this issue, this was standard procedure).

Yes, the line side of the holding contact is run through an ice block relay, which is an interlock to make sure that the equipment down the line from it turns on first.

This issue arose a month or so ago. The daytime plant operator always had to paperclip the start button. However, when we came in for a night shift, it would function fine. Then they had several times it started up fine; however, at some point when running it would drop out. Now it just never engages.

I really appreciate all of your comments/suggestions! I have looked through this forum with several projects which I’ve had through the years. This issue has just stumped me, so I made an account and asked this question.
 
Seems like there must be a safety or limit that is open somewhere that is momentarily out of the circuit when the start button is engaged. You need to identify the entire path that power must flow through to get to the coil. Use a test lamp in place of the coil for visual and while you troubleshoot if you have to.
 
Check for voltage across the holding contacts. There should be the control voltage, which should drop to zero when you hit the start button, and remain at zero when you release it.
 
Disconnect the load wires from the starter. This will eliminate any interference/ voltage drop due to the motor starting. Press your start button. Does the contactor stay engaged? If so, your voltage drop due to the motor may be the issue. If the starter doesn't stay engaged, the holding circuit isn't 100%. It could be somethin as simple as a loose wire.
 
CoolWill said:
You need to identify the entire path that power must flow through to get to the coil.
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Agree with this. If other limits or aux contacts from other starters are in that circuit, any intermittent connection in that path can be the culprit. The start button possibly bypasses some items while pressed, but those items need to have continuity through them for the holding circuit to do it's job once the start button is released.
 
ptonsparky said:
Some conveyors we worked on had rotary switches that would detect a shaft rotation. Load end. Not the driven.
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Yeah. The start button needed to be held in until the motor was turning at the correct speed, also saw this on a saw blade. In another case a lube pump needed to be pulled in.

Definitely time to play detective.
 
There is a zero speed on the conveyor. I hadn't payed much attention to it as the issue because someone already jumpered it out. However I just discovered that the zero speed goes to a relay which is in series with the holding contact.
 
Billy H. said:
There is a zero speed on the conveyor. I hadn't payed much attention to it as the issue because someone already jumpered it out. However I just discovered that the zero speed goes to a relay which is in series with the holding contact.
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Probably to shut the conveyor down if it jams.
 
Thank you a ton for leading me to the answer. It appears to have been a loose screw on a terminal strip which was related to the zero speed relay.

I cannot explain why this has anything to do with it deciding to work yesterday when I had the breaker off; However, I'll take it!
 
Billy H. said:
Thank you a ton for leading me to the answer. It appears to have been a loose screw on a terminal strip which was related to the zero speed relay.

I cannot explain why this has anything to do with it deciding to work yesterday when I had the breaker off; However, I'll take it!
Click to expand...
Perhaps no vibration when the motor and driven load isn't running or ambient temp is lower when things aren't running?
 
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