Inter capacitance

  • #1

Main Question or Discussion Point

A pump did not stop when operator pressed the emergnecy push button. He ran towards another start/stop station and pressed the stop button. Even then the pump did not stop. After a while according to operator the pump stopped on its own. Electrical expert says that this can happen because of intercapacitance due to distance between 230v contactor and push button station. The distance was hardly 30 metre.
Can intercapacitance effect 230 V contactor to latch momentarily?

Rajesh
 

Answers and Replies

  • #2
4,662
5
Before you can determine why the emergency push button did not work properly, you need to get a schematic of the circuit and the electrical properties of the components, and analyze it. Very roughly a cable has ~ 100 picofarads per meter of capacitance (depending on cable choice), or ~3 nanofarads for 30 meters. For safety reasons, the pushbutton switches should be normally closed (NC) and open when pushed. It would be safer if each pushbutton had a separate cable to the contactor, but they could be wired in series.

Bob S
 
  • #3
sophiecentaur
Science Advisor
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You might question the 'expertness' of your expert.
Presumably the equipment runs on AC so it is hard to see how this 'momentary' holding-on could last more than 10ms (half a cycle of the mains). Also the time constant of any power circuit, loaded with an extra few nF is of the order of ns!
Could the delay be deliberately built-in for some reason?
It sounds to me that you could have something mechanically 'sticky' which is delaying the breaker from opening after the control current stops.
 
  • #4
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And since it is an emergency stop button that isn't working properly, I suggest the pump is not run until this is figured out. I hope nobody got hurt...
 
  • #5
dlgoff
Science Advisor
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You might question the 'expertness' of your expert.
Yep!
 
  • #6
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It is possible that your pump is running on a mechanical latching relay, and not one that releases with a break in the relay coil circuit. See Fig. 3 in

http://www.eleinmec.com/article.asp?24

The relay shown in Fig. 3 is a SPST. You could use a DPST relay and use separate contacts for the holding circuit.

Bob S.
 

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