# Weird issue with motor protection



## BlazeTaco (Jul 20, 2011)

At the facility I am working at we are having a problem with the motor overload fuses blowing even though the motor, gear box, and conveyor are not bound up. The overload is properly rated for the motor along with the fuses. My question is that I noticed the the contactor engages and disengages in a second or less then a second randomly. Can this cause the line side fuses before the contactor to blow when it is doing this?


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## JRaef (Mar 23, 2009)

BlazeTaco said:


> At the facility I am working at we are having a problem with the motor overload fuses blowing even though the motor, gear box, and conveyor are not bound up. The overload is properly rated for the motor along with the fuses. My question is that I noticed the the contactor engages and disengages in a second or less then a second randomly. Can this cause the line side fuses before the contactor to blow when it is doing this?


Yes, *absolutely*. 
When a contactor closes on a motor circuit, there is an instantaneous "inrush" of current of up to 20X the FLA for the first cycle or so, then followed by a longer surge of current at about 6X the FLA until the motor accelerates. There is a LOT of energy in those surges, all seen by the fuse elements as heat. They are designed to handle that on a limited basis, but if your contactor is randomly closing and opening, that is stressing those fuses needlessly (among other very bad things). So when you WANT to start the next time on purpose, the fuses is already stressed and melts.

You need to *immediately* troubleshoot your control system to find out what is causing that, it is NOT normal and must be avoided.


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## wendon (Sep 27, 2010)

Is your control circuit powered off the line side of the starter? What's the voltage? What size of motor? I'd check the voltage on the control circuit while the motor is starting.


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## Fixastang (Sep 4, 2012)

JRaef said:


> Yes, absolutely.
> When a contactor closes on a motor circuit, there is an instantaneous "inrush" of current of up to 20X the FLA for the first cycle or so, then followed by a longer surge of current at about 6X the FLA until the motor accelerates. There is a LOT of energy in those surges, all seen by the fuse elements as heat. They are designed to handle that on a limited basis, but if your contactor is randomly closing and opening, that is stressing those fuses needlessly (among other very bad things). So when you WANT to start the next time on purpose, the fuses is already stressed and melts.
> 
> You need to immediately troubleshoot your control system to find out what is causing that, it is NOT normal and must be avoided.


I with JRaef. I would look specifically at whatever sends a signal to the conveyor telling it either to run or that it's ready for it to run (equipment downstream of the conveyor or the PLC set points).

Also let us know if this is happening on the hand or auto mode.

Please keep us posted regarding what you find.


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## BlazeTaco (Jul 20, 2011)

Thanks guys, thats kind of what I was thinking too, but when I mentioned it to the controls engineer he dismissed me and said it was normal, like I didnt know what I was talking about, kind of a arrogant guy. It is controled by a PLC using a flex IO sourcing card and 24 volts. It does also have a supression diode on the coil for the contactor(someone did replace the contactor so it wasnt that) too. I am kind of leaning toward something goofy going on in the program, like a scan time issue or an input not being seen correctly. It is in auto, the machine has no way of running conveyors in manual.


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## Steve01 (Mar 8, 2013)

wendon said:


> I'd check the voltage on the control circuit while the motor is starting.


I agree with checking the voltage while the contactor is engaging. Also keep an eye on the flex i/o status light. If your 24V power supply has foldback protection you could loose 24v for a moment if the contactor inrush is overloading the power supply. The status light will blink off if you loose power.


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