Motor thermals & VFDs

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ptonsparky

ptonsparky

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Hooking up a couple small pumps to a vfd. Each of the motors has a thermal switch with a diagram on the pecker head cover that indicates the thermal switch should be in the control circuit of the motor. I am using the VFD control to start and stop the drive (2 wire control) and am having a hard time with the thought of including this thermal in a circuit that will be subjected to the noise a vfd creates. We could isolate the control conductors all the way to the motor but feel that is most likely a waste of time considering where the thermal is laced into the motor windings.
 
it depends on what the "thermal" is. A lot of drives have an analog input designed for the PTC thermistor that is often used in these kind of motors for thermal protection.

if it is just a dry contact type thermostatic snap switch, wire it into the enable circuit of the drive.

it is not something that one should just "tape off". it is there to help protect the motor.
 
petersonra said:
it depends on what the "thermal" is. A lot of drives have an analog input designed for the PTC thermistor that is often used in these kind of motors for thermal protection.

if it is just a dry contact type thermostatic snap switch, wire it into the enable circuit of the drive.

it is not something that one should just "tape off". it is there to help protect the motor.
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Agreed, in fact in some motors, it is REQUIRED that you use the thermal device if the motor is run from a VFD, because if they were using the fluid flow to keep it cool, and the VFD gets set to run it too slow, the motor thermal OL based on current will not know that the motor is over heating due to lack of flow through it. So that thermal device is the last line of defense.

If your drive has a PTC Thermistor input, you can wire up to that either way: ie if it is a PTC or a Klixon, because the Klixon opening is the same as high resistance from a PTC Thermistor. The reason why you want to do this is because that way, the VFD display will indicate an over temp on the motor and you know why it isn't running, as well as it showing up in the trip history.

If your drive does not have that input, you will need to know if that thermal device is a thermistor or a Klixon. If it's just a Klixon, you could just put it in the Run command circuit, but you will not know why it is not running if it trips. My suggestion then is to get a Thermistor Trip Relay and use it either way, just as I suggested if the VFD has it. That way you can use a separate set of contacts on that Thermistor Trip Relay to annunciate that the reason it is not running is that the motor over heated.
 
please consider carefully where you put it, for safety sake....

please consider carefully where you put it, for safety sake....

if you just wire it in series with the run or enable of the vfd, that means when the motor overheats it shuts off of course - as you want. BUT IT ALSO MEANS IT WILL POTENTIALLY AUTOMATICALLY RESTART THE MOTOR WHEN IT COOLS OFF! Depending on the run/enable circuit of course.

Not good idea unless that circuit is latched and will not restart when the motor cools..... use a thermal input or programmable input on the vfd instead that will not allow restart without human intervention.
 
mike_kilroy said:
if you just wire it in series with the run or enable of the vfd, that means when the motor overheats it shuts off of course - as you want. BUT IT ALSO MEANS IT WILL POTENTIALLY AUTOMATICALLY RESTART THE MOTOR WHEN IT COOLS OFF! Depending on the run/enable circuit of course.
]

I was thinking of utilizing something besides the Enable or Run but had not put that twist on it yet. I was thinking more on the line of the customer not knowing why it had stopped.

As always, good suggestions.
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Aren't some drives capable of accepting an "external fault" input that would need operator intervention to reset?
 
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