# Thermal overload tripping on a motor?



## ToddH (Apr 27, 2008)

Hi guys

I am working on a commercial kitchen exhaust fan on a roof. It has a variable speed control next to the stove on the wall. Another guy from my company went there because the fan was not working and said the motor was bad so he went and matched it up with a new one and I went back to replace it. It ran fine then the thermal tripped after about 20 min. Now I don’t know if it is just not the same hp but it seems to move the fan no problem. I think it has to do with the variable speed control. Does the motor have to be a variable speed motor? Also how does the variable speed control work does it change the sign wave like a dimmer? The motor is just a 115v.


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## McClary’s Electrical (Feb 21, 2009)

A dimmer changes the voltage, a drive changes the frenquency. They do not work the same. The motor does not have to be an inverter rated motor,,,,but that does help. My guess is there was already a mechanical problem that lead to the failure of the first motor. Take amperage readings and see if the motor is working too hard. Then correct the mechanical problem. Also, I have seen bad thermistors that open before their predesignated temp.


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## brian john (Mar 11, 2007)

Check the current based upon the Nameplate current.
Check the OL based upon the running and nameplate current.


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## micromind (Aug 11, 2007)

What type of motor is it, and what type of speed control? 

If it's a shaded-pole or PSC motor, and the proper speed control is used, the current will decrease with decreasing speed. 

If it's capacitor-start or split-phase, any type of speed control will cause problems. 

Rob


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## westernexplorer (Jun 20, 2009)

After you check the Overloads for proper sizing., look for drag on the drive. Could be a pully or belt rubbing, that would cause the motor to overload. Also, since your saying it's 115 volts, check the internal connections of the windings to be sure it's connected 115 volts and not 230 volts.....all of these things will cause the motor to overload and will cause overloads to trip as well.....


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## MDShunk (Jan 7, 2007)

If this is a NEMA type starter with eutectic overloads, I've seen the little pawl that latches them in wear to the point of tripping for no good reason. Just a thought. If this thing has been reset a billion times in the past, it could be worn and not latch in well.


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## John Valdes (May 17, 2007)

mcclary's electrical said:


> A dimmer changes the voltage, a drive changes the frenquency. They do not work the same. The motor does not have to be an inverter rated motor,,,,but that does help. My guess is there was already a mechanical problem that lead to the failure of the first motor. Take amperage readings and see if the motor is working too hard. Then correct the mechanical problem. Also, I have seen bad thermistors that open before their predesignated temp.


A drive changes volts and frequency. Thats what makes them work. You cannot use an inverter type control (VFD) on a single phase motor. But you can run a three phase motor on single phase supply with one.

What exactly do you have?


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## Larry Fine (Oct 24, 2007)

ToddH said:


> I think it has to do with the variable speed control.


A simple test would be to see how it runs with the controller bypassed, either just jumped out or temporarily replaced with a switch.

If it works, and they don't really need the variable speed, replace it with the switch permanently. The motor will probably last longer, too.

I've taken 2-speed switches out of kitchen-hood systems because the fan might be on low when the system trips, and the motor doesn't like both wires energized at the same time.


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## Richard Rowe (May 25, 2009)

Had it given any trouble before it went out? If not then check the motor plate for amp load on the old motor vs the new one to be sure they are the same. Put your amp meter on it and see what it is pulling when it starts-- running-- and when it trips.... This should get you headed in the right direction to find your problem. You might want to check your voltage while the motor is running as close to the motor as you can just to make sure you don't havea drop.


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