Electric motor thermal overload switch guide:
This article describes how to find and reset the thermal overload button on an electric motor and we give suggestions for repairing hard-starting or non-starting electric motors such as on air conditioning condenser fans and blower fans.
We describe where to find reset switch on most electric motors. We explain about automatic thermal overload switches on motors - switches that reset themselves when the motor cools down. And we describe what can go wrong with automatic thermal overload switches on electric motors. We include tips for diagnosing electric motors that keep having thermal overload symptoms.
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An electric motor used in lots of equipment may include overload reset button on the motor body. If the motor is overloaded or if it overheats, this button will pop "out" indicating that an internal electric safety device has tripped,shutting the motor off.
Once the motor has cooled sufficiently it can be re-started, usually by pushing the popped-out reset switch back down.
If the motor turns off again on re-set you should not keep running it as it may be damaged and the system may be unsafe.
The page top photo was taken of of an oil burner electric motor not an air conditioning blower fan motor or pump motor, but you'll see that all of these electric motors look a lot alike. Sometimes the reset button on an electric motor is hard to find, and sometimes there is no reset button!. But this photo shows the red reset button most clearly.
Find the electric motor that operates the device that has been shut down. For example on a warm air heating system or central air conditioning system look in the air handler/blower compartment.
On the blower motor itself, look for a red or yellow button which is normally flat with the motor surface but which will pop up to show that the motor has been shut off by its internal overload protection circuit.
If the installer rotated the motor so that the button is facing away from you and impossible to see, feel around on the bottom and back side of the motor for the button's presence. Especially if the motor's internal protection has tripped, it should be easy to feel the button since it'll be sticking up about 1/2".
When the air conditioner fan or blower motor has cooled sufficiently this button can usually be simply pressed back down to "reset" the switch. If the motor overload
switch won't reset (stay depressed) either the motor is still too hot (wait) or there is another failure that needs diagnosis.
Sometimes the reset button is present but hard to find, depending on the position in which the motor was bolted in place.
You can see that in this picture the button is flush with the motor surface. Sometimes these buttons are hard to find but they are usually present on heating and air conditioning system electric motors for fans and blowers.
While manual-reset thermal-overload switches are most common on heating systems such as oil burners and some fan motors, air conditioner electric motors and compressor motors and submersible well pump motors are an example of motor designs that may be use a thermal overload switch that resets itself automatically when the motor has overheated.
Electric Motor Thermal Overload Switch Trip Temperatures
|Class A electric motor||140oC / 284oF|
|Class B electric motor||165oC / 329oF|
|The reset temperature is not precisely specified but the automatic thermal overload switch will reset after the motor cools down. |
If a well pump motor is overheating for any reason (low voltage, bad start/run capacitor, damaged motor, damaged pump impeller parts, loss of water in the well, pump running dry) it may be a model that will turn itself off when too hot. A thermal sensor inside the pump motor housing handles this job. With the motor off for a cool-down period, the thermal sensor automatically resets and the pump will run again. Typically the "off" time is 15-20 minutes. If the pump on-off activity is erratic or very long, it could be that the problem is a bad thermal sensor switch.
Note that other electric motors may have a thermal reset switch that is mechanical and "pops out" to show a red "reset button" that must be pushed back in manually. If the motor is still too hot, pushing the button won't work - the button won't stay in.
Table of Causes of Electric Motor Thermal Overload Switch-Off
Things to Check if an Electric Motor Starts but Overheats and Trips its Reset Button
|This first table is for electric motors use in air conditioners, air handlers, blower units, oil burners on boilers, steam boilers, water heaters etc. - HVAC equipment in general. A second section of defects (below) addresses oil burner motors.|
|Bad line voltage||Use a VOM to check the voltage level at the pressure control switch||
If voltage is too low, check voltage at the electrical panel and check that the proper size wiring was used for the ampacity and length of run and that there are no partial shorts or damaged wires or connectors
|Incorrect motor wiring||Check the actual electrical wiring against the motor wiring diagram or the installation manual for the equipment||Reconnect wiring properly|
|Motor is too hot due to surroundings - inadequate ventilation||Check the air temperature where the motor is located. If the air temperature is over 100 degF, the pump may be too hot and its thermal overload switch tripping because of the environment, not a pump problem.||Install adequate ventilation, or if needed, shading, or relocate the motor/equipment to a cool location|
|Motor subjected to repeated overloading, overheating||A motor subjected to overloading due to very long duty on-cycles may run hot and trip the thermal overload protection device. Eventually the thermal overload switch may itself fail and the motor will no longer restart nor can you reset the thermal overload switch.||Check the recommended duty cycle and loading on your motor; check also for abnormally low voltage levels in the electrical supply. If necessary, replace the motor with a heavier-duty unit.|
|Motor for a water pump operates too long at low water pressure||If the well recovery rate is too poor and the pump is operating at low water pressure, possibly because a tailpiece is installed to prevent air injection and pump burnup, the pump may be overheating.||
Install a valve on the water discharge line and reduce water flow to increase water pressure inside the pump itself.
Adapted from various sources including Betta-Flo Jet Pump Installation Manual from the National Pump Co. and Adapted & expanded from Beckett (1989)
Table of Causes of Oil Burner Thermal Overload Switch-Off
|Bad oil pump (fuel unit)||Oil pump has seized or is binding, overloading the electric motor||Disconnect the coupling between fuel unit and electric motor and manually turn each shaft to inspect for binding or seizure. Replace a bad fuel unit.|
|Misaligned components||Connections between the electric motor drive shaft, coupling, and driven parts are loose or improperly mounted||Check for misalignment due to loose mounting bolts of motor, blower, fuel unit; check for improper seating of the motor against the oil burner housing such that the motor is askew. Re-mount.|
|Bad electric motor start switch||The electric motor will overheat if the start-switch does not disconnect at the proper or "throwout" rpm.||See Electric motor start switch operation|
|Jammed blower wheel||If the oil burner air blower wheel has jammed, possibly due to debris, mechanical damage, obstruction, the motor may spin the coupling or may not rotate at all, overheating & shutting down||Inspect the blower assembly for obstruction, free spinning, and repair|
|Undersized electric motor||Load requirements exceed the motor horsepower Hp nameplate ratings. [Unlikely on factory-assembled heating or air conditioning system but possible if someone performing a repair substituted a lower Hp motor].||
Using an Amp probe ammeter check the current draw of the motor against its nameplate rating. If the actual current draw when the motor is running exceeds the rated run-amps by more than 10% and if there is no binding or misalignment in the system we suspect that the motor is under-sized.
See TEST EQUIPMENT, ELECTRICAL GUIDE - separate article
|Bad coupling||Jammed or defective coupling between motor and driven components; improper belt tensioning on belt-driven components||Usually a bad coupling inside the oil burner will fail by becoming loose and just spinning, so the motor runs but the driven components do not spin or spin at a too-low speed. But a bent, damaged, or jammed coupling might overheat a motor - OPINION - Ed.|
|Electric Motor Lubrication Failure||If the electric motor is a model that requires periodic lubrication and is not properly maintained its drive shaft or bearings may run dry, causing binding and overheating.||
Inspect the motor to determine if it is a model that requires lubrication. Check its maintenance history; check for shaft binding; lubricate the motor if appropriate; check motor drive shaft and bearings for excessive side play, wear, or looseness.
Watch out: do not try to lubricate an electric motor that does not have lubrication fittings.
Details are at Electric Motor Lubrication Specifications
Adapted & expanded from Beckett (1989)
For complete electric motor diagnostic procedures see ELECTRIC MOTOR DIAGNOSTIC GUIDE - separate article
If your electric motor won't start, vibrates or is noisy, see our diagnostic guide to problems with electric motors at ELECTRIC MOTOR DIAGNOSTIC GUIDE.
Air conditioning & Heat Pump Compressors & Other Motors
Heating system reset switches: If you are looking for the main oil burner reset button on heating equipment you'll want to see:
Controls on well pumps and water supply equipment that may require reset or repair are discussed at
And see CAPACITORS for HARD STARTING MOTORS for advice on adding or replacing a start/run capacitor for an electric motor that has trouble starting.
See USING DMMs VOMs SAFELY. Example: testing a blower fan motor winding: referring to the electrical diagram for your equipment, unplug electrical connectors at the fan motor. Measure the resistance between each lead wire with a multimeter or VOM. The multimeter should be set in the X1 range. For accuracy, don't measure when the fan motor is hot, allow it to cool off.
When the resistance between each lead wire are those listed in the specifications for your equipment the fan motor should be normal. Zero resistance or infinite resistance are indicators of a problem.
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Im am rewiring a 1953 Singer Featherweight 222K with original 220 volts motor. I plan installing some small thermostatic circuit breakers to protect the motor, the foot switch and machine arm (with the wiring for the Singerlight lamp) from owerheating.
I will use silicone-insulated cables standing uo to 200 C heat, so the first component to be damaged from exess heat is the motor windings, insulated by dipping in with shellac insulating varnish, which was the standard industry procedure in 1953.
If one chooses breakers with a too low trip value they will stop the machine at normal operating temperatures, if one chooses breakers with too high trip value, the insulation could burn.
Now, what would be a suitable trigger temperature for those circuit brakers, or in other words,
what is your informed oppinion on the maximum recomendable and safe internal operation temperature for a A-insulated electrical motor from 1952 in original condition? - Gunnar Pettersson, Finland 3/10/2014
G.P. Thank you for an interesting question; with regrets, I do not have an answer immediately at hand. I'm a bit nervous about citing a specific "safe temperature" for a 1952 electric motor as I suspect that there was a range of acceptable temperatures based on motor design and application.
I can suggest some references that might help narrow the question to help a bit, for which Farrell's article might be most helpful.
... this question seems to be difficult to find an answer to on the web. I am very far from beeing an electricien (or how one puts in English) but I understand here are two things in play: the normal calculations and safety margins for a modern motor with modern materials, and then the older designs, using older materials and thereupon the ageing factor of those old time materials. Kind of industrial archeology. In my basement I have a thick book, I think it is Swedish, but it may be in Finnish, named "Electricity". I have it after my uncle whom I never saw, as he was killed as a young officer in the war, I think it was 1941. He was a quite promising electrical engineer, and he wrote nice and clear formulas in his workbooks, where I write as we say crow's feet. He must have had a good mathematical sensem where I am almost illiterate.
Among his books was one with the title "Introduction to higher analysis" (in Finnish) by Ernest Lindelöf:
Some professor in mathematics heard that my sister, professor H. had this book and begged for the favor to keep it for some time. It was well before the internet era.
So, still we do not now, what the temperature limit inside a Singer external, pre-1960 motor for home sewing machines model NN should be. We do, however know, what solder one should use in the rewiring: "“63-37” means it’s 63% tin and 37% lead. This specific percentage is what Singer 221 guru Graham Forsdyke’s motor specialist uses, and he used to work in the Singer factory in Kilbowie, so I’m sure he knows his stuff."
I am sure the information on "aged shellac safe" internal motor temperature can be found in the books you name, maybe as a tabel, or formula in them. Or maybe I find it in "Elektricity". I read the book in my childhood and remeber it beeing full of illustrations of vintage applications. But as it was a brave new world and the technology of tomorrow for the writer, they probably were not observant of the problematics concerning ageing of insulation materials.
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