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Overheating electric motor diagnosis and repair:
This article provides a table of the causes of and cures for an electric motor that overheats or runs too hot. If your electric motor keeps tripping the thermal reset button this table helps find the cause and cure.
This series article describes A/C electrical motor troubleshooting: here we provide an electric motor diagnostic table, a troubleshooting guide that helps diagnose and repair most electric motor problems for motors found on HVAC equipment in buildings such as air conditioners, furnace or air handler blower fans, oil burner motors, well pumps, and condensate return pumps.
Provided here: a Table of Air Conditioning or Heating System Electric Motor Troubleshooting Procedures for a Motor that Overheats or Trips its Reset Button or Runs at Abnormal Current or Voltage.
Fatal Shock Hazard Warning: Inspecting electrical components and systems risks death by electrocution as well as serious burns or other injuries to the inspector or to others. Do not attempt these tasks unless you are properly trained and equipped.
See DMMs VOMs SAFE USE OF for help in making safe use of electrical test equipment before you start poking your meter probes into anything.
At page top our photo illustrates the motor as typically found in a direct-drive HVAC blower or air handler assembly. (BLOWER FAN OPERATION & TESTING)
Table B: 7 Things to Check if an Electric Motor Starts but Overheats and Trips its Reset Button or Runs at Abnormal Voltage or Current Levels
See ELECTRIC MOTOR OVERLOAD RESET SWITCH for how to find and reset this button as well as a description of how the thermal overload switch works, the use of automatic-reset thermal overload switches, and related information.
If your electric motor won't start or won't run, see ELECTRIC MOTOR WON'T START / RUN.
|1: 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|
|2: Incorrect electric motor WIRING||Check the actual electrical wiring against the motor wiring diagram or the installation manual for the equipment||Reconnect wiring properly|
3: Electric MOTOR runs too HOT due to surroundings - inadequate ventilation, operating conditions
The electric motor is overloaded.
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.
Operating conditions can cause a motor to overheat, such as a well pump that runs continually because of other problems in the water system
An air conditioner compressor motor that is overheating may sometimes be re-started by cooling the equipment with water (watch out for shorting electrical components). This is a diagnostic step not an adequate repair measure.
Any electric motor that is being asked to operate equipment whose forced exceed the motor's rated capacity will run hot and will ultimately burn-up, or fail to start.
If the motor starts and runs normally when disconnected from its load but has trouble starting when re-installed, the problem could be on the load end
. A common example is trouble at an electric motor on a heating oil burner whose fuel unit (oil pump) drive shaft is binding.
Causes of electric motor overloading include and too-hot operation include
Low supply voltage
Damage or binding in the driven-mechanical system that the motor is operating
Higher ambient temperature than the motor is rated to handle
Excessive duty cycle
Internal damage in the motor: shorted or grounded winding (may show up only after the motor has spun up to full operating speed)
Install adequate ventilation, or if needed, shading, or relocate the motor/equipment to a cool location
Look for abnormal conditions that cause the motor to keep running (bad control, loss of well water, bad pump controls, water left running in the building, valve closed on pump outlet side, strainer clogged at pump inlet or outlet, pump running continually due to improper voltage, pump running backwards due to electrical damage or mis-wiring or starter capacitor short)
A well pump motor may run continually and be unable to reach shut-off pressure due to a damaged impeller or loss of well water.
|4: Electric MOTOR runs too long or WON'T SHUT OFF||
Check for a bad switch or motor control assembly.
Example: 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.
Also see step 6 in this table.
Install a valve on the water discharge line and reduce water flow to increase water pressure inside the pump itself.
|5. Excessive or Abnormally HIGH current draw at electric motor||
Running motor amps measurements (current draw):
Check the motor name plate data and note the motor's RLA amps rating. Or if available, obtain from the manufacturer detailed specifications for the motor's operation and during start-up.
LRA tests: (Lock Rotor Amps) Using an ammeter, measure the current draw of the motor during start-up.
Abnormally high current draw at the start of a motor on-cycle can indicate mechanical damage to the motor, a developing short in windings (? citation needed), or a bad start-run capacitor.
Typical motor start-up current draw in amps will be 5-6 times the RLA but very quickly (seconds) the motor current draw should fall back to RLA.
RLA Tests: (Running Load Amps) Using an ammeter, measure the actual current draw of the motor during operation.
While the specific tolerances will vary depending on motor, motor design, and application, if the current draw measured in Amps is significantly higher than the manufacturer's specifications (typically RLA or the amperage expected when the motor is running under load) the motor may be failing or its driven parts may be failing .
Don't get fooled:
Bad crankcase heater: Attempting to start a failing or failed air conditioner/heat pump compressor motor can draw very high current.
But a failed crankcase heater on a heat pump compressor motor trying to start in cold weather may deliver similar symptoms.
A faulty overload safety relay control on a motor (not present on all motors) can shut down a motor that is working normally, fooling you into thinking that the motor is drawing excessive current.
Excessive LRA: if present, try replacing the start-run capacitor.
For air conditioners & heat pumps see BURNED-OUT COMPRESSOR
For heat pumps, also see CRANKCASE HEATERS
Excessive RLA: Further investigation may be needed by direct examination of the motor and the mechanical components that it is driving.
|6. Abnormally LOW current or Amps draw at an electric motor||
Low amperage draw may indicate internal motor wear or more likely, worn parts mechanically driven by the electric motor
Example 1: in testing an air conditioner or heat pump compressor motor, unlike a high-amp current draw which indicates that the compressor is danged internally in a way that its piston(s) is (are) tight in the cylinder, a low-amp current draw, if below normal, may confirm internal wear on the compressor parts, and would support the diagnosis that the compressor is worn and inefficient.
Example 2: a well pump or heating circulator pump or oil burner motor whose coupling to the mechanically-driven components has broken completely may draw low current
Example 3: a well pump or water pump that is running "dry" (no water in the well) will draw abnormally low current.
Where there are no gauge ports to actually measure compressor low side and high side vacuum and refrigerant pressures, this simple electrical test is a useful first step.
|7. Bad or jammed internal TEFC electric motor cooling FAN||
TEFC electric motors use a totally-enclosed fan for cooling. The fan blades are behind metal guard on the back of the motor. The fan itself should be securely fastened to the shaft, should not wobble nor be bent or damaged, and the cooling air inlet openings should not be clogged. - WikiHow 
Note: you won't find this design on residential air conditioners, heat pumps, well pumps, nor most other home appliances. TEFC motors have external cooling fins that are quite distinctive.
If the motor has not already failed, clean the motor fan and fan air inlet openings if clogged and observe whether or not this solves a motor overheating problem.
If the motor has not already failed, replace the fan assembly if it is bent, damaged, jammed, will not spin freely - or replace the entire motor assembly.
|8. NOISY electric motor||
Various possible defects could cause an electric motor to jam or bind, such as loose mounts, bad bearings, loose couplings, defects in the driven-assembly or part.
If pressing the reset button starts the motor but it runs hot or noisy see the motor noise diagnostic article at right.
Check for high current draw in a binding motor.
Adapted from various sources including Betta-Flo Jet Pump Installation Manual from the National Pump Co. and Adapted & expanded from Beckett (1989)
See also ELECTRIC MOTOR OVERLOAD RESET SWITCH
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Questions and answers about selecting, installing, diagnosing or fixing electric motors that were posted originally on this page are now found at ELECTRIC MOTOR DIAGNOSTIC FAQs and also ELECTRIC MOTOR DIAGNOSTIC FAQs-2 and also ELECTRIC MOTOR DIAGNOSTIC FAQs-3
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