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Air handler blower assembly(C) Daniel FriedmanElectric Motor Overheat Diagnosis

Overheating electric motor may trip thermal reset button

  • POST a QUESTION or COMMENT about how to troubleshoot electric motors such as air conditioning compressor motors, heating equipment burner or fan motors, swimming pool motors, water well pump motors

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.

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- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Overheating Electric Motor Troubleshooting

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.

Watch out: Never simply replace a burned-out electric motor without also finding and fixing the cause of the burn-out. Otherwise you'll simply be replacing the motor again. And again.

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 find and reset this button

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

See:

AMPS MEASUREMENT METHODS

DMM DIGITAL MULTIMETER HOW TO USE

DMMs & VOMs, Using Safely

VOLTS / AMPS MEASUREMENT EQUIP

SAFETY for ELECTRICAL INSPECTORS

TEST EQUIPMENT, ELECTRICAL GUIDE

 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

Or

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

Worn bearings

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.

See ELECTRIC MOTOR RUN DIRECTION

See ELECTRIC MOTOR OVERLOAD RESET SWITCH
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.

Air conditioner & heat pump motors: check refrigerant level.

An air conditioner or heat pump compressor motor may overheat or even be destroyed by having run with very long on-cycles due to a refrigerant leak and a lost refrigerant charge.

Water pump motors: Install a valve on the water discharge line and reduce water flow to increase water pressure inside the pump itself.

See WATER PRESSURE PROBLEM DIAGNOSIS TABLE

See Air Conditioner Long on cycle - Insufficient Cooling - Loose or Worn Compressors
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.

See:

AMPS MEASUREMENT METHODS

DMM DIGITAL MULTIMETER HOW TO USE

DMMs & VOMs, Using Safely

VOLTS / AMPS MEASUREMENT EQUIP

SAFETY for ELECTRICAL INSPECTORS

TEST EQUIPMENT, ELECTRICAL GUIDE

Excessive LRA: if present, try replacing the start-run capacitor.

See CAPACITORS for HARD STARTING MOTORS.

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.

Examples:
BLOWER FAN OPERATION & TESTING - check for bad blower assembly bearings
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.

See:

AMPS MEASUREMENT METHODS

DMM DIGITAL MULTIMETER HOW TO USE

DMMs & VOMs, Using Safely

VOLTS / AMPS MEASUREMENT EQUIP

SAFETY for ELECTRICAL INSPECTORS

TEST EQUIPMENT, ELECTRICAL GUIDE

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.

See Air Conditioner LONG ON-CYCLE - Insufficient Cooling - Loose or Worn Compressors
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 [36]

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.

AMPS MEASUREMENT METHODS

RESET BUTTON, ELECTRIC MOTOR

ELECTRIC MOTOR NOISE DIAGNOSIS

Notes to the table above

Adapted from various sources including Betta-Flo Jet Pump the National Pump Co. and Adapted & expanded from Beckett (1989) [1]

See also

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-02-10 by (mod) - Fluke's list of most-common causes of electric motor failures

I really wish I could help but with so little information I don't think that even by speculation would be useful. I assume that you checked obvious such as improper voltage being delivered.

But courtesy of Fluke, we have a list of the most-common causes of electric motor failures:

Power Quality Causes of Electric Motor Failure

  1. Transient voltage - from the electrical supply, possibly weather or load- related.
  2. Voltage imbalance - in 3-phase systems
  3. Harmonic Distortion - un-wanted high frequency AC voltages or currents to the windings

Variable Frequency Drive Motor Failure Causes

  1. Reflections on drive output PWM signals [Pulse Width Modulation]

    Excerpt: Variable frequency drives employ a pulse width modulation (PWM) technique to control the output voltage and frequency to a motor.

    Reflections are generated when there is an impedance mismatch between the source and load. Impedance mismatches can occur as a result of improper installation, improper component selection or equipment degradation over time.
  2. Sigman current - stray currents resulting from signal frequency, voltage level, capacitance and inductance in conductors may cause nuisance tripping or overheated windings.
  3. Operational overloads - check for abnormal current draw. Fluke says that 30% of motor failrues are caused by overloading!

    See more at

    WATER PUMP PROTECTION SWITCH

    WATER PUMP VARIABLE FREQUENCY / VARIABLE SPEED DRIVE (VFDs)

Mechanical Causes of Electirc Motor Failures

  1. Misalignment - may be anguar or parallel or compound misalignment
  2. Shaft imbalance - often caused by

    Dirt accumulation
    Missing balance weights
    Manufacturing variations
    Uneven mass in motor windings and other wear-related factors.
  3. Shaft looseness - may be caused by misalignment between the motor's drive shaft and the driven load
  4. Bearing wear, often caused by failure to lubricate the motor properly or by

    A heavier load than designed for

    Inadequate or incorrect lubrication
    Ineffective bearing sealing
    Shaft misalignment
    Incorrect fit
    Normal wear

    Induced shaft voltages

Improper Installation Causes of Electric Motor Failure

  1. Soft foot - rubber or other motor mount components are worn or missing
  2. Pipe Srain - excerpt:

    misalignment or long shaft wobble between a drive motor and the driven equipment. We have seen this with heating circulator pump motors and have gone to sites where there was literaly a pile of "dead" electric motors repeatedly replace on the zone circulator because the installer never noticed that the motor and circulator pump itself were mis-aligned!
  3. Shaft voltage - excerpt:

    When motor shaft voltages exceed the insulating capability of the bearing grease, flashover currents to the outer bearing will occur, thereby causing pitting and grooving to the bearing races.

Related Electric Motor Start Failures

  1. Bad start or run capacitor (this is not necessarily a motor failure; often a failing motor bearing causes hard-start motors)

On 2021-02-09 by Eeng.M

we are facing broken fan issue in our SQ induction motors, we couldn't find out the root causes of these failures...

On 2021-02-09 by Eeng.M

we are facing broken fan issue in our SQ induction motors, we couldn't find out the root causes of these failures...

On 2020-07-13 by (mod) - why did my variable speed Mini-Mill electric motor seize?

Ken

I would start by checking the wiring and the power supply to be sure that the motor is being provided with correct voltage.

If there's not a power or voltage is too I'm concerned that there may be a bad bearing. It says no bad bearing, is it possible that the motor was overloaded in use. Overloading causes about 30% of small electric motor faillures.

If none of those suggest to you that we're on the track of problem it might make sense to take the device or the motor itself to a local motor testing show where the motor will be run and measurements made of its current draw or amps and other features.

Higher on this page we provide an abbreviated version of Fluke's list of causes of motor failiures - be sure to take a loooik.

On 2020-07-13 by len

I just brought a small variable speed Mini Mill for small Engineering Jobs
is was new , ran it for a few hrs , then noticed it started to make a few noises in the Motor, which then lead to motor running hot ,

I let it cool down for a few hrs can back and ran it , for about 10 minutes , it clicked out my overload button , I tried to rotate the motor , but it seems it is now seized.

I brought from Interstate , very happy with use of the Machine but sure what I may have don't or not done.
the seller cannot help me from his end , willing to pay for repairs , any suggestions would be appreciated.

On 2020-02-28 by - by (mod) -

friedman (mod)

Anon

Let's start checking out your overheating electric motor by looking through the common causes listed on the page above. Let me know what you think.

On 2020-02-28 by Anonymous

Hey guys, l have a dc motor but it's heats

On 2019-09-21 by - by (mod) -

friedman (mod)

I'd check the supply voltage, the electrical connections, the controller, and lastly a bad winding.

On 2019-09-21 by Aamir

Yellow phase terminals of my motor always get heated. .the motor is run by frequency drive. .

On 2019-07-22 by - by (mod) -

friedman (mod)

Is it a start problem?

Some fans want to be started at hi speed then cut back.

Why did you try a smaller capacitor?

On 2019-07-21 by Alex

I have bought a Gfc Pedestal fan recently. It becomes overheated at 3 gear (at slow speed), I have replaced the Capacitor to 2.5 uf from 4.5uf. Please, help me solve the problem of overheating at the lowest speed. Thank you.


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