How to Diagnose & Repair Electric Motors on Building HVAC Equipment
     

• ELECTRIC MOTOR DIAGNOSTIC GUIDE - Electric Motor Troubleshooting Guide - Diagnostic Table - how to troubleshoot and fix electric motors
  • Components of an A/C Electric Motor
  • Electric Motor Lubrication Specifications
  • Electric motor start switch operation
• Table A: 14 Things to Check (in order) if an A/C Electric Motor Will Not Start
• Table B: 6 Things to Check if an Electric Motor Starts but Overheats and Trips its Reset Button
• Table C: 7 Things to Check if an Electric Motor is Noisy - separate article
• General advice: Electrical Tests to Check HVAC Blower Fan Motor or Outdoor Compressor Fan Motor Winding on Heating or Cooling Equipment or on Other Electrical Motors
• CAPACITORS for HARD STARTING MOTORS - separate article
• ELECTRIC MOTOR OVERLOAD RESET - separate article
• ELECTRICAL TOOLS BASIC - separate article
• TEST EQUIPMENT, ELECTRICAL GUIDE - separate article
• Questions & Answers 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
• References

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Uni-directional electric motors run just one way: clockwise (CW) or counterclockwise (CCW) but not both.

In many applications the equipment driven by the electric motor will not work properly unless the motor drive shaft spins in a pre-determined direction: clockwise or counter-clockwise.

Examples include HVAC unit blower squirrel cage fans whose blades won't move air if the fan runs backwards and oil burner or well pump motors whose oil pump or water pump won't move oil or water if the pump motor is driven in the "wrong" direction.

In a fixed-direction electric motor such as on an HVAC blower fan or an A/C or heat pump compressor, each time the motor starts its start capacitor and start winding give the motor a "kick" in the right direction.

Watch out: it is indeed possible for some electric motors to run "backwards" following damage to the motor's start capacitor or windings. We've had an occasional report of an HVAC motor running "the wrong way" or sometimes starting to run the the wrong way.

Bi-directional & self-reversing electric motors run in either direction, CW or CCW

Bi-directional electric motors run in either direction and are used in applications in which the direction in which the motor spins doesn't matter to the equipment it's driving. "What the heck?" you're saying. Well here's a cute example - your home microwave.

Electric motors require a kick or to put it more like an engineer, a starting torque to overcome the resistance when the motor is to begin spinning from a stopped position. Compared to electric motors used in oil burners and air conditioning compressors, we don't need such a costly and heavy-duty electric motor to spin the rotating table in a microwave oven.

Instead, microwave (and some other self-reversing) electric motors use a clever design that allows for elimination of the start winding winding and also allows use of a lower-cost and less powerful electric motor.

When the motor has stopped the mechanism it drives remains pressed against the motor's drive shaft, providing a "driven load" ( the microwave turntable) against the direction the motor's shaft was last rotating. When the motor is asked to start again it encounters this extra load due to the resistance provided by a mechanical mechanism in the turntable that was itself "wound" or loaded by the table's prior rotation.

The microwave motor, encountering this load, reverses itself to begin rotating in the opposite direction, one of less load, which gives a brief "no-load" interval that lets the microwave's turntable motor get up to speed. In sum, the combination of a load or rotation resistance provided by the turntable mechanism and an automatic direction reversing feature in the electric motor gets the microwave motor spinning and the turntable rotating - which is why each time you start your microwave it is likely to rotate in the opposite direction from that of its previous cycle.

Note that oil burner fuel units can be purchased as CW or CCW devices. All three components have to be designed to rotate in a common direction: the electric driving motor, the oil burner combustion air blower fan assembly, and the oil burner fuel unit or oil pump.

In our photo at left you can see the notation on this electric motor data tag indicating the the motor is non-reversing and rotates counter-clockwise - designated by that CCW (red arrow).

The blue oval marks the motor's rotating speed - 3450 RPM - this is a high speed oil burner.

The green circle marks other useful data in the data tag for this motor, made by Emerson Electric: it's a 1/7 hp motor, designed for 115VAC, drawing 2.35A. A temperature rating (40C) and other data are given as well, including an explanation that this motor is. thermoally protected and that should the motor's thermal protection switch trip off the user needs to press the red button. (ELECTRIC MOTOR OVERLOAD RESET).

This motor's data tag also includes oiling specifications indicating the required lubrication schedule, discussed below.

Air Conditioner, Heat Pump, or Heating System Electric Motor Lubrication Schedule

Notes:

How much oil is required when lubricating an electric motor? The answer is I can't say because the quantity varies according to the motor design.

For the oil burner motor whose data tag is illustrated above, re oil instructions specify that each bearing is to be lubricated with 150 drops (about one teaspoon) of SAE 20 oil.

Lubrication should be with an oil and at frequency specified by the equipment manufacturer. If you don't have this data use the table above. Lubrication typically is with SAE 20 oil, droplets or tube-fed into the motors (usually two) oiling ports.

Watch out: Do not soak the motor in oil, do not spray the motor windings with spray oils like WD40, do not try to lubricate permanently-lubricated motors that don't have an oil port.

Check that all service switches for the equipment are in the "on" position.

Check for voltage at the pump motor or pump controls. If no voltage is found, check for voltage at the electric panel.

If power is on to the building, check fuse or circuit breaker serving the electric motor that won't run.

Check for local reset button on the motor (popped out = off )

Check for other power reset or power off buttons such as an access door compartment safety switch that turns off power to the equipment (found on A/C blower compartment doors)

Turn on "off" switches.

Replace bad fuse. Reset circuit breaker; if necessary replace bad circuit breaker.

Let hot electric motor cool down, then push in its reset button.

Be sure all safety interlock switches such as on compartment doors are depressed and that the doors are securely shut. Replace a bad safety switch.

See
ELECTRICAL POWER SWITCH FOR HEAT
FAN AUTO ON Thermostat Switch
WATER PUMP ELECTRICAL SWITCHES
CIRCUIT BREAKER FAILURE
ELECTRIC PANEL INSPECTION

Replace the fuse or re-set the breaker - does the electric motor now run and keep running normally?

If the fuse/breaker blow/trip problem repeats check for abnormal current draw (Table B), binding mechanical parts, damaged electrical wiring, internal short in motor, seized electric motor (such as an air conditioner compressor).

Be sure proper breaker or fuse size in ampacity is installed - ELECTRIC PANEL INSPECTION

At CIRCUIT BREAKER FAILURE we provide an example of a failure traced to the breaker itself.

At BURNED-OUT COMPRESSOR we explain that the compressor motor itself may be seized

Some motors such as submersible pumps include an automatic self-reset once the motor cools down. Other motors use a manual button that must be reset.

See ELECTRIC MOTOR OVERLOAD RESET SWITCH for how to find and reset this button. In that article we also list the reasons that a motor may have shut down on thermal overload.

At BURNED-OUT COMPRESSOR we describe the diagnostic of cooling down a hot A/C or heat pump compressor with water from a garden hose

Details are in the table below at Things to Check if an Electric Motor Starts but Overheats

Check with VOM or DMM at the pressure control switch or at the motor wiring. Incorrect voltage can prevent motors from starting or may cause slow "weak" electric motor operation or may prevent the motor from starting at all.

Frequent operation at low voltage can damage some motors.

See:
DMM Digital Multimeter, How to Use
DMMs & VOMs, Using Safely
AMPS MEASUREMENT METHODS
VOLTS / AMPS MEASUREMENT EQUIP
TEST EQUIPMENT, ELECTRICAL GUIDE to

Be sure the proper size of wire is used for the ampacity and length of circuit;

Test for low voltage to the building.

Example: At WATER PUMP REPAIR GUIDE we describe weak well pump operation due to low voltage or due to a partial short to ground in the pump wiring.

Check wiring against the motor installation manual diagram, check all connections to the motor for tightness, shorts, burns, damage

Check the control switch contacts for burning or wear.

Example: If the electric motor control is a well pump pressure control switch,check the pressure control switch settings - cut-in and cut-out; inspect for burned, pitted switch contacts or for dirt or wear.

For water pumps, adjust or replace the pressure control switch.

Temporary emergency repair by cleaning the switch contacts may be possible. See WATER PUMP PRESSURE CONTROL

A control that is intended to turn the motor on or off may itself be defective, such as a failed or mis-wired thermostatic control, timer, or pressure control switch.

Try temporarily bypassing the control switch to assure that power is being delivered to the motor.

Clean, repair, or replace the switch.

Example: Clogged or leaky tubing connecting a water pump pressure control switch to the water system results in failure to properly sense and respond to water pressure.

Example: debris clogging can also occur in the bottom of a water pump pressure control switch where it mounts or connects to the tubing.

Check and clear tubing blockage (blow air through tubing). Or install new tubing.

Be sure tubing is proper diameter and type to seal properly with other fittings.

Tighten tubing fittings to be sure there are no water or air leaks. Soap solution may help find air leaks in tubing fittings.

Clear or replace clogged pressure control switch if the bottom sensor opening is clogged and cannot be cleared.

See  
 WATER PUMP PRESSURE CONTROL REPAIR
 PUMP PRESSURE CONTROL REPLACE

Turn off electric power to motor, and disconnect the motor from whatever mechanical assembly it is driving.

See if you can move the motor shaft

1. if the electric motor shaft won't turn the motor has bad bearings or has otherwise become jammed or damaged

2. if the electric motor shaft will turn when disconnected from whatever it is driving, then look for a binding or bearing or damage problem in the driven mechanical parts such as a water pump impeller assembly or a furnace blower fan assembly.

1. For a frozen electric motor itself, replace the motor

2. Remove obstruction in mechanical components, inspect for and replace damaged parts

Replace the starting capacitor.

See CAPACITORS for HARD STARTING MOTORS

10: Bad electric motor internal winding, brushes, or other internal wiring

-

How to distinguish bad electric motor bearings

from bad driven mechanical components

from an electrical problem such as open or shorted electric motor windings

Watch out: To avoid risk of shock or death, turn off electric power to motor, and disconnect the motor completely from any power source as well as disconnecting it physically from whatever mechanical assembly it is driving.

See:
SAFETY: ELECTRICAL INSPECTION SAFETY
DMM Digital Multimeter, How to Use
DMMs & VOMs, Using Safely
AMPS MEASUREMENT METHODS
VOLTS / AMPS MEASUREMENT EQUIP
TEST EQUIPMENT, ELECTRICAL GUIDE to

Visually inspect the motor for evidence of overheating or burning, such as discolored paint, and check for external damage (loose motor mount, broken parts) as well as for dirt or debris that may also enter the motor assembly.

See if you can move the motor shaft

1. Jammed electric motor / bad bearings: If the electric motor shaft won't turn the motor has bad bearings or has otherwise become jammed or damaged.

If the motor turns with difficulty and/or makes grinding or scraping noises there is internal damage to the motor or its bearings. If the motor shaft wobbles the bearings are shot; if the motor shaft will move back and forth (at right angles to the direction of rotation) more than about 1/8" there may also be internal bearing or shaft damage.

1.a. Shorted motor windings: Check the motor for shorted internal windings; typically if the motor windings are shorted to the frame or shorted together the motor will draw very high amps and usually will trip a circuit breaker or blow a fuse when you try to turn it on - Row J in this motor diagnostic table.

- With the VOM/DMM set to highest resistance scale and set to measure resistance (Ohms) if you find that there is zero resistance (continuity) between the each individual motor power lead and the motor case or ground connection (test one lead at a time) then a winding is shorted to ground.

3 phase motor leads are numbered T1, T2, T3 etc.

- If you see very high resistance, say 500,000 ohms or more, the motor may be OK.

1.b. Open motor windings: Many (not all) single phase and 3-phase electric motors such as those used in equipment and appliances found in homes can be tested "across the leads" for an open winding in the motor.

Check the motor for an open winding - a wire has broken inside the motor but has not shorted to the motor case or to ground as follows:

With the VOM or DMM set to its lowest resistance setting and to measure Ohms if you see very low resistance, close to zero but not actually zero, the windings may be OK. If you see higher resistances there is a problem in the motor's windings.(Check the wiring diagram to confirm that the meter is measuring across each winding.)

Watch out: this "static test" of an electric motor's windings can miss a broken wire inside the motor that opens (fails) only when the motor is spinning.

Note: an electric motor may have both open winding and shorted winding damage at the same time - something that can happen if the motor's internal parts are internally damaged e.g. by debris falling into the motor.

2. Jammed external driven parts: If the electric motor shaft will turn freely and without wobbling or scraping when disconnected from whatever it is driving, then look for a binding or bearing or damage problem in the driven mechanical parts such as a water pump impeller assembly or a furnace blower fan assembly.

When an electric motor won't start and we have confirmed that power is being delivered, usually we suspect that the motor windings or the start switch have failed.[1]

Be sure you've checked for thermal overload first.

1. For a frozen electric motor itself, or for a motor with open or shorted internal windings, replace the motor

2. If the mechanically driven components are not jammed or binding, and if the motor does not use a start/run capacitor, or if it uses a start / run capacitor and you have replaced that device without solving the problem, it is possible that an internal wire or winding is open in the motor.

If or restoring power the motor runs, this is most likely the case. Order a new motor and replace the old one the next time that the motor won't start. - thanks to Paul Galow for these notes

Fuse blows or breaker trips as soon as the motor tries to turn on. Also see the electrical diagnostic suggestions in row 10 above.

See:
SAFETY: ELECTRICAL INSPECTION SAFETY
DMM Digital Multimeter, How to Use
DMMs & VOMs, Using Safely
AMPS MEASUREMENT METHODS
VOLTS / AMPS MEASUREMENT EQUIP
TEST EQUIPMENT, ELECTRICAL GUIDE to

If the external wiring is ok (no short circuits) the motor is shorted internally or has suffered internal mechanical damage.

Check that the pump pressure control switch is trying to turn on the pump and that there is voltage at the pump wiring

For electric motors that operate additional mechanical assemblies, such as a well pump motor that drives a pumping impeller assembly, before condemning the electric motor itself, separate the motor from the components that it drives and check for damaged mechanical parts that are binding the motor.

Replace the electric motor or have it repaired and rebuilt by a specialist

Check out the rear bell housing of the motor to see if the motor uses a centrifugal switch to switch the start / run capacitor or other windings in and out of the circuit at a specific RPM.

If a centrifugal switch is present, check that its switch contacts are not welded closed or contaminated with dirt and grease. The switch mechanism should can move freely. - WikiHow [36]

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. A bad cooling fan on an electric motor or clogs that prevent cooling of the motor can lead to motor failure. - WikiHow [36]

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.

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

Use a VOM to check the voltage level at the pressure control switch

See:
DMM Digital Multimeter, How to Use
DMMs & VOMs, Using Safely
AMPS MEASUREMENT METHODS
VOLTS / AMPS MEASUREMENT EQUIP
TEST EQUIPMENT, ELECTRICAL GUIDE to

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.

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.

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.

See WATER PRESSURE PROBLEM DIAGNOSIS TABLE

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

Running motor amps measurements:

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: 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.

RLA Tests: 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 .

Examples: 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.

See:
DMM Digital Multimeter, How to Use
DMMs & VOMs, Using Safely
AMPS MEASUREMENT METHODS
VOLTS / AMPS MEASUREMENT EQUIP
TEST EQUIPMENT, ELECTRICAL GUIDE to

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

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

See:
DMM Digital Multimeter, How to Use
DMMs & VOMs, Using Safely
AMPS MEASUREMENT METHODS
VOLTS / AMPS MEASUREMENT EQUIP
TEST EQUIPMENT, ELECTRICAL GUIDE to

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

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.

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