Electric Motor Won't Start Diagnosis

Troubleshoot Electric Motors

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 ) - see step 3 below.

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 RATES

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 RATES 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.

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.

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

Detailed diagnosics for overheating motors are at ELECTRIC MOTOR OVERHEATS

Check the line voltage at the motor with VOM or DMM motor or its 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. Typically an electric motor requires that voltage remain within 10% of the voltage rating given on the motor's data tag.

Frequent operation at low voltage can damage some motors.

AMPS MEASUREMENT METHODS

DMM DIGITAL MULTIMETER HOW TO USE

VOLTS / AMPS MEASUREMENT EQUIP

TEST EQUIPMENT, ELECTRICAL GUIDE

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.

If the motor is a three phase unit check that power is being delivered on all power wires.

Note: some electric motors designed for multiple phase power will run on a single phase but run hot and at reduced power.

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 motors having trouble starting see

CAUSES of HARD STARTING ELECTRIC MOTORS

where we explain how a failed starting capacitor OR depending on the motor design, a bad centrifugal switch can prevent a motor from starting.

See ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD

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.

1. Failing electric motor bearings or motor shaft - the motor turns but runs hot, noisy, with vibration, and/or has trouble starting. The motor may show these symptoms:

motor noise

hot or overheated motor bearings

vibrations at the motor may indicate failing bearings or drive shaft damage

sparking at the brushes (not all motors have this component)

ultimately a seized motor drive shaft

See if you can move the motor shaft.

2. If the electric motor shaft won't turn at all, the motor has bad bearings, dirt in the bearing or slipring, the motor has otherwise become jammed or damaged

3. 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

10: Bad electric motor starting capacitor

Or

Bad Run Capacitor

Single-phase electric motor hums but won't start on its own.

In some cases giving the motor a spin will enable it to start and keep running.

Use a VOM in ohms setting to check resistance across the capacitor.

When the motor is switched "on" the ohms reading should immediately drop to zero then slowly climb again towards infinite resistance.

If the meter does not move (no current flows) the capacitor is "open".

If there is very low or zero resistance the capacitor is shorted.

Note: not all electric motors use a starting capacitor. Multi or three-phase motors won't have a starting capacitor.

Replace the starting capacitor.

See TEST a MOTOR START or RUN CAPACITOR how-to

Also see HARD STARTING COMPRESSOR MOTORS

See CAPACITORS for HARD STARTING MOTORS

See VOLTS / AMPS MEASUREMENT EQUIP

See HUMMING MOTOR SOUNDS

11: Resistance or Ohms Tests

Bad electric motor internal winding, 

Bad or sparking brushes

Other internal wiring damage

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 for ELECTRICAL INSPECTORS

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.

When the motor is running, (or was running) did you see sparking at the brushes?

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.

Electric Motor Winding Tests

Watch out: First turn off electrical power & then remove all external wires connected to the motor.

1.a. Shorted motor windings: Check the motor for shorted 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.

1.a.1 Motor windings shorted internally: With the VOM DMM set to measure resistance (Ohms) check for continuity between the motor treminals.

Make a note of the resistance that you read. You will need the motor brand, model, possibly serial number, and then with that data you will need to check with the motor manufacturer to see what is the proper resistance across the motor windings.

Zero resistance or infinite resistance across the motor windings are easy to diagnose:

1.a.2. Motor windings shorted to ground: 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. If a motor winding has broken or is "open" there will be no electrical continuity across the winding wires.

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:

Continuity checks on motor windings: First remove all external wires connected to the motor. Then check for continuity between the motor terminals 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.)

If you see infinite resistance then the winding is definitely "open" or "broken".

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.

Normal electric motor winding resistance:

At BASIC ELECTRICAL TESTS for BURNED OUT COMPRESSOR MOTORS

we discuss Ohms measurements across the windings terminals of a good electric motor such as is used in an air conditioner or heat pump system where you should see approximately:

Pin 1 to Pin 2 – 3 ohms, Pin 2 to Pin 3 – 6 ohms, Pin 1 to Pin 3 – 9 ohms

The lowest electrical resistance reading should be below 10 ohms (this is the electric motor run winding).

The second highest reading should be about 2-4 times higher than the lowest reading (this is the motor start winding).

The highest reading should be the sum of the resistance read across the two smaller motor windings

Watch out: a motor winding wire may be damaged but may "open" or "break" only when the motor is trying to spin or only when the motor gets up to full speed.

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

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

For sparks at the electric motor see SPARKING ELECTRIC MOTOR

See these electric motor diagnostic articles

BASIC ELECTRICAL TESTS for BURNED OUT COMPRESSOR MOTORS

BURNED-OUT COMPRESSOR

CAPACITORS for HARD STARTING MOTORS

COMPRESSOR / CONDENSER DIAGNOSTICS

TIGHT or SEIZED AC COMPRESSORS

3. Sparking electric motor brushes: if the motor uses brushes and you see a shower of sparks from the brush/armature area when the motor is running the cause may be

Worn brushes: some electric motors have replaceable brushes. If the brush is worn down to a short stub or such that its spring is contacting the rotor, that could be the trouble.

To inspect the brush(es), remove the setscrew covering the brush assembly and remove each brush (typically there are at least 2). Replace worn or damaged brushes with a matching brush unit.

Brushes are worn unevenly and are jamming in their moving sleeve; replace the brushes and check that the brush moves freely in the sleeve. You may need to clean the sleeve opening. I use a q-tip.

A worn or broken brush spring can also cause brushes to jam.

A worn, corroded, or broken slip ring can cause sparking at the brushes

An overloaded electric motor can result in rapid rush wear, burning, sparking

A damaged rotor that is gouged, has worn shaft ends, or has lost a balancing weight (on larger electric motors) will vibrate and can cause brush wear and sparking; that rotor will need to be balanced or repaired.

Inspect the brushes for wear, damage, sticking, bad springs.

Inspect the rotor for vibration, lost weight, out of balance, damage.

Check the motor capacity against its load.

See SPARKING ELECTRIC MOTOR

4. Sparking electric motor due to burned or scored armature assembly

If you see sparks spewing from the electric motor (we often see this on electric drills and circular saws) the motor brushes are probably being damaged by a burned, scored commutator.

Remove the part, clean and remove scores, replace the motor brushes.

See SPARKING ELECTRIC MOTOR

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:

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 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]

See

ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD

See
CAUSES of HARD STARTING ELECTRIC MOTORS

where we explain how a failed starting capacitor

OR depending on the motor design, a bad centrifugal switch can prevent a motor from starting.

Replace the centrifugal switch assembly if the switch is damaged or not moving freely.

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.

Various possible causes of electric motor noise include: 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 - see the amps measurement methods article at right.

If the motor hums and is not starting, turn it off right away and see the causes of electric motor not starting listed earlier in this table.

See AMPS MEASUREMENT METHODS

RESET BUTTON, ELECTRIC MOTOR

ELECTRIC MOTOR NOISE DIAGNOSIS

HUMMING MOTOR SOUNDS