Electric motor test & repair guide:
This 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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In this article we provide a diagnostic guide to determine and repair problems with electric motors. 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.
At left our photo illustrates the motor as typically found in a direct-drive HVAC blower or air handler assembly. (BLOWER FAN OPERATION & TESTING)
[Click to enlarge any image.]
While our page top photo shows the red reset button most clearly, the reset button on the motor at left may be harder to spot. Sometimes the reset button on an electric motor is hard to find, and sometimes there is no reset button!
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.
Before discussing how to diagnose air conditioner or heating system electric motors let's be sure we know what motor parts might be involved. (Or skip right to Table A if you prefer).
The electric motor has quite a few parts if examined in detail, switches, wires, possibly capacitors, oiling ports and more, but there are four basic parts to every HVAC electric motor:
In addition to the basic electric motor components above there are two other features to know about when troubleshooting a motor.
Details for this topic have moved to ELECTRIC MOTOR RUN DIRECTION.
In short: check the motor label: uni-directional electric motors run just one way: clockwise (CW) or counterclockwise (CCW) but not both. Bi-directional & self-reversing electric motors run in either direction, CW or CCW. Some electric motors can start and run "backwards" following damage to the motor's start capacitor or windings.
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 the words CCW ROTATION (red arrow).
If you enlarge the photo [Click any image to see an enlarged, detailed version] you will see text above the red arrow noting that this is a NON-REVERSING motor.
See ELECTRIC MOTOR RUN DIRECTION
The blue oval marks the motor's rotating speed - 3450 RPM - this is a high speed oil burner. Older oil burners and equipment motors run at 1725 RPM. Some HVAC equipment uses a variable-speed electric motor.
The green rectangle marks other useful data in the data tag for this motor, made by Emerson Electric in St. Louis MO. This is a 1/7 hp motor, designed for 115VAC, drawing 2.35A. These data are helpful when diagnosing electric motor problems: using a DMM or VOM we can detect unusual current draw above that 2.35A as a sign of trouble and we can check that the voltage level delivered by the electrical supply is close to 115VAC 60 cycle current single phase.
The motor's model number (SD55GYJTK-5181 in this example) is useful when replacing the motor or contacting the manufacturer for assistance.
An Electric Motor Time Rating designation is specified as CONT (continuous duty) - this motor is able to run continuously without damage or overheating under normal conditions.
A temperature rating (40C) and other data are given as well, including an explanation that this motor is. thermally protected and that should the motor's thermal protection switch trip off the user needs to press the red button.
See ELECTRIC MOTOR OVERLOAD RESET
This motor's data tag also includes oiling specifications indicating the required lubrication schedule, discussed
at ELECTRIC MOTOR LUBRICATION
Watch out: when buying replacement electric motors, fuel units, and blower fan assemblies to be sure they all are compatible. For example on oil fired heating equipment, the oil burner fuel units (the mechanical heating oil pump driven by the oil burner electric motor via a coupling) can be purchased as CW or CCW devices. All three components have to be designed to rotate in a common direction:
If the fuel unit is not rotated in the proper direction the heating appliance won't run - it won't receive fuel, and the driving motor and coupling parts may be damaged.
If a squirrel cage blower fan on an oil burner or inside of an air handler is spun backwards it will not move much air and equipment will not function properly.
See ELECTRIC MOTOR RUN DIRECTION
For article loading speed we have moved this data t
o ELECTRIC MOTOR LUBRICATION
at ELECTRIC MOTOR OVERLOAD RESET - separate article.
The start switch connects power to the start winding to start the motor spinning. This feature is necessary because depending on the position in which the rotor stopped when the motor last turned off, the rotating electrical field created by the run winding can't start the motor.
At CAUSES of HARD STARTING ELECTRIC MOTORS we explain how a failed starting capacitor OR depending on the motor design, a bad centrifugal switch can prevent a motor from starting.
A trained service technician may sometimes diagnose a failed start winding or failed start switch (centrifugal switch) by spinning the motor manually (potentially dangerous!). If the motor keeps running we suspect a bad start winding or bad start switch (see diagnostic table details
at Table A: 14 THINGS to CHECK (in order) if an A/C Electric Motor Will Not Start.
When the electric motor has reached about 75-80% of its full speed the centrifugal switch opens, thereby disconnecting AC electrical power from the start winding. Power was already connected to and remains connected to the run winding.
So if the motor will start but won't keep running, we suspect a bad run winding or bad wiring to the winding.
For electric motors used in most HVAC applications motor full speed is usually 1725 or 3450 rpm, though some equipment may use variable speed motors as well. The centrifugal switch will open ("throwout") at about 2800 rpm for a 3450 rpm electric motor, and the centrifugal switch will open at about 1400 rpm for a 1725 rpm electric motor.
Table A: 14 Things to Check (in this order) if an A/C Electric Motor Will Not Start
This table describes "offline" - power off, disconnected - inspections and tests used to diagnose electric motor problems & failures and also limited Online tests (online MCA) such as measuring current and voltage properties when a motor is running. 
Using a well pump motor as an example, most of these troubleshooting tips pertain to other electric motor applications in buildings too such as in an air conditioning air handler blower compartment. Note that DC motors have different operating properties, so while some of the test procedures listed here will help troubleshoot a DC electric motor, other test procedures such as resistance measurements and distinctions between brushless and brush type electric motors may be different. Some of the electric motor troubleshooting suggestions in this list pertinent to well pump problems are from Betta-Flo Jet Pump Installation Manual, National Pump Co. 
Electric motor repair general note: on HVAC equipment the electric motor is not normally field repaired. The motor is replaced as a non-serviceable item. However in the hands of an expert, most electric motors can indeed be repaired.
|Motor Trouble Cause||Diagnostic Procedure - Offline Motor Circuit Analysis (MCA)||Repair Procedure|
|1: Electrical Power is Off to the electric motor or system||
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.
|2: Blown fuse, tripped breaker on electric motor circuit||
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
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
|3. Tripped thermal overload reset switch on electric motor||Ambient temperature may be too high, or other failure conditions (defective control switch, loss of well water for well pumps, binding driven mechanical components, low voltage) can cause the motor to run too hot or too long leading to overheating.||
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
|4: Low voltage to the electric motor||
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.
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.
|5. Lost phase in power supply to a multi-phase motor||
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.
|6: Loose, improper, or broken electric motor hook-up wire||
Check wiring against the motor installation manual diagram, check all connections to the motor for tightness, shorts, burns, damage
|Rewire or repair or replace wiring|
|7: Bad electric motor control switch||
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
For water pumps, adjust or replace the pressure control switch.
Temporary emergency repair by cleaning the switch contacts may be possible.
|8: Bad electric motor control switch or control sensor||
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.
|9: Bad mechanical parts being turned by the electric motor - e.g. blower assembly or well water pump impeller||
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:
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
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.
11: 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.
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.
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.
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.
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
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
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.
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.
|12: Electric motor is shorted out, jammed, burned out, or defective||
Fuse blows or breaker trips as soon as the motor tries to turn on. Also see the electrical diagnostic suggestions in row 10 above.
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
|13: Bad or incorrect starter coil on electric motor||Wrong starter coil for the power supply can prevent the motor from starting.||Replace the coil or motor|
|14: Bad electric motor centrifugal switch||
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 
Replace the centrifugal switch assembly if the switch is damaged or not moving freely.
|15: 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. A bad cooling fan on an electric motor or clogs that prevent cooling of the motor can lead to motor failure. - WikiHow 
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.
|16. Noisy electric motor||
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.
Some of the electric motor troubleshooting suggestions in this list are adapted from and can be found at the Betta-Flo Jet Pump Installation Manual from the National Pump Co.
Also see Electric Motor Startup Ddiagnosis flowchart, Koldwater Training Software, retrieved 2016/11/06, Tel: 1-573-547-5630 Email: support @ koldwater.comoriginal source: http://koldwater.com/electricmotortroubleshooting.html
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.|
|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
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:
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.
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
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)|
Table of Air Conditioning or Heating System Electric Motor Troubleshooting Procedures for a Motor that is Noisy
For document loading speed we moved this data.
Please see separate article: ELECTRIC MOTOR NOISE DIAGNOSIS
Reader Question: 12/22/2014 sparks when motor is running said:
When a table saw is turned on I see sparks... what is the cause and how to fix it
Some sparking is normal within many electric motors including table saws and drills, but no sparks ought to be seen exiting the device or its motor for obvious safety reasons.
Typically when I've seen sparks spraying out of a drill or table saw I've found that the motor brushes need replacement. A burned commutator in the electric motor is often the root problem. Some light cleaning of the commutator may be needed. Take care not to score it. Remove the motor armature and sand it with very fine emery paper. Inspect the amature while cleaning it
Example of an electric motor test: 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.
Most single phase motors have two sets of windings.
The main or primary windings are directly connected to the power lines while the motor in running. The second windings are usually thinner wires physically offset from the main windings inside the motor.
The purpose of the secondary windings is to provide directional information and an initial strong kick to get the motor started turning. Once the motor is started, the main or primary windings can keep the motor running just fine.
Less common, these secondary windings are directly powered from the power lines through a run capacitor that provides a continuous time or phase shift to the windings.
Also see HOW to CHOOSE a START / RUN CAPACITOR
Far more common, the secondary windings and capacitor are powered through a centrifugal switch that is closed for approximately 1/2 second on starting. As the motor gets up to 2/3 speed, the centrifugal switch opens and disconnects the secondary windings.
This switch is usually behind and part of the connection plate where you attach the power cables in the end of the motor. Any little piece of dust can keep this switch from closing when the motor is stopped. At this point, the motor just sits there and hums, not knowing which way to go or how to get started. Simply knocking this one little dust particle off makes the repair, and the motor will run fine henceforth. Most of the time you have to pull the back end of the motor off to get to this switch.
Reversing the direction of the current in the secondary windings reverses the direction of the motor. This can usually be done by switching the leads to the secondary windings. Sometimes one of the secondary leads is attached to a primary windings and you have to dig into the windings to find it and separate it out.
You can buy electronic phase converters to generate a third leg which work reasonably well, but generally works for just one electric motor at a time. The converter must be somewhat closely matched to the motor in size. It converts AC to DC, then creates an artificial third leg electronically. This approach is about 80-85% effective in that you need to derate the driven motor by 15% or so.
You can use any 3 phase motor to generate the third leg if you can get it started. I have read about people using a small single phase motor to get a bigger 3 phase motor going, or using a pull rope wrapped around the motor shaft to pull start the big 3 phase motor.
Once running, a 3 phase motor will generate a third leg which you can use to run other 3 phase motors. Y wound motors work much better than a Delta wound motors in this approach. This is about 65-70% effective.
I think you can use a capacitor to the third leg much like single phase motors to generate enough of a phase offset to get the 3 phase motor started. The shocked client may have used the 3 phase motor to run the compressor directly. He probably needed a good unloader to prevent the motor from getting bogged down by the compressor until the motor got up to speed. The center of the Y windings may have been attached to the neutral or ground wires, but probably would be shifted towards the third leg.
You can buy commercial rotoconveters designed to generate 3 phase from 2 phase. They look like a motor with no output shaft and have an attached big box containing a batch of capacitors. The rotating armature of a 3 phase motor on its own will generate a weak third leg that will probably be somewhat phase shifted towards the 2nd leg.
The capacitors are attached between the first and third leg, which will generate a leading current towards the first leg, somewhat offsetting the shift towards the second leg. Adding a few more turns would generate a larger voltage output from the generated third leg. I have never taken a rotoconverter apart, so this is just a good guess. Rotoconverters make a very clean 3rd phase from single phase , approaching is 90-95% effectiveness.
You can run multiple 3 phase motors of different sizes simultaneously. The motors actually help each other. Rotoconverters are somewhat noisy, maybe two-three times that of an equivalent motor. Otherwise they work very well. I have use a rotoconverter for 20 years to run my mill and metal lathe. I am running a 5 hp lathe on a 3 hp converter and have never managed to slow it down. Rotoconverters are often used oil companys to run remote oil extraction pumps
It is cheaper to buy the converter than to pay for the miles of copper for the third leg from the power company. - 2017/03/31 from Ozzie
Also see LOST NEUTRAL SHOCKS HOMEOWNER - a 3-phase motor "converted" to single phase becomes unsafe
Continue reading at ELECTRIC MOTOR OVERLOAD RESET or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see ELECTRIC MOTOR DIAGNOSTIC FAQs-2 - more questions & answers about fixing electric motors
Or see ELECTRIC MOTOR HORSEPOWER
Or see ELECTRIC MOTOR RUN DIRECTION
Or see these
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This article describes a lot of diagnostic test procedures for A/C electric motors such as those found in air conditioners, heat pumps, washing machines, clothes dryers, condensate pumps, well pumps, even garage door openers. But what about direct current motors such as used by hobbyists and in some automotive or boat applications?
Hobbyist LB Miller has described the design and function of a simple test fixture useful for determining the electrical resistance of DC motors by providing a 1A current to the motor and measuring the voltage drop across the motor, thus giving motor resistance in milli-ohms. Note that his approach is for DC motors. 
(Feb 18, 2014) Anonymous said:
I have a Bosche table saw and the motor has malfunctioned. It is a 120volt motor. When switched on it "hums" but does not spin. What could be the cause of this? Is it repairable?
I would remove the motor from the saw completely - for safer testing.
With the motor out of the saw, see if you can rotate the shaft by hand.
If you can then the shaft is (probably) not bound and the problem may be a bad start capacitor (if there is one). There are also some simple tests in the article above that you might want to try when the motor is out of the saw.
(Apr 7, 2014) Jan said:
My furnace 1/2 hp blower motor starts and runs quietly from cold but will not start and makes a loud buzzing sound when hot until the thermal breaker kicks in. The motor is hot to the touch when this happens. It will go through one heating cycle, turn off and not restart. BTW the motor is only 1 yr old.
Buzzing is often a bad relay, switch, or control board. Hot to touch motor sounds like a failing motor or motor start capacitor, possibly showing up when the system is hot. It's time for a call to your heating service tech. Let us know what s/he finds as it will surely help others.
Sometimes a hot electric motor won't start because the motor shaft is binding in the bearing.
Sometimes a hot electric motor won't start because at the higher temperature a winding or electrical connection has opened.
(May 14, 2014) Jaime (from Huntington WV) said:
Hi! In 2010 we replaced our outside central air unit, without replacing the furnace inside. The forced gas heating inside works great for heat in the winter, but in the summer when it gets humid, or newer AC in combination with our older fan/motor inside the house do not work well together. I have very cold air coming out, but by the end of the day when the thermostat is set on 69, it just keeps going up and up to about 71.
From what I have been told, the humidity makes the air thicker which in turn the old fan/motor in the furnace part can't handle the cool air from the new AC, which in turn my duct work under the house sweats terrible. Can I replace the motor in the house with a newer/more powerful motor without replacing the entire furnace inside? I hope you understand what I meaning. =) Thank you! Email me or comment back BillandJaime2004@gmail.com
would look for an operating problem, possibly improper refrigerant charge, or dirty filter, that is causing the coil to freeze - that could explain the reduction in cooling capacity as the day wears on. Look for frost or ice on the cooling coil.
Another direction of diagnosis: if the system is oversized then it will cool the home too quickly, failing to adequately dehumidify.
(June 16, 2014) Robert holt said:
Hi. I have a cooker hood motor that hums when fan speed is selected but does not turn. I have taken it out from the impeller casing, and if I give ithe shaft a quick turn whilst power is on, the motor spins, but as soon as turned off, does not restart on button. Any help please
It sounds as if the fan motor needs a new start or start-run capacitor.
(July 23, 2014) B said:
Hey guys - I'm having a problem with the blower motor in my carrier air handler. I keep it oiled regularly every yr to 2 yrs as equipment suggests. Started squeaking on me when we turned our ac on this summer. After cleaning / lubricating the motor, restarted ac and it ran fine for 4-6 hours then wouldn't not kick on when tstat showed a higher inside temp then set at. I check ed the motor and all was clear and well lubed and restarted.
Ran OK for a period then same issue. Motor became noisy again and will only run intermittently and then get noisy and shut itself down. Is this a case of bad bearings / windings in the motor or could it be a wiring issue? Any helps would be great. Thanks in advance.
Check first that the blower assembly itself is spinning freely, not binding, and doesn't have worn bearings.
Then consider that the motor may be failing internally, possibly a bad bushing or bearing.
You might extend its life with a hard start capacitor kit.
(July 23, 2014) Jeff Wilmsmeier said:
Can you oil a Emerson K55hxkwa-9803
Jeff I had trouble finding a manual for this fan motor online too - so I don't know the answer.
Look closely at the motor assembly. Oil ports will be obvious, often capped with a hinged cap or a hole facing up marked "OIL" on the steel body of the motor.
Or send us some sharp photos of all sides of the motor and we'll comment further.
See ELECTRIC MOTOR LUBRICATION for detailed advice
(Aug 22, 2014) Blower fan internal heat switch cuts motor after 30 min. said:
Had an HVAC CO provide a free inspection. A/C was working fine for prior 1 yr. Tech added refridg. and change dirty filter in attic. Now Blower Fan only runs 30 min and cuts off. Back on in 20 min. After 4-5 more visits, Tech wanted to sell me a new A/c at $8-$10,000. Found bent blower door and improper/wrong filter. Also, refridg. was 2 lbs low so none had been added by Tech.
What type of Blower Fan filter provides "0" airflow resistance...?
Thank very much,
New motor, cage bearings oiled and spinning freely. New fan belt. I removed the "new" filter and motor is now running cool enough that it is not cutting off from overheating. So I guess I am looking for a filter with no resistance.. any thoughts?
I'm surprised that a tech would add refrigerant if the system was working properly - though we should keep in mind that a skilled HVACR tech may still not be an English major - so may not have explained what s/he was doing or why. Now I'm not sure what's wrong, but that offer of a costly new system is disturbing.
Ask to be shown and told what expensive parts are shot and how that diagnosis was made.
Then call a second HVAC company and ask for a second opinion before we accuse anyone of something further. If your system is an old one it might be at end of life, but working up until the time the tech touched the system is indeed worrisome.
A filter that gave zero air resistance would probably also not filter air passing through it.
A very very low air flow resistance air cleaner that is widely used is an electrostatic air cleaner. However usually we see these used in combination with a more conventional particle filter.
Kudos on getting the new motor going.
There are no zero-resistance air filters - since filtering will always create some resistance. But filters are rated for flow resistance. About the lowest flow resistance type is an electrostatic air cleaner. Typically those are preceded by a washable filter and some by a low-resistance fiberglass or other paper filter.
Take a look at inspectapedia.com/aircond/Best_Air_Filter.php
(Aug 30, 2014) Mike said:
Blower motor in Nordyne heat pump will not turn on. I have tried overriding thermostat but still won't kick on. I checked for voltage at transformer (240v-28vac) and looked good. I wanted to know how to test the blower motor relay. I thought I should see ~240vac across black and white wires of motor.
Check the motor data plate - it more likely is a 120V unit.
A 28VAC transformer output is more likely being used to operate the blower's controls.
(July 18, 2015) Allen said:
Thanks for your prompt reply. After checking the voltage, the main and start windings are within specs. I did however receive a reading suggesting the motor is grounded. Could a damaged wire in the well casing be causing the symptoms?
Double-faults can occur but Occam's razor says we should look first for the simplest explanation as it's most likely.
Is it possible that during the repair a pipe connection simply was not perfectly tightened. For example I've found air leaks into piping (and thus potentially loss of prime) when a hose clamp on plastic piping at joints was not quite tight.
A problem is that air leaking in to the lines may not be so easy to detect unless your system has a clear plastic water filter in which you'd see bubbling and frothing. But you might start by tightening or double-hose-clamping every accessible connection.
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