Photograph of the overload reset button on an electric motorElectric Motor Troubleshooting
How to Diagnose & Repair Electric Motors on Building HVAC Equipment

  • POST a QUESTION or READ FAQs 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

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

A/C Electric Motor Troubleshooting Guide - Test Procedures

Air handler blower assembly(C) Daniel FriedmanIn 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. More about this air handler fan is at BLOWER FAN OPERATION & TESTING.

[Click to enlarge any image.]

Article Series contents

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.

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.

Basic Components of an A/C Electric Motor such as used on heating and air conditioning equipment

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).

Becket oil burner electric motor (C) Daniel FriedmanPhoto: electric motor on a modern oil burner.

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:

  1. Electric motor rotor: the rotor follows (turns in the direction impelled by) the rotating magnetic field and thus spins the motor shaft
  2. Electric motor stator: the stator consists is a device or core containing start and run windings (of copper wire) wound around a central core to create a magnetic field.
  3. Electric motor windings: the two windings are used to create an electrical field in the stator.
    1. Definition of Start winding: in an A/C (alternating current) electric motor electrical current flowing through the start winding is used just to get the motor spinning from a stopped condition.

      The start winding is disconnected, usually by a centrifugal switch, when the motor is up to speed.
    2. Definition of Run winding: in an A/C electric motor the run winding is what keeps the motor spinning once it has started.

      Current flowing through this winding produces a rotating magnetic field in the stator that keeps the motor shaft turning after the start winding has turned off.
  4. Electric motor start switch: a centrifugal switch connects the A/C electrical power to the motor to the start winding on the stator until the motor has reached a speed typically of 75-80% of its full run speed (typically that's1725 rpm or 3450 rpm on newer high-speed oil burners).

In addition to the basic electric motor components above there are two other features to know about when troubleshooting a motor.

Which way does an Electric Motor Run - Can Electric Motors run Backwards? Information found on the electric motor's data tag.

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.

How to Read the Information on an Electric Motor Data Tag

Electric motor data tag for oil burner (C) Daniel FriedmanIn 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.

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.

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

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:

  1. the electric driving motor,
  2. the oil burner combustion air blower fan assembly, and
  3. the oil burner fuel unit or oil pump.

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.


Electric Motor Lubrication Specifications & Schedules: when, how much oil, where to oil

For article loading speed we have moved this data to ELECTRIC MOTOR LUBRICATION

Electric Motor Thermal Overload Switch - the Thermal Switch

Details are at ELECTRIC MOTOR OVERLOAD RESET - separate article.

Electric Motor Start Switch Operation in Electric Motors

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 of Air Conditioning or Heating System or Other Electric Motor Troubleshooting Procedures for a Motor that Will Not Start

Table A: 14 Things to Check (in this order) if an A/C Electric Motor Will Not Start is now found at ELECTRIC MOTOR WON'T START / RUN - 14 Things to Check

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

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 is now found at ELECTRIC MOTOR OVERHEATS

Table C: 7 Things to Check if an Electric Motor is Noisy

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

Sparking Elecric Motor Repair

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

Electrical Tests to Check HVAC Blower Fan Motor or Outdoor Compressor Fan Motor Winding on Heating or Cooling Equipment or on Other Electrical Motors

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.

Repair Article Recommendations by System Type

Working with single phase vs. 3-phase electric motors

Reader comments on single phase & 3 Phase motors

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.

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 (HUMMING MOTOR SOUNDS from A/C or Heat Pump system), 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

Electric Motor Wiring Diagrams, Parts, Resources, Research


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.


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