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BOOKSTORE - ELECTRICAL
BUILDING SAFETY HAZARDS GUIDE
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CIRCUIT BREAKER SIZE for A/C or HEAT PUMP
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CORROSION in ELECTRICAL PANELS
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CUTLER HAMMER PANEL FIRE
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DIRECTORY OF ELECTRICIANS
DMM Digital Multimeter HOW TO USE
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ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
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HEATING COST FUEL & BTU Cost Table
HEAT TAPE USAGE GUIDE
Hertz - Definitions of KHz MHz GHz THz
KNOB & TUBE WIRING
LIGHTING, EXTERIOR GUIDE
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LIGHTNING PROTECTION SYSTEMS
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
MAIN ELECTRICAL DISCONNECT
MAIN DISCONNECT AMPACITY
MOISTURE SOURCES in PANELS
MURRAY SIEMENS Recall
PHOTOVOLTAIC POWER SYSTEMS
PUSHMATIC - BULLDOG PANELS
REMOTE ELECTRIC POWER, PHOTOVOLTAIC
RUST in ELECTRICAL PANELS
SAFETY for ELECTRICAL INSPECTORS
SE CABLE SIZES vs AMPS
SIEMENS MURRAY Recall
THERMAL EXPANSION of HOT WATER
THERMAL EXPANSION of MATERIALS
UNDERGROUND SERVICE LATERALS
VOLTS / AMPS MEASUREMENT EQUIP
VOLTAGE MEASUREMENT METHODS
WIND ENERGY SYSTEMS
WIND TURBINES & LIGHTNING
ZINSCO SYLVANIA ELECTRICAL PANELS
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.
What Are the 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).
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.
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.
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. (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. (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 (ELECTRIC MOTOR RUN DIRECTION) it will not move much air and equipment will not function properly.
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.
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.
A trained service technician may sometimes diagnose a failed start winding or failed start 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 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 Electric Motor Troubleshooting Procedures for a Motor that Will Not Start
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
|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||
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.
|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 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. 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.
|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
See these electrical test tool & equipment articles
Example: 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.
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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. 
Questions & answers or comments 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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