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BOOKSTORE - ELECTRICAL
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Classified CIRCUIT BREAKER WARNING
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CORROSION in ELECTRICAL PANELS
DEFINITIONS of ELECTRICAL TERMS
DIRECTORY OF ELECTRICIANS
DMM Digital Multimeter HOW TO USE
ELECTRIC METERS & METER BASES
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRIC PANEL AMPACITY
ELECTRIC PANEL INSPECTION
ELECTRIC PANEL MOISTURE
Electric Power Frequency Table
EMF RF FIELD & FREQUENCY DEFINITIONS
ELECTRICAL GROUND SYSTEM INSPECTION
ELECTRICAL SERVICE DROP
ELECTRICAL SERVICE ENTRY WIRING
EMF RF FIELD & FREQUENCY DEFINITIONS
FIRE SAFETY Checklist, CPSC
GFCI PROTECTION,Testing GFCIs AFCIs
HEATING COST FUEL & BTU COST TABLES
HEAT TAPE USAGE GUIDE
Hertz - Definitions of KHz MHz GHz THz
KNOB & TUBE WIRING
LIGHTING, EXTERIOR GUIDE
LIGHTING, INTERIOR GUIDE
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 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.
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:
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.
For article loading speed we have moved this data t
Electric Motor Thermal Overload Switch - the Thermal Switch
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
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 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 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
General advice: Electrical Tests to Check HVAC Blower Fan Motor or Outdoor Compressor Fan Motor Winding on Heating or Cooling Equipment or on Other Electrical Motors
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
Continue reading at ELECTRIC MOTOR OVERLOAD RESET or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
Question: are there different test procedures for DC electric motors?
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. 
Question: table saw motor hums, doesn't run
(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.
Question: blower motor buzzes and won't start if it's hot
(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.
Question: AC not working to cool the home - change the blower motor?
(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.
Question: cooker hood motor hums but runs if I give the fan a spin
(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.
Question: Carrier air haandler blower motor repairs - noisy, intermittent run
(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.
Question: how to oil an Emerson K55hxkwa-9803
(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
Question: free AC inspection, said working perfectly, added refrigerant, recommends zero resistance filter
(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,
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/AirFilters2.htm
Question: Nordyne heat pump blower motor voltage checks
(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.
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