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). Photo: 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:
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.
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 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. Continue reading at ELECTRIC MOTOR OVERLOAD RESET or select a topic from the More Reading links or topic ARTICLE INDEX shown below.