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Electric motor run direction and backwards-running motors:
Why would an electric motor suddenly start running backwards? Here we explain the causes and cures for a motor that is running backwards and we also discuss motors that are designed to reverse direction.
This article series describes A/C electrical motor troubleshooting: 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.
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)
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:
Electric motor rotor: the rotor follows (turns in the direction impelled by) the rotating magnetic field and thus spins the motor shaft
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.
Electric motor windings: the two windings are used to create an electrical field in the stator.
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.
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.
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.
Uni-directional electric motors run just one way: clockwise (CW) or counterclockwise (CCW) but not both.
In many applications the equipment driven by the electric motor will not work properly unless the motor drive shaft spins in a pre-determined direction: clockwise or counter-clockwise.
Examples include HVAC unit blower squirrel cage fans whose blades won't move air if the fan runs backwards and oil burner or well pump motors whose oil pump or water pump won't move oil or water if the pump motor is driven in the "wrong" direction.
In a fixed-direction electric motor such as on an HVAC blower fan or an A/C or heat pump compressor, each time the motor starts its start capacitor and start winding give the motor a "kick" in the right direction.
Bi-directional & self-reversing electric motors run in either direction, CW or CCW
Bi-directional electric motors run in either direction and are used in applications in which the direction in which the motor spins doesn't matter to the equipment it's driving. "What the heck?" you're saying. Well here's a cute example - your home microwave.
Electric motors require a kick or to put it more like an engineer, a starting torque to overcome the resistance when the motor is to begin spinning from a stopped position. Compared to electric motors used in oil burners and air conditioning compressors, we don't need such a costly and heavy-duty electric motor to spin the rotating table in a microwave oven.
Instead, microwave (and some other self-reversing) electric motors use a clever design that allows for elimination of the start winding winding and also allows use of a lower-cost and less powerful electric motor.
When the motor has stopped the mechanism it drives remains pressed against the motor's drive shaft, providing a "driven load" ( the microwave turntable) against the direction the motor's shaft was last rotating. When the motor is asked to start again it encounters this extra load due to the resistance provided by a mechanical mechanism in the turntable that was itself "wound" or loaded by the table's prior rotation.
The microwave motor, encountering this load, reverses itself to begin rotating in the opposite direction, one of less load, which gives a brief "no-load" interval that lets the microwave's turntable motor get up to speed.
In sum, the combination of a load or rotation resistance provided by the turntable mechanism and an automatic direction reversing feature in the electric motor gets the microwave motor spinning and the turntable rotating - which is why each time you start your microwave it is likely to rotate in the opposite direction from that of its previous cycle.
Manually-reversible Electric Motor Run Direction
By swapping wire connections on some electric motors it is possible to change or reverse the motor direction.
Typically purple and yellow wires or wire pairs permit this change. Purple to purple = rotate clockwise
Purple to purple = rotate clockwise
Purple to yellow = rotate counter-clockwise
For an example of electric motor wiring that includes wiring to select electric motor speed options as well as direction of rotation see
Can an Electric Motor Run Backwards by Accident or Because of Damage?
Watch out: yes it is indeed possible for some electric motors to run "backwards" following damage to the motor's start capacitor or windings.
We've had an occasional report of an HVAC motor running "the wrong way" or sometimes starting to run the the wrong way. For example the blower fan motor in an air handler can start and run backwards as can some air conditioner compressor unit cooling fan motors.
Typically an electric motor running in reverse direction is caused by (in order of probabiliy)
A failed electric motor starting capacitor
A single speed single-phase motor leads are reversed on the motor's start widing connections
A single-speed 3-phase electric motor has two of its supply leads reversed
Belt driven air handler motors attached to a blower fan in a multiple-blower HVAC system can be turned backwards with the motor OFF and the squirrel cage fan rotating backwards because of air currents created by other fans that are on in the same system
(For 2-speed electric motors you'll need to contact your motor supplier)
Read the Information on an Electric Motor Data Tag to Determine the Motor Run Direction
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.
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:
the electric driving motor,
the oil burner combustion air blower fan assembly, and
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 (ELECTRIC MOTOR RUN DIRECTION) it will not move much air and equipment will not function properly.
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
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.
Thermal overload relays on electric motors used on air conditioning, heating, water supply and other building equipment may shut off an overheated motor and (if not automatic) may require a manual reset.
See ELECTRIC MOTOR OVERLOAD RESET SWITCH explanation (how to reset a motor off on reset).
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Questions & answers about the run direction of electric motors & why a motor might run backwards, posted originally in this article are now found at ELECTRIC MOTOR RUN DIRECTION FAQs
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Simpson 260® Series 6XLM
Volt-Ohm-Milliammeter Instruction Manual, retrieved 9/5/2012, original source: http://www.simpsonelectric.com/uploads/File/datasheets/260-6xlm.pdf, [copy on file as Simpson_260-6xlm.pdf]
 Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN. Mr. Hankey is a past chairman of the ASHI Standards Committee. Mr. Hankey has served in other ASHI professional and leadership roles. Contact Roger Hankey at: 952 829-0044 - firstname.lastname@example.org. Mr. Hankey is a frequent contributor to InspectAPedia.com.
 Arlene Puentes, an ASHI member and a licensed home inspector in Kingston, NY, and has served on ASHI national committees as well as HVASHI Chapter President. Ms. Puentes can be contacted at email@example.com
 ASHI Technical Journal, Vol. 2. No. 1, January 1992, "Determining Service Ampacity," Dan Friedman and Alan Carson,
 ASHI Technical Journal, Vol. 3. No. 1, Spring, 1993, "Determining Service Ampacity - Another Consideration," Robert L. Klewitz, P.E.,
with subsequent updates and additions to the original text ongoing to 2/19/2006. Reprints of the originals and reprints of the Journal are available from ASHI, the American Society of Home Inspectors www.ashi.com.
 "Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
 "Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
 "How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
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 "Home Wiring Inspection," Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
 "Evaluating Wiring in Older Minnesota Homes," Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
 Jim Simmons: Personal communication, J. Simmons to Daniel Friedman, 9/19/2008. Photographs contributed to this website by Jim P. Simmons, Licensed Electrician, 360-705-4225 Mr. Electric, Licensed Master Electrician, Olympia, Washington Contact Jim P. Simmons, Licensed Master Electrician, Mr. Electric, 1320 Dayton Street SE
Olympia, WA 98501, Ph 360-705-4225, Fx 360-705-0130 firstname.lastname@example.org
 Kenneth Kruger: Original author of the sidebar on testing VOM DMM condition: Kenneth Kruger, R.A., P.E. AIA ASCE, is an ASHI
Member and ASHI Director in Cambridge, MA. He provided basis for this article penned by DJ Friedman.
 LB Miller, "A simple Do-It-Yourself test fixture that will allow you to measure the DC resistance (Rm) of RC Model Electric Motors", San Marcos C, HobbyKing.com, retrieved 9/12/12, original source: http://www.rcgroups.com/forums/showthread.php?t=580151 [copy on file as Miller_Test.pdf]
 "Electrical Systems," A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
 "Basic Housing Inspection," US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
 Wikipedia provided background information about some topics discussed at this website provided this citation is also found in the same article along with a " retrieved on" date. NOTE: because Wikipedia entries are fluid and can be amended in real time, we cite the retrieval date of Wikipedia citations and we do not assert that the information found there is necessarily authoritative.
WikiHow: "How to Check Out an Electric Motor", retrieved 9/12/12, original source: http://www.wikihow.com/Check-an-Electric-Motor
 Andy Page, "The Basics of Motor Circuit Analysis", Reliable Plant, (Noria Corporation), 1328 E. 43rd Court, Tulsa, OK 74105
Tel: 800-597-5460; Email: email@example.com, retrieved 9/13/12, original source: http://www.reliableplant.com/Read/10686/motor-circuit-analysis, [copy on file as Page_Andy_The basics of motor circuit analysis.pdf] - Quoting:
MCA online [tests performed while the motor is operating] can be further split into two categories - current analysis and voltage analysis. Current analysis is primarily focused on the rotating components. Loose or broken rotor bars, cracked end rings, rotor eccentricity, misalignment and coupling/belt problems are some of the "big-hitter" failure modes detected in the current signature. Power quality issues like harmful harmonics, voltage imbalances and under/over-voltages are among the issues identified with voltage analysis.
MCA offline is most famous for the resistance-to-ground measurement. But other measurements make motor circuit defects easy to find. Measuring electrical characteristics like impedance, inductance and capacitance tell the analyst plenty about the condition of the windings. Inductance is a great indicator of turn-to-turn shorts. Capacitance to ground measures the amount of winding contamination (water, dirt, dust, etc.). Changes in each of these affect impedance (total resistance of an AC circuit). These characteristics are measured phase to phase and phase to ground and compared to each other and to percent change from baseline to identify motor circuit defects.
Motor circuit analysis (MCA) is often and easily confused with motor current analysis (MCA), which is an abbreviated version of motor current signature analysis (MCSA).
BETTA-FLO JET PUMP INSTALLATION MANUAL,[PDF] National Pump Company, 7706 North 71st Ave., Glendale, AZ 85303, Tel: (800) 966-5240 Email: firstname.lastname@example.org, website: http://www.nationalpumpcompany.com, retrieved anew 9/13/12, original source: http://www.nationalpumpcompany.com/pdf/Betta_Flo_IOM_Jet_Pump.pdf
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The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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