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Illustration of the internal components of a 3-phase electric motor (C) InspectApedia.com Mamatha3-PHASE Electric Motor Troubleshooting
Diagnose & Repair Electric Motors on Building HVAC Equipment

Troubleshooting procedures for 3-phase electric motors.

Page top illustrationof a 3-phase electric motor is provided courtesy of Mamatha K, IndMALL Automation cited inour 3-Phase Motor Selection article found the end of this page.

This article series 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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3-Phase Electric Motor Troubleshooting Procedures

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.

Multi-Phase or 3-Phase Electric Motors

Two-phase or three-phase electric motors use the rotating magnetic field produced a two-phase or three phase design both to start the motor and to keep it spinning.

See details at ELECTRIC MOTOR TYPES - 8 types of single phase, split phase, 2 & 3 phase motors

3-phase electric motors use three separate windings out of phase with one another, also producing a rotating magnetic field to spin the motor.

Working with single phase vs. 3-phase electric motors

Watch out: if you are not trained in proper, safe electrical work, stay out of your electric motor - you could be shocked or killed.

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. Also

see HOW to CHOOSE a START / RUN CAPACITOR

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

see details at 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.

See details at ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD

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 rotoconverters 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 by oil companies 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

 

3-Phase Motor Diagnostic Guides

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Reader Comments, Questions & Answers About The Article Above

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: 3 phase motor hums and runs slowly

(June 25, 2015) Tim said:
3 Phase motor hums and doesn't reach full speed. Is it single phasing or is the fan plugged?

Reply:

Check for lost power on one of the phases. I think your motor is single phasing but this isn't my expertise.



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