When an electric motor fails to start, runs intermittently, runs hot, or continually trips its overcurrent device, there my be a variety of causes. Sometimes the trouble lies within the power supply, including branch circuit conductors or motor controller. Another possibility is that the driven load is jammed, binding or mismatched. If the motor itself has developed a fault, the fault may be a burnt wire or connection, a winding failure including insulation deterioration, or a deteriorating bearing.
A number of diagnostic tools – a clamp-on ammeter, temperature sensor, Megger, or oscilloscope – may help illuminate the problem. Preliminary (often definitive) tests generally take place using the ubiquitous multimeter. This tester is capable of providing diagnostic information for all sorts of motors.
If the motor is completely unresponsive, no ac humming or false starts, take a reading at the motor terminals. If there is no voltage or reduced voltage, work back upstream. Take readings at accessible points including disconnect(s), the motor controller, any fuses or junction boxes, and so on, back to the over-current device output at the entrance panel. What you are looking for is essentially the same voltage level as measured at the entrance panel main breaker.
When there is no electrical load, the same voltage should appear at both ends of the branch circuit conductors. When the circuit electrical load is close to the circuit capacity, the voltage drop should not exceed 3% for optimum motor efficiency. In a three-phase hookup, all legs should have substantially equal voltage readings, with no dropped phase. If these readings vary by a few volts, it may be possible to equalize them by rolling the connections, taking care not to reverse rotation. The idea is to match supply voltages and load impedances so as to balance the three legs.
If the electrical supply is satisfactory, examine the motor itself. If possible, disengage the load. This may restore motor operation. With power disconnected and locked out, attempt to turn the motor by hand. In all but the largest motors the shaft should turn freely. If not, there is an obstruction inside or a seized bearing. Fairly new bearings are prone to seizure because the tolerances are tighter. This is especially true if there is ambient moisture or the motor has been unused for a while. Often good operation can be restored by oiling front and rear bearings without disassembling the motor.
If the shaft turns freely, set the multimeter to its ohms function. The windings (all three in a three-phase motor) should read low but not zero ohms. The smaller the motor, the higher this reading will be, but it should not be open. It will usually be low enough (under 30 Ω) for the audible continuity indicator to sound.
For proper motor operation, all windings must have megohm readings to ground, i.e. to the motor enclosure. If a winding is grounded out, the winding insulation is broken down or the armature is touching the stator, unless there is a possibility of a loose or chafing wire inside.
Small universal motors, as for portable electric drills, may contain extensive circuitry including a switch and brushes. In the ohmmeter mode, connect the meter to the plug and monitor the resistance as you wiggle the cord where it enters the enclosure. Move the switch from side to side and, with a trigger switch taped so it remains on, press on the brushes and turn the commutator by hand. Any fluctuation in the digital readout may point to a defect. Often a new set of brushes is what is needed to restore operation.
Amperage readings are useful in all kinds of electronic and electrical work. With a voltage reading, you know the electrical energy available at the terminals, but you don’t know how much current flows. Multimeters always have a current function, but there are two problems with it. One is that the circuit under investigation must be cut open (and later restored) to put the instrument in series with the load. The other difficulty is that the multimeter is not capable of handling the amount of current present in even a small motor. All the current would have to flow through the meter, instantly burning the probe leads if not destroying the entire instrument.
An excellent tool for motor current measurement is the clamp-on ammeter (trade name Amprobe). It circumvents such difficulties by measuring the magnetic field associated with that current, displaying the result in a digital or analog readout calibrated in amperes.
The clamp-on ammeter is absolutely user friendly. Just open the spring-loaded jaws, insert either the hot or neutral conductor, then release the jaws. The wire need not be centered in the opening and it is OK if it passes through at an angle. However, an entire cable containing hot and neutral conductors cannot be measured this way. That is because the electrical current flowing through the two wires travels in opposite directions so the two magnetic fields cancel out. Consequently, it is not possible to measure the current in a power cord, as is often desired. To deal with this situation, make up a splitter. This is a short extension cord of adequate rating with about six inches of jacket removed so that one of the conductors can be separated and measured.
Digital and legacy analog clamp-on ammeters work well and are capable of measuring up to 200 A, which is adequate for most motor work.
The basic procedure is to measure the start-up and running current for any motor while it is connected to a load. Compare the reading to documented or nameplate specifications. As motors age, the current drawn generally rises because winding insulation resistance drops. Excess current causes heat, which must be dissipated. Insulation degradation accelerates until there’s an avalanche event, causing motor burn out.
The clamp-on ammeter reading will tell you where you stand on this continuum. In an industrial facility, as part of routine motor maintenance, periodic current readings can be taken and put into a log posted nearby so damaging trends can be spotted in advance to avoid expensive downtime.
Okay, Call me a dummy. I have a Sears 10″ radial arm saw that I pulled the motor off about twenty years ago and didn’t record where the three wires went. The red goes to the capacitor, but the device the B & W hook to doesn’t look like the diagram (below). My gut instinct says it doesn’t matter if they’re switched, but I’d prefer not to blow it up.
Your guidance is greatly appreciated.
http://c.searspartsdirect.com/lis_png/PLDM/1307294P-00012.png
Normally the black only is switched and that will safely power down the tool. Some tools have double-pole devices that switch off both the white and black. What is definitely prohibited is switching just the white. That would shut down the motor but half the circuit would still be energized, making a shock hazard if someone goes to work on it. Your radial arm saw diagram shows a double-pole switch. The thinking behind this is that if the wires get reversed somewhere between the entrance panel and your tool, it will be certain that the hot wire can be shut off. To understand what is actually going on, check it out with an ohmmeter with power disconnected.
Best Regards
David
I obtained a 150 psi 6.0 scfm Porter Cable air compressor that had been thrown away. It has a start and run capacitor on the motor. Both capacitors were good.The motor did nothing. The power cord has a white, black and green wire. With the unit plugged in there was 122 volts between the ground and the black and also between the ground and the white. It does not matter if the switch is turned on or off the voltage is still there. This was a very perplexing situation to me, I could not understand why the breaker was not tripping. I finally decidd that it must be the black and white wires shorted in the power cord so I took apart the plug that had been put on the end of the cord. I could not believe what I saw. Someone had wired the black wire to the ground terminal!!
David….I enjoyed this article…I have a 4 pole (1750 rpm) 2 hp air compressor , 12A…..Its not starting. Just makes a humming noise.The capacitor checked out OK. Motor spins freely by hand. Suspect windings are shorted. The ohms reading on a regular meter cant read high enough when both wires are checked at ground. Even a Megohm meter cant get a read. Funny thing is…when I feed 20 volts into the windings via one wire at time and checking for voltage at the case, I get a reading of 2.6 volts !!! Yet no omhs readings through the case!!! What a mystery. A little back ground….before the problem occured I covered the compresor with a box so it wouldnt spay oil …Which clearly inhibited cooling…..What do ya think??? This is the best compressor ive ever owned…Fini…made in Italy
David Herres says:
Yes, it definitely sounds like the insulation is fatigued.
The low voltage you are getting at the case is probably what is known as phantom or ghost voltage, and it is observed when you would expect a reading of 0 volts. As soon as you connect to a real voltage, the phantom voltage goes away. It is a consequence of the very high impedance of the meter, so high that a voltage will appear across it. This voltage is due to static electricity in the instrument, one or both probes acting as an antenna or capacitive coupling to nearby house wiring.
Since the motor is turning freely, it won’t do any good to lubricate the bearings and since the capacitor checks out and the motor is humming, it means you’ve got voltage to the terminals. The only thing you can check is that you’ve got the proper line voltage with a heavy load, such as the humming motor, connected. Otherwise, it is a job for a motor repair shop.
I hope this helps.
Best Regards,
David
Thank you for this article.
i have a newly rewound 6 Pole 4kw 3 phase brake motor….motor readings are 11.5A per phase when not on load..motor nameplate read 440V/60Hz 7.6A ….I have a 400V/50hz 3 phase supply for this motor??? any suggestions to why its drawing a current this high without load…bearings seems normal and theres no humming noise coming from the motor when its running.
Nameplate reads 60Hz, but supply is 50Hz.
I have a 30 gallon Husky vertical air compressor that builds up pressure to approximately 50 psi and then starts to slow down and overheat. Motor is new, check valve is new and tank has been drained and flushed. What do I look for as the cause.
An air compressor of that sort should go up to 125 psi. I would check the pressure with a tire gauge because your pressure gauge may be misreading. Also, the pressure switch could be out of adjustment. Directions for adjusting it (two screws) are usually printed inside the removable cover. The other thing is that the points in the pressure switch may be corroded. You say the motor is new. Does that include the compressor? If not, maybe the compressor is going hard.
Infact I like this page its very educative my regards to you engineer’s
I have a clothes dryer with a new ac 110V 1/3hp universal motor with no capacitor. The starter switch lights up when it is pressed so there is a continuity on the starter switch. I checked for the wire harness that is attached to the input of the motor and has 110V AC reading for the power input when I turn on a dial timer. However, when I tested for the 2 wires coming from the starter switch to the said harness and I press the starter switch that lighted up, I only have around 30 to 35 Voltage of AC current and the motor won’t start, not even a hum. The motor is new and it was tested on my other dryer unit. Shall I have the same Voltage reading for the wires connecting to the starter switch of 110V AC so the motor will start? Or having a reduced voltage is fine? If yes, any guess where could be the problems why my motor is not starting? Thank you and I appreciate your inputs.
Nicely written articles. Easy to comprehend and full of valuable info. I will be visiting more often.
Question: I watch a ton of YouTube video’s of people fixing laptops, TV screens, iPhones, etc. All these items are using DC low voltages which is why I assume they use touch to sense overheating components. As well, they use benchtop powersupplies up to 30v. What do people use to troubleshoot pcb’s that power loads like motors and pumps? Can’t be poking around in diode mode with 115v applied via powercord to wall. Are there benchtop AC powersupplies for these situations?
My car starter motor is failing, not the solenoid. There is a commutator and brushes but I do not want to break the thing open to check them out. The item is on my bench now. It will spin but needs power plus a gentle tap on the end cap most times to wake it up. In simple terms what might be wrong and why, what will I likely find when I reluctantly take it apart?
Kevin
Kevin,
Giving your starter motor a whack with a tire tool is an old way to “fix” a balky starter, and it works because the contacts are worn enough to allow a complete gap when the rotor stops in just the right (or wrong) place. While I personally chose to replace the starter myself — easy job!— in a 17-year-old Toyota when this happened to me, I expect that if you open the motor housing you’ll find worn brushes. I realize this was posted weeks ago, but I’m sure there’s a You Tube video out there showing the process.