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| Subject - Troubleshooting Alternating Current Motors | |
| Scott Vickrey |
After an extensive search requested by a member of this site I have been unable to find any page about troubleshooting ac motors. This is a great Topic since troubleshooting ac motors and there control circuits is not well documented already on the net. This is why we are here! Troubleshooting Alternating Current Motors and Their Control Circuits The trouble shooting process is greatly dependent upon the discrepancy. We will list some common discrepancies and walk through the thought process of troubleshooting them. Since there are many possibilities, This topic may take some time to develop. Discrepancy: Overloads are tripping on a 3 phase motor circuit. Overloads are installed in an motor control circuit to prevent the motor from overworking to the point of total failure(or literally working itself to death). When a motor is running it is producing heat this heat is normally removed by the motors cooling fan. In an overloaded condition the heat is produced faster than the fan can remove it and if this condition is allowed to continue the insulation on the windings will eventually degrade to the point that it can no longer hold back the voltage in the windings. The resulting short circuit and it's arcing ruins the windings rendering the motor useless. So,If the overloads on a motor starter are tripping then you should automatically be wondering why is this motor being over worked? Something has this motor in a bind. Are the bearings shot making the motor hard to rotate? What is the motor turning? Is it fouled, plugged or otherwise unable to turn with ease? I have seen many causes of this from destroyed bearings to something plugging the pump the motor was attached to. After locking out the motor to prevent inadvertent or accidental starting Try to turn the motor's shaft if it's really hard or feels rough then you will have to disconnect the motor from whatever it is attached to. Then spin the motor and what it was driving to determine where the resistance is. If the motor is hard to turn it will have to be replaced. If it's the equipment that won't turn then breath a sigh of relief because it's the mechanic's equipment that is at fault.  Another possibility is that both turn freely. This is caused by a portion of the windings are bypassed from previous insulation failure. The motor still runs but, it's operating extremely inefficiently and will continue to trip the overloads or fail completely unless the motor is replaced. You will usually find this in a motor that has a long history of overload discrepancies. You will only be able to test for this condition by using a high-pot or a megger. These two pieces of test equipment read resistance at a very high voltage. The reason for the high voltage is the higher voltage will bridge high resistance shorts in the motor windings that your 9 volt resistance meter cannot see. Read from phase to phase they should all read the same. Read from any phase to ground and it should read infinity or multi-megohms. |
| JimmyDee | One of the neat ways I was shown for one type of problem with three phase motors is to do a quick test to see if the motor is shorted to ground. This test will also check if the short is through a carbon track. It isn't 100 % but it gets it most of the time. You use your Wiggies for this test. Put one probe of the Wiggies to a hot terminal of the starter. Motor is off. Then check each T terminal (load side of the starter) with the other probe. If the Wiggies pull in, you have a ground. I do this on all three incoming line terminals and to each of the load terminals. This puts enough voltage to do the check through carbon tracking and will add enough resistance to the circuit as not to cause a serious dead short and blow something up. If the Wiggies pull in, the motor needs to be replaced or you have a bad feed line to the motor. Jim |
| JimmyDee | A recording amp meter is a good tool for motor troubleshooting. It will show the actually amperage of the motor and I have seen the time that there was no problem with the motor, a faulty overload heater caused the starter to trip. I have also seen by using the recorder that an operator, started and stopped a motor multiple times to trip the motor on purpose. This was clearly shown on the recorder. Jim |
| ricky | If you have a blown fuse or tripped circuit breaker,ohm each winding to ground to check for shorts before restoring power.Check to see if the motor is hot.If you can't touch it for 5 seconds,it's probably too hot.If it is,spin the shaft to check for tight bearings.Lots of 3-phase motors are pulled in by a contactor only since they don't need start components like single phase motors do.There are no heaters for overload protection.The contacts on these contactors wear out and should be checked and replaced if nessesary.Motor starters have contacts in them too.A bad contact can cause single phasing of the motor and burn it up.Rotation is important also.When replacing any controls,wiring,or motor,check the rotation of the motor.Reversing any two of the three phases will reverse the direction of the motor.Check the amp draw of the motor with an amp clamp and compare it with the nameplate rating.Check each phase.The tighter a fan belt is the more current it will pull.A belt that is too tight will also make the bearings go bad.Oil that motor if it has oil holes,and replace the plugs.If it's an open motor,make sure openings aren't clogged with dust.Blower wheels also can collect dust and cause unbalance.Check for cracked or bent fan blades.These things will cause the bearings to go bad also.Check the wiring connections at the motor and control.Crimp connectors are a common cause of a burned off connection. |
| kingofmars | I have always found that the best tool for troubleshooting any motor, coil, or transformer is a megger, this tool will cost a few hundred bucks but is invaluable in saving time in troubleshooting, the megger induces a preset dc voltage into the windings and measures loss at the other end, just be sure that the winding you are testing is isolated at both ends and do not get caught between the leads of a megger, it jolt will only last for a second but on the one I have it can be as many as 1500 volts dc. |
| wood111 | (Step1)First remove power Lockout Tagout!!! (Step2) Check each lead with an ohmeter at the contactor (motor side) to ground very simple very fast you should not have a reading.If you have a reading go to step(4).(Step3) Then check the contactor for continuity by pressing it in with a screwdriver and checking each leg with an ohmeter you should get a short reading across each of the three legs. If not replace the contactor and try the motor.(Step4) Remove all of the wire connections making sure you write them down at the motor so all you have is 9 wires. Using an ohm meter check for continuity between wires 7,8,9 also 1&4 2&5 3&6 you should have a reading. Then check 4&7 5&8 6&9 there should be no continuity. Also check each wire to the motor ground there should be no continuity. If the motor checks OK check each wire feeding the motor to ground the could be a bad wire feeding the motor. Hope this helps. |
| cs409 | any of yall have "Electric Motor Repair book by Robert Rosenburg"? great book for anyone doing a lot of motor repair.... |
| Jason Cole | I would like to see more on the topic of industrial motor control,plc's,cardreaders,control centers starters,...etc |
| JimmyDee | Jason, I would like to welcome you the this site. We need questions and they will be answered. We have many here that are very proficient in the areas you are asking about. Topics such as these are generated by questions from people like you. If there are any areas you have questions in particular, just ask. I am currently writing an article on elementary PLCs and should be ready in a week or so. This site is new and it takes a lot of time to accomplish even a little. It is growing and I think will be one of the best help sites for an electrician. Jim |
| Kraze | Yes it wuold be good to see a lot more topics on motors and motor controls.I'm a student in the residential and commercial/industrial electrical feild but apond commpletion I intend to go in the industrial feild.so I would also like to get some input on PLC's and all the newer high tech stuff. So any on who can share any knowlege about this stuff it would be great. |
| Ship Shock | Ahh, my bread and butter. If you sign on a ship as an electro, your meat is 3 phase 440 motors and their pesky habits, your taters is lighting...which can get interesting...(think masthead lights going out in a Force 12). Meggers- AEMC makes a nifty little battery powered digital megger.It runs about $300 from Grainger's. I remember the model fondly because I used it to help put a drunken bum of a first engineer I sailed with ashore. He had already ordered a new $25k 40hp motor for a cargo hold exhaust fan, not realizing that tube axial fans at sea that are not regularly exercised will corrode into the fan race and blow the overloads. To an untrained eye, it'll look "electrical", when in fact it's time to call the deck gang with wire brushes...and for Blotto the First to pack his seabag. Any of the crank-type meggers will serve...forever...and should be cheaper than digi-meggers. The Japanese and the Chinese have pretty much got the market sewn up. After LO/TO, and checking for no voltage with your Fluke, clip the "earth" or "ground" lead to a good ground...and check it for continuity, (paint can be a good insulator). I always try and check the motor on the load side of the starter, removing each line, and checking the windings. I always carry a skinny little screwdriver with a test lead clipped to it, I clip the other lead to the same ground as the megger is clipped to. Before unclipping your megger's "load" lead from the winding line, ground the electrostatic charge off of it...unless you need practice with your profanity. If you get a bad reading, (out here, anything under 1 MegOhm to ground is cause for alarm), leave the pigtails free of the starter and repeat the procedure at the motor peckhead. Maritime service can be hard on wiring, and I've sometimes found that a "bad motor" is actually "bad wires". That kind of discovery should mean MAJOR attaboys for ya, depending in direct proportion to the price of a new motor. Do NOT use your megger to check resistance of the windings phase to phase. A big 440-480 volt beast, brand new, will measure very low resistance across the windings...you don't want to be the guy who throws 1000 volts across the puppy. Your DMM or Simpson will give you phase-to-phase just fine, without creating unpleasant little side effects. You can also use your hand-crank megger to fish with...and find out information from Iraqi prisoners...the uses for a portable hand-crank genny are limited only by your imagination. |
| zapped208 | Ahh, those were the days, taking apart a bad motor on a exh fan next to a screaming forced draft blower on 1200 psi plant, or taking out a bad 2 hp motor on a running 1500 psi main feed pump, for the lube oil supply. |
| Ship Shock | quote: Sounds like an SL-7,(now known as a Fast Sealift Ship), t'me. Yeah, those are/were fun. Hand-fired the Capella from Hatteras to Jax once, found out later that I had had no low-water alarm system for the starboard boiler. The Chief would carry a flashlight during Coast Guard and ABS inspections and when they'd need an "alarm" the Chief would flash the First, who would hit the toggle switch "cheater". Clever lads, they. On a 60,000 hp boiler, too. There was another one of those ships where they managed to wire the forced draft fan in one of the boilers to spin backwards...they got it steaming down the Mississippi, even, and couldn't understand why the boiler automation wasn't "up to snuff"... 
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| zapped208 | Ship Shock,- I was in the USN for 7 years and 2 ships, as an electricians mate. |
| Allphase | http://www.reliance.com/prodserv/motgen/h7000.htm |
| phase 1 | Hello. I am new here and iwould like to ask a ? about amotor.Iam rebuilding ablower motor and can not stop the brushs from arcing or sparking . all the winding read good andare free of shorts and grounds if anybody has any ideas that could help i would mutchly appreciate it thank you |
| JimmyDee | Not sure what it is called but there is a white stone used to clean up the commutator. Run it on the commutator for a few seconds and it will probably cut it a lot. Jim |