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Thread: 3 Phase motor troubleshooting...
12-31-2010, 04:20 PM #1
3 Phase motor troubleshooting...
Happy New Year Everyone!
I would appreciate some guideance on how to troubleshoot a 25hp motor that is not running.
My friend called and said his pump motor was out and could I take a look at it... the holiday and all... he didn't want to call a real electrician and pay triple time.
He thought it was the thermal overover load part of the starter....
I pushed the contact bar on the starter in and all I got was a hum. The motor rotates freely.
Not knowing much on troubleshooting motors I procede to switch the thermal overload as requested... I did so and I am still getting the hum.
I am leaving off several important parts of the story.
- Of the three leads going from the starter to the motor, two were so loose that when I reached into pull on them both were removed w/o a screwdriver
- I failed to check all three fuses in the switch box.
With the two loose wires leaving the starter I'm thinking I may have a blown fuse or two that I have overlooked???
I'd appreciate am good old PM advice.
12-31-2010, 04:33 PM #2
If I were to guess I'd say the loose connections popped a fuse and the humm is telling you the motor is single phasing. Check your fuses and make sure you have good connections so you don't blow another one. Is this a location that has vibrations? You should check your motor leads and any connections between the starter and motor.
Just a guess
12-31-2010, 05:49 PM #3
01-02-2011, 05:25 PM #4
I went back and checked the fuses. One of the three was blown. I thought I had overlooked the obvious. I replaced the fuse expecting all to be back to normal. Not!
The starter still is humming?
01-02-2011, 07:21 PM #5
It would not be unusual for a motor to single phase itself to death from a blown fuse, especially if the thermals were not correct. The best way to know for sure is check the voltage at the motor on all three legs. That way you can rule out the starter and all the conductors to the motor. Then with the power off check for continuity between all three feeders to the motor...they should show continuity. Then check all three to ground...they should not show continuity to ground. If you have full volts to all three legs and the motor still just hums it is toast. Ed.
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01-02-2011, 07:42 PM #6
01-02-2011, 07:43 PM #7
If the thing is a down-hole pump, maybe you won't be measuring "at the motor" unless you and the meter are real skinny...... and like diving....
but you can ohm the motor leads from the top..... they should all ohm the same, between each pair. Any big differences are a sign of a problem, either with motor or cable... with D-H pump it don't matter which, it's coming up anyhow.
But, the OP said there was a humming...... if the motor is "remote", it must be the contactor. if the motor ohms out OK, you can try pushing the contactor closed with a plastic pusher of whatever shape will push it.
Motor should start. if not, then it could be jammed, etc, or your ohm reading is not right.
01-03-2011, 09:37 AM #8
Thanks for the excellent pointers. This pump is on an auotmatic drivethru car wash.
I don't have a megger... I remember the stories of using those to shock fish in the fish ponds.
I have a Fluke multimeter and Amprobe & Greenlee meters to measure amperage.
01-03-2011, 09:42 AM #9
01-03-2011, 10:36 AM #10
I would not rely on ohm readings for finding turn to turn short, the best way is using a surge tester for testing out a winding. I would check your wiring from the circuit breaker panel box to the motor, making sure everything is good and tight. Check the contacts on the starter ( burnt, pited, melted..ect ). If everything checks out good, then we are down to the motor itself. Bad winding, open rotor endring, bad bearing journal or bearing itself.
01-04-2011, 04:39 PM #11
I went down to this job with full intent on running a few tests you guys had suggested.
I turned the power back on the machine and pushed in the contact bar.
The motor fired up just perfect. I cycled it several times. We operated the wash and then it started doing the same thing?
The starter is a Schneider TeSys
01-04-2011, 05:24 PM #12
#1, don't do that any more. You are not likely able to push that in hard enough. It may LOOK like you are, but when current flows, magnetic forces are trying to force the contacts open again and without enough strength, the contacts chatter, even on an almost microscopic level, which causes them to heat up VERY fast and they can weld in a matter of seconds.
#2, all you are doing is bypassing the control circuit for that contactor. The control circuit is also tied into the motor protection circuit. So manually pushing in that bar will defeat some of the things that are preventing a fire in that motor.
It appears as if you have some sort of interruption of your control circuit. This can be either an overload that has tripped or some other protection device, it could also be a failure of some other control components ahead of it, or it could be a bad coil on the contactor.
Find the terminals labeled "A1" and "A2" on the Tesys unit. Using your VOM, give a Run command and see what the voltage is at A1 and A2. Check first to see what it is SUPPOSED to be by the way, a lot of those use 24VDC. If the proper voltage is getting to A1 and A2 at the proper time, then you have a bad unit. If it's small, I think those are throw-away, no replaceable parts.
If the proper voltage is NOT getting to A1 and A2 when you tell it to, then you need to start trouble shooting backwards using a schematic wiring diagram. Keep going backwards until you find the right voltage and see what it stops after that.
01-04-2011, 05:37 PM #13
Did you ever have to reset the overloads? I don't know if they are manual or automatic. If auto they reset themselves after they cool off. The work/ no work is scary. Intermittent troubles are the worst. I wouldn't bother with the fuse tester, Ohms is a verry good way to check for bad fuses. It may be the contactors or the coil that actuates them. Maybe some one here can come up with an Ohms value on the coil if you can post the voltage and the part numbers. Does the humm come from the starter or the moter? If it is coming from the starter it is a good sighn that the contactors or the coil is going south.
01-04-2011, 08:10 PM #14
if teh coil IS DC, and you get only humming and no pull-in, then possibly the control signal that was "sposed" to be DC is actually AC now....... rectifier problem, either shorted and giving AC, or half-waving with too much ripple to pull in.
if coil is AC (it should SAY on it what it wants) humming may be a stuck plunger, or low voltage.
An AC coil needs a LOT of current to pull in, and afterwards a lot less to hold. So sometimes tehre are tricks used to reduce teh holding current. If the "trick defeater" that allows pull-in is bad, then it won't pull in on the "hold" current.
01-05-2011, 03:53 AM #15
When the starter is humming, is the motor humming?
01-05-2011, 05:54 AM #16
Presumably if the motor ran OK several times when you pushed, the motor has no "permanent problem".
01-05-2011, 11:02 AM #17
01-05-2011, 05:21 PM #18
01-05-2011, 06:25 PM #19
The starter was the problem! The contacts were badly pitted. Maybe due to the poor connection.
Thanks guys for allowing to pick your brains and generously sharing your knowledge!
01-05-2011, 10:20 PM #20