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# 3ph motor problem

Discuss 3ph motor problem in the Commercial Electrical Advice area at ElectriciansForums.net

#### catdog

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Hi there,

Have a 3ph motor that is giving problems, when truned on it attempts to go backwards sort of but when given a turn in the right direction with a rope (safety...) it goes fine at seemingly full speed.

Now all phases are live but havent checked the voltage differences.

Question would be if 1 phase is out phase more than usual, would that cause this problem? dont have any other motors just now to check.

thanks.

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M

#### mdshunk

That sounds pretty classic for a lost phase. Are you certain all three phases are present and in good order (ie, measure from each phase to ground with the motor disconnected, measure phase-to-phase voltages)? A 3 phase motor will start with 2 out of the three hot legs present with assistance, and appear to idle at full speed. There will be a dramatic loss of horsepower once the motor is put to work doing whatever it's connected to.

#### catdog

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yeah im thinking that. there is definetly 3ph present but dont have a meter at the moment so cant check, as for horse power it seems to be ok but since it is a beast of a motor would be hard to tell unless put under huge stress. thanks.

S

#### Shakey

yeah im thinking that. there is definetly 3ph present but dont have a meter at the moment so cant check, as for horse power it seems to be ok but since it is a beast of a motor would be hard to tell unless put under huge stress. thanks.
Hi, almost certainly a phase down

I used to teach motors and generators, we had a wound rotor induction motor with a resistance starter as a fault finding rig. We used to take out one of the phases, and the rotor would either go forwards, backwards, or just 'rock' back and forth, and then would start in whichever DOR you gave it a nudge in (I have seen students crying over that one)

Be careful with saying that all three phases are there - I had a fault once, motor wouldnt run up to speed. Checked three phases to deck at incoming to contactor - all fine. Checked three phases to deck on motor side of contactor - all fine.

Measured between phases, no voltage between two of them

Found problem was loose connection on motor side of contactor on one of the phases. (Lets say it was Phase 1, or red in old money)

When I measured phase to deck on phase 1, i was actually reading the open circuit voltage from Phase 2 (or yellow) that was feeding back through the motor windings. When I measured phase to phase L1 to L2 I got zero volts beacuse it was effectively the same phase

All it was was the terminal not tightened onto the cable on the contactor, but jeez, had to think about that one for a while

C

#### Carter

If the protective devices check out OK it may be volt drop across a burnt contact(you said t was a beast), it could be at the local isolators or controlling contactors/overloads, check for good clean, bright, crud free contacts at the supplying switch-fuse blades. If you measure off load you find full voltages but as soon as the load comes on and current is required to flow that's when the volt drop (which can be considerable) is manifested. I found clamp meters to take measurements of each phase and with a peak hold function for comparisons of inrush can be v helpful with motor/supply faults.
If the supply is pukka only then turn attention to the motor.

#### ian.settle1

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Mentor
Arms
yeah im thinking that. there is definetly 3ph present but dont have a meter at the moment so cant check, as for horse power it seems to be ok but since it is a beast of a motor would be hard to tell unless put under huge stress. thanks.
How do you know if you haven't got a meter?

#### catdog

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well still dont have a volt meter but just used mains tester.

C

#### Carter

well still dont have a volt meter but just used mains tester.
Well not knowing what you used precisely I'm guessing that at best all it will give you is a qualified 'go-no go' and an indication that voltage (of some sort) is present. A "voltage indicator" is what it says, it's just an indicator, it can't give any qualitative info so there'll be a limit to what can be achieved with it.
Who is responsible for/owner of the motor in question? Have you been called in as an external party to look at it or are you part of staff?
You really need some sort of multimeter to get further with this if basic visual inspection and remedial fettling fail to restore rotation. Word of advice, it doesn't need to be a whizz bang, bells and whistles job but neither should you go poking around 3ph with a £5 market stall meter or Maplin bargain bin special. It might say "600v" on the dial but the thought of applying those mickey mouse test prods and leads across phases makes me feel very wobbly inside.

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