New three phase motor winding burn out

[brizospark]

We have been involved in the installation of a dust extraction system which is powered by a fairly large 3 phase motor (18A).

The motor was pre-wire to a control box on the side of the extraction housing and the star/delta starter was also pre-wired. The only wiring that was required was the 2 sets of cables from the extraction junction box to the star delta starter. These cables were to be connected to the main contactor and the star contactor, anyway we followed the drawing to a t.

The overload was set at 22a and we have used a 32 TP type C MCB at the DB.

The motor was first operated yesterday, it ran for a few hours and then the MCB tripped. I couldn't understand why this would trip before the O/L but anyway it did.

So I had a look at the motor and it was red hot, couldn't even touch it for any period of time so we waited a bit and checked everything, our wiring was spot to drawing and everything in the starter looked ok. I took the cover off the motor when it was cool enough and checked the resistance across the set of windings and one set was Open circuit.

Any ideas what could have caused this apart from a faulty motor?

 

[brizospark]

The starter did change from star to delta I have verified this by running once motor has been disconnected and all works fine voltage checked through ok through contactors and overload.

Regarding the motor being rated for star/delta the extraction unit was all supplied as a kit from a supplier so I can only assume that they would have surely produced a piece of machinery which was compatible with the motor they supplied

 

[stevethesparks]

You need to check that no damage has occurred to the contacts / OL of which may not be apparent with no load connected.

 

[Knobhead]

If it ran for a few hours I very much doubt a faulty motor. What I do suspect is a bad connection/contact in or around the run contactor.

 

[Bobster]

Personally I think it been wired wrong.

As it's changed over to delta, it's ran on 2phases.

 

[Darkwood]

What happened to the old motor ...did it burn out by any chance?

 

[brizospark]

No old motor complete new dust plant

 

[Darkwood]

What i don't get is you say its got an FLC of 18amps ad you have a O/L in the delta leg set at 22amps..is this correct?

An O/L in this set up would be 0.58 x FLC which you state to be 18amps so approx 10.5 to 11 amps off top of my head.

There seems to be something amiss with your story it just dosen't ring the right bells.... did the motor sound ok when running in delta or did it make quite a noise?

 

[brizospark]

It sounded ok when first commissioned until before it tripped the mcb it sounded noisy

 

[Darkwood]

If you can answer my last post regarding the plated Full Load Current (flc) of the motor and what your O/L was set at it may explain why your O/L didn't trip.

I suspect the motor was running at over-current from the start (many reasons can cause this) and a few hours of this burnt it out as your O/L setting was over double what it should have been.

The mcb may have been noisy due to progressive winding breakdown ....

I would also make a note to check the mcb bus pin connections are all on the correct side of the mcb clamps and cables are also secure... this also would create arcing but at the mcb.

 

[brizospark]

Sorry dark wood the running current is 18a per phase which is why the o/l was set to 22a. We checked that the motor was free and spins freely no problem. I have no idea why the mcb tripped before the overload

 

[Marvo]

Sorry dark wood the running current is 18a per phase which is why the o/l was set to 22a.

Was the 18A measured or was that the value on the info plate?

 

[brizospark]

That information was in the documentation supplied with the motor

 

[Darkwood]

Ok get you now ... this would make it a 18.5 kw motor then if im correct making the motor O/L setting of 22amps be correct at 0.58 x flc

I would still check the mcb and especially the busbar pins are clamped correctly to mcb.

 

[brizospark]

Darkwood sorry you are misunderstanding the motor was noisy not the mcb

 

[showerman]

Hi, have you checked that the winding id tags in the 6 terminal block are correct ?
It is possible that they are not connected to the correct stud and what might have
happened is that you were effectively running with 2 windings in phase and the 3rd
one acting as a brake if it had the ends joined the wrong way round . I recently had
a motor that had u1 , u2, v1 v2 and then w1 and another w1 ! Motor supplier said
" you are not the first mate " , I noticed this when checking before motor was run !
The noise you mentioned was probably the brake effect and my theory would also
account for the heat, hope this helps . Ken .
( check connections and do you own phasor diagram , to make sure )