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]

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 )

 

[Darkwood]

Your post 23 was ambiguous it can be read both ways - but that aside a motor with a failing field may be noisy depending how it failed what i was getting at was a reversed field winding, this would still run but very noisy but it would be evident from the start as it is it was commissioned and sounded ok so not the cause.

Did you not clamp test it on commission to ensure everything was as should be?...

Id get the suppliers in to fit and commission new motor don't take any second options as they have a responsibility here at a minimum to send a replacement motor...can you can you confirm the Kw rating also it may show other probabilities as to the cause.

Im just trying to establish the motor FLC is fully protected by the O/L

 

[Marvo]

So what readings did you get for line and phase current?
......

...As per Tony's question what were the running currents when you commissioned? Did you check rotation was correct?

......You still didn't give info on the run currents, how close were they to the maximum allowed 18A?

.......Did you not clamp test it on commission to ensure everything was as should be?...

Good luck with that question, it's been asked several times and still no answer so I'm assuming there were no run current tests taken.

 

[Darkwood]

In defense of the OP here .. if he's just connected the supply up to an all in one manufacture commissioned unit then although i always check myself its not always routine to double check motor running loads when they are delivered in such a set up ...but i somehow don't think this is the issues ... its a factory supplied already connected unit and just needs the OP to tag the motor up with well labelled cables the O/L is pre-set at factory and sounds correct for running current he specifies and at a guess its an 18.5Kw motor (OP to confirm). We either have a loose connection or motor fault IMHO as the cause.

 

[Marvo]

My first guess was a 10kW if the FLA is stated on the plate as 18A assuming it's a 4-pole@ 50Hz.

I'm not sure how we can rule out common or garden overload damage without a measured run current.

I'd pop the rear bearing housing off the motor and check the windings. If they're all dark brown then probably overload, if only two windings discoloured then possibly poor connection on one phase or one phase went completely open circuit. The problem I have with the loose connection theory is that any connection carrying 18A or thereabouts that was loose for any length of time would almost certainly be obviously damaged with discolouration of the termination and the wire itself. I'd also need more evidence for the faulty motor theory, if it ran fine for hours and the bearings are still free then I'd have it down as a long shot.

 

[Darkwood]

Its star/delta and O/L in delta side so will be max 0.58 of flc thus my 18.5kw guess which will have flc of approx 38amps meaning current O/L setting will be max 22amp which relates to what op says .

 

[Marvo]

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

That information was in the documentation supplied with the motor

I understood this 18A figure was the maximum permissible run current from the motor specs so the overload rating should be 18A X 0.58
Apologies if I have the wrong end of the stick, the English terminology sometimes causes me confusion

 

[Darkwood]

agree slight confusion but factory set OL at 22 amps must be .58 of flc as he said it was on delta config so thats where i get my 18.5kw motor ...... instructions say running current 18 amp which can lead u to believe thats on the supply so contradicts the OL setting hence i keep asking what size motor is ..... we are both right without further info ..and op disappeared.

 

[Knobhead]

Unless we get some meaningful figures we may as well give up. I don’t guess when it comes to motors.

Re an earlier post:
A reversed phase is an unusual fault. I’ve had it only twice in 40 years, each time the motor was a rewind.
A reverse phase on a Y∆, the motor wouldn’t start as the torque would be far too low to get a fan moving.

 

[Bobster]

I don't think it was even a reversed phase, as you say Tony it's a rare fault.

I think somewhere it has been wired so that when the delta contactor kicks in, only two phases are connected correctly. The motor would carry on running single phasing. From what I can see the overload was set incorrectly so it carry on running. Then burned out.

 

[Darkwood]

But that doesn't tally with the info he received it has a running current of 18 amp and a O/L setting of 22amp, given his info and the fact the Star/delta starter has the preset O/L set at 22amps in the delta format (assuming the flc of motor) then this would make the motor an 18.5kw with a full flc of about 38amps thus 0.58% O/L setting 22amps and the running load 18amps