Motor Fault

Most likely is the cause. You can easily investigate. Power down at DB. Measure resistance across contacts with contactor manually engaged. It should be zero. Alternate method, tag & remove all terminals from motor. Power up & measure voltage put out from contactor. It should be same as your normal motor supply.

Heres where I begin to have doubts about the contactor being faulty... I opened it up to inspect the contacts and they have signs of wear, but not excessive... with power off I forced the contactor in and all the contacts are touching.. surely I would have a very vissible problem for example a missing contact or a badly worn contact with a vissible gap between contacts... (ps we are talking about a 200KW 400 Vac motor with a LC1 F400 contactor.

Look for short in the motor winding. You would need a Milli ohm meter.

Please do not power up the motor till the fault is conclusively located, proved & made good.

GA

Good common sense.

GA JRaef!

I also agree with other posters who advise to switch off the power and check the resistance across the contacts of each phase of the contactor while closing the contactor manually. All three readings should be equal and indicate a low resistance. If the one reading obtained is unstable or different then this contact set would be suspect. Check also for free movement of the contactor as it closes - it is possible that dirt has infiltrated the contactor and is preventing it from closing properly providing equal contact pressure across each of the three sets of contacts.

The same goes for checking the winding's of the motor - not only test insulation between the winding's and also winding's to frame (earth/ground) but also measure the resistance of each winding.

Apart from the above please do not exclude a damaged rotor where one of the bars may be cracked. I use a very easy method to determine this but as the original poster has called the motor winders to site to test the motor they should be able to exclude this.

Re the motor changing direction - this would occur on any three phase motor that is 'single phasing' i.e. being supplied by only two of the three phases. I normally check this by spinning the motor shaft in one direction by hand and then starting the motor. Then after letting the motor coast to a stop, spinning the motor by hand in the opposite direction before starting the motor. If the motor runs in both directions then a 'single phase' condition exists.

Lastly, when a fuse blows it is because of an over-current problem and this should be cleared before replacing fuses. Fault finding is a process of looking until the fault is identified and unfortunately there are no shortcuts! Fault finding is a combination of technical knowledge, use of instruments, experience, thinking 'outside of the box', dealing only with facts and lastly patience!

I have never heard the term peckerhead referred to as part of an electric motor. What is it?

I have been called a peckerhead when continually replacing fuses without finding out why they are blowing!

definition of peckerhead

To save time, I am copying and pasting something I wrote in another forum, replying to someone thinking my use of the term was not "politically correct" because of their mind being in the gutter...

• Hey, there is no "political correctness" needed here! "Pecker Head" is a now commonly accepted term in much of the US for a motor termination box. As I learned long ago, the term was derived from "Picker Head" which was a protrusion off of a drum in old farm machinery which caught the material to be picked, be it an ear of corn, a sunflower head, a cucumber etc., and separated it from it's stalk or vine. Because the termination box on an electric motor looked very similar, it was called (probably) "that there picker head thang on the 'lectric motor"! Interestingly enough, in the South the term "Peckerhead" already meant a dullard and was also apparently derived from "Picker Head", but meaning someone too dumb to do anything on the farm but be a picker. With a Southern twang, "picker" can easilly end up sounding like "pecker".
  
  The non-PC meaning of either term likely came along a lot later.
  
  Hey what can I say. I did an etymology of common electrical terms once as a term paper for a class and the prof liked it so much he gave me extra credit to research oddball terminologies. I've forgotten most of them now but that one stuck because I later used it a lot.

No, it's because the usual slope of the motor junction box flange resembles the shape of the glans.

--Ed. C.

"No, it's because the usual slope of the motor junction box flange resembles the shape of the glans."

<Looks in pants>

Huh, doen't look like anything I've got down there...

Roundhead or Cavalier?

None of your GD business.

But I was born Cavalier, I know that.

If the contactor is faulty (not your situation, but you asked) it is possible that one phase doesn't get any supply. But in such a case, the motor will not rotate, since this is what is referred to as a 'single-phasing' situation and there is no rotating field. The motor will draw a high current in the two phases, and the overload relay will usually trip, since the current will be in the overload region, not quite a short-circuit (which would blow the fuses). Perhaps the fuses are undersized, or have aged (not normal if they are from a reputed manufacturer).

Absolutley.

Thanks for your replies..

Just to clarify

I always replace the 3 fuses even if just one blows.. today I will replace the three contacts in the contactor just to remove any doubts.

I have a motor repair company comming out today just to verify the motor.

I meggered and supply cables at the contactor.

Thanks all

Maybe we have to focus on why the rotation of the motor reversed! So there must be changes in phase sequence not single-phasing. Suggest to use phase sequence tester for both the line side and load side where terminals of the motor are temporarily disconnected.Sorry I cannot believe a possibility of interchanging of two phases inside the contactor.

it seems that motor has an internal winding short-circuit. to find out you have to check both winding resistance & inductance. they must be equal for all 3 windings.

you forgot to mention what kind of a starter you are using, where diagnosis will be more. But any way if the motor terminations winding continuity and insulation test readings are good and you have 3 phases in order the motor MUST run.In this case your insulation is good than it has to be motor windings / terminal fault or the starter contactor is not giving 3 phase, after all the load had already been decoupled.pls do confirm upstream voltages as well.

"you forgot to mention what kind of a starter you are using, where diagnosis will be more." First, please read post....

#2 we are talking about a 200KW 400 Vac motor with a LC1 F400 contactor.

#8 today I will replace the three contacts in the contactor just to remove any doubts.

Hi

Once again thanks for the replies...

Motor was tested (Inductance and Resistance) with a decent tester and not the normal fluke tester that we use.. one set of windings had a short circuit between the windings but not to earth.

Thanks all

In my opinion your rotor has broken bars in it. To verify connect a lower voltage (1/2 rated to two of the wires of the motor and put a clamp on ammeter on one line.Energize the supply and turn the shaft very slowly. The ammter wil go crazy when you are on the broken bars otherwise the reading will be steady. A rotor costs more that the entire motor so keep the peckerhead and throw the rest