Motor tripping

[Frosty69]

Hi people,

Just been out to a customer who is having problems with a motor. Basically it used to be fed from an old rerwireable db and for about three months it has been blowing the yellow phase about four times over that three months. The old db has now been changed for a wylex three phase db.

The problem
The motor is started and runs fine whilst in star for about five seconds then as soon as it switches over it pops the mcb, the motor is rated at 30kw and when load tested before the db was changed the highest readings were 50A at full speed. The motor is now protected by a 63A t/p type c circuit breaker.
The motor itself is about five years old and never serviced as its in a loft area in a factory that isnt very easily accessed.

All the electrical supply to the motor etc is fine so just wanted the opinion of somebody that works with motors on a regular basis.

I did the following calculations whilst I was there just to check loadings please let me know if im wrong.

1.732x400v=692.8A
30,000/692.8= 43.3A
43.3A/0.8= 54A

Any help would be greatly appreciated, I was going to try a 63A type D but I am worried that I will be spending money on materials that wont cure the original fault that was blowing the rewire able fuse?
They had a guy there today to check the motor and he says its fine although ive found out he never went up to the motor just checked the start stop unit and has told the company it is a loose connection in the new wylex db, which I checked before and the connections are as tight as a nuns!

Thanks

 

[stef]

Hi Frosty

Has it been disconnected recently? It could be that it changes direction when switching from star to delta. Are youngetting all three phases to the load terminals of the delta contactor. Try the following: disable the star/delta timer so that the motor runs in star only. Measure the current on all three phases see whether the windings are balanced. You could have a partially shorted out winding, a rare but possible fault. Changing the breaker won't help in this case. Resistance test is a bit awkward with assync motors so you have to measure the running current.
I hope the motor is rated for the supply, check the rating plate the voltages must be
240 in star and 415 in delta, or something similar.

 

[Frosty69]

Hi Stef thanks for your reply, If I test the Delta side of the contactor and it shows there is only two phases could that cause the mcb to trip rather than just run slowly? When the motor is started it runs the correct way.
The guy who has looked at the motor today doesnt seem overly confident and is meant to be a motor specialist, I have done a bit of reading on internet and does a dol motor have its highest current at start up or change over? I asked what loadings he had taken but just said I dont know so I think hes just blamed it on a loose connection

Thanks again

 

[Frosty69]

Also stef it has not been disconnected recently although where it has been supplied from has gone from a rewireable fuse to a 60898 T/P mcb.

 

[stef]

If the motor is running on two phases only (which it can) then the maximum load wi
l be distributed through the remaining two phases which will exceed the rated current of the circuit breaker. For example, a healthy motor pulls 10 amps on each phase, one winding fails then the remaining two pull then 15 amps each. The maximum current you will get is if you start the motor from stand-still, basically a dead short circuit till the magnetic field has built itself up ( i think this is called saturation, not sure though). The current will increase when switching over from star to delta but not dramatically in your case probably from 30amps to 54 amps witch a slight surge on change over as -again the magnetic field need some time to build up again. Hope this helps.

 

[Frosty69]

That does help thanks, I am hoping that their might be a member from manchester on here that is good with motors who fancies a bit of cash in hand for an hours work. I just feel that if the motor in question has been taking a 3036 fuse on a regular then there must be something wrong on the motor side, I am just not confident enough to start taking the controls apart.
If anyone is reading this who fancies having a look for cash in hand either sunday 19th or monday 20th please get in touch.
[email protected]

 

[stef]

You can check the windings for continuity with a multimeter, there must be six wires plus an earth going to the motoe, three to the main contactor and three to the star delta contactors. Test any agains each other and if you get three continuity readings ( something as low as 3 Ohms ) then the windings are ok, then the fault must be with the contactor, probably a burnt ou contact, loose connection, broken wire, ...

 

[Frosty69]

Thanks Stef I will give that a blast tomoz and see what happens, thanks again for your help!

 

[snowhead]

Forget the "Expert" he's not.

Someone needs to get up to the motor, you can't assume all is O.k.

It may be 1 bad connection on the 6 pole isolator at the motor, assuming it's got one.

Loose connection in the motor, bare cable earthing.

The motor has always had a fault, changing to an MCB has just highlighted it.

Stick a bigger MCB in or a d type and you'll probably hide the fault again ******* NOT RECOMMENDED**

Check the load in Star as Stef's post above check on all 6 cables to the motor first, then get to the motor.

 

[ipf]

Are there no overloads involved ?

 

[stef]

Are there no overloads involved ?

Probably there is but in this case the fuse disconnects faster than the overload which pints towards a missing phase in the delta supply.

 

[Frosty69]

There are overloads etc and I am going there tomoz to do the tests that stef said and check all connections etc and see what comes of it. I hope the guy is wrong as hes a tool aswell and thinks hes gods gift.
Thanks everyone

 

[Strima]

If it hasn't been serviced in five years has it had one arranged? Could be build up of carbon dust around the bushes etc.

 

[Knobhead]

The MCB isn’t big enough, a 80A type C should sort it.
All tests on the motor can be done from the starter as all 6 terminals are there. Testing from the starter also checks the cables and isolator. You need to do both IR and CR, the CR readings should be within a % or two of each other.
In the starter make sure the interlocks between the star and delta (mechanical and or electrical) are working.

There are nine tests to do on the motor as follows:

View attachment 14108

 

[Knobhead]

If it hasn't been serviced in five years has it had one arranged? Could be build up of carbon dust around the bushes etc.

? ? ? ? In an induction motor ? ? ? ?