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

 

[Strima]

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

Ooops, missed that bit...

I shall now go stand in the corner and call myself names whilst flicking poo at myself... :lol:

 

[Knobhead]

Strima, some induction motors do have brushes like the one in my avatar. I should have said squirrel cage...... Silly Tony.

Frosty, have a look in the commercial / industrial forum “sticky” threads for Motors.

 

[oldtimer]

You need to start at the beginning
1. Measure the winding resistance ie all 3 should measure the same
2. megger the windings again all 3 should be the same
3. Check the local isolator a guy spent 2 weeks trying to figure out what was wrong only for me to find one phase was burnt out on it.
4. ensure the wiring and all connections are ok.
5. Check the contactors ie when they close you get a low resistance.
6. If the motor state 37 amps delta then you need to 60% it and thats your overload setting.
7. Get a large 300mm screwdriver put the blade on or near the front bearing put the handle end up to your ear then give the motor a turn by hand if you hear a metalic grinding noise then the bearing is worn.
8. Repeat the above step for the rear bearing
9. Make sure the motor is mechanically disconnect from what it drives ie take off V belts.
10. Check the operation of what the motor drives ie that it turns easy and check the bearing on this as well.
11. While the motor is mechanically disconnected give it a no load test run this will test all the control side.
12. Remember to measure the running current of each phase of the motor when running.
13. if everything checks out connect the motor mechanically and again measure the running current and compare with overload settings
14. If the motor is tripping a breaker this does not mean its too small it could be a bad connection on one of the contactors making the motor run on 2 phases hence the trip.
15. What ever the motor drives make sure it is free to turn when connected up if too tight or badly set up then the motor could trip or trip the breaker

 

[Strima]

Strima, some induction motors do have brushes like the one in my avatar. I should have said squirrel cage...... Silly Tony.

Frosty, have a look in the commercial / industrial forum “sticky” threads for Motors.

I should still read the thread fully though... :lol:

 

[stef]

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.

Asunc motors don't have brushes

 

[micknew]

Asunc?

 

[Knobhead]

Asynchronous?

 

[Strima]

Asunc motors don't have brushes

Don't rub it in, I feel bad enough already...

But at least I can spell async... :lol:

 

[Marvo]

I agree with Tony, the existing MCB is too small to handle the changeover transient (voltage/current surge) when changing from star to delta. Rather up-size the MCB according to the supply cable and let the overload or manual motor starter protect the motor.

 

[JD6400]

I would have said the 5 second star - delta change over is a wee bit fast , would you not want a figure of more like 10 -15 seconds at least ?

 

[frank3323]

Hi Gents,

i am just trying to get the attachment for a couple of Tonys posts here on motors, but it does not seem that the attachments are working?? or am i just being a dope?

thanks.

 

[Silly Sausage]

They don't work anymore for some reason! <shrugs shoulders>

 

[clematis432]

I would have said the 5 second star - delta change over is a wee bit fast , would you not want a figure of more like 10 -15 seconds at least ?

Yes this time is a bit tight, I WOULD up this time to around 10s and see how it affects the changeover current, you will do no harm and it takes little time to do. I will probably get shot down, but would think twice before considering to upsize the c/b as the problem was there before the new 63A c/b was fitted and the c/b is primarily to protect the circuit cabling. What size fuses were blowing in old CU ?
Has the motor load increased due to mechanical/process issues recently ?
Oops just seen the age of this post, apologies !! Frosty what was the outcome ?