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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

Regular EF Member
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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.
 
OP
F

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
 
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F

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

Regular EF Member
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576
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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.
 
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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

Regular EF Member
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576
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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, ...
 
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Frosty69

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

snowhead

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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.
 
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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

Electrician's Arms
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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.
 
OP
K

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
 

Strima

Electrician's Arms
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? ? ? ? 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:
 
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K

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

Regular EF Member
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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

Electrician's Arms
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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:
 

Strima

Electrician's Arms
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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

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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

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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

Regular EF Member
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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.
 

clematis432

Regular EF Member
Messages
175
Location
Ayr
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 ?
 
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