Any HV Motor 'Experts' around?

30 Jun 2008
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United Kingdom
Called to a site today to look at a 7500Kw slip ring motor. Stator is 6kv and rotor is 2.5kv. Problem is that the brushes on the centre slip ring are running so hot they are 'welding' themselves into the brush boxes before being due for replacement. Carried out 'Surge' tests, Hi-Pot tests, Ducter resistance tests and motorised megger tests. All test results were satisfactory and I can find no reason for these brushes burning the way they do. Does anyone have any ideas? 12 new boxes and 36 new brushes have been fitted, bedded in and all boxes aligned and now I will keep a weekly check on them for the forseeable future but if it starts doing it again it would be nice to be able to explain to the customer what the problem may possibly be. As you would imagine, taking a machine this big out of situ for repair would be VERY expensive! :eek:

Thanks in advance.
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Theres got to be a lot of current flowing though the rotor windings - is it run with resistances in place? Are the resistances ballanced?
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what sort of conditions is it running in? is there much vibration around it?
brush pressure on the slip rings is quite often overloooked
Yes its a slip ring motor, I think the 3 big slip rings give the clue! :LOL:
Seriously though it is a bar wound rotor connected internally in star, (wye), it was 48 brushes not 36 as I stated in O/P, speed is 423rpm, full load current of motor is 1020amps. It is run through a 'liquid start' system, i.e. 3 prongs in a brine bath act as a resistance and once upto speed the rotor runs similar to a squirrel cage rotor. I don't fully understand the exact science of it just the basic principles. It is a metal recycling plant and this machine is one of the main shredders. Mounted about 60 feet in the air it is cooled by 2 large fans mounted on the top bulkhead, internally there are a number of anti-condensation heating elements, (low wattage), and the windings of the stator are protected by thermistors. Went down today as I was in the area and the site manager is very pleased with what we have done and it is running at full speed/capacity at the moment catching up on the backlog from last week.
The whole thing is covered/surrounded by a corrugated tin hut type of affair so is quite sheltered from the elements. Brush springs were checked using a spring balance to determine all pressures were equal and all brushes were bedded to sit 100% true to slip rings. All leads and cables are in very good condition with no chafing or severe bending.
It is just this middle ring which always seems to have problems. Did think today that maybe there are some dry joints on the internal connections but surely this would produce uneven resistances or unbalanced currents.

Thanks for your thoughts so far lads.
How are the prongs in the bath arranged? In a triangle fashion or 3 parallel bars?
Haven't seen them myself but I think they are parallel.
Does it make a difference?
Not 100% sure, I'm thinking unless they are arranged in a triangle formation then the resistances between them will not be the same.
I'm more used to resistor banks - they have all 3 resistors arranged in series and two contacts at each step which in effect move the star point closer to the motor depending on speed / torque requirements.
Thanks Spark. Will make enquiries and let you know.

Still doesn't answer my question as to why the centre ring seems to have problems though! :LOL:

Where's that fountain of knowledge BAS when you need him? :LOL: :LOL:
Still doesn't answer my question as to why the centre ring seems to have problems though! :LOL:

My chain of thought - if the bars are not arranged correctly i.e. in a triangle then the distances between L1 - L2, L2 - L3, L3 - L1 will not be equal hence the resistances between them will not be equal.
If they are arranged as parallel bars then L1 - L2 will be the same distance as L2 - L3, but.. L3 - L1 will be twice the distance so double the resistance (there'll be more to it than that!!). So not as much current will flow through L1 and L3 as there is flowing through L2.

holmslaw - mind you don't let the plate touch the bottom! :LOL:
No this is not a wind up, it is a 6,000v stator and 2.500 v rotor.
Will talk to my boss tomorrow, he should have more information than anyone regarding this.
Will let you know.

Cheers lads.
Problem is that the brushes on the centre slip ring are running so hot they are 'welding' themselves into the brush boxes before being due for replacement.

If only one rotor phase is affected, I would suspect an earth fault on the rotor. Have you tested the insulation on the rotor at twice the working voltage? Is there any eccentricity on that slipring? Is there a mixture of brush grades? Is the centre slipring of the same material as the others? Is that slipring surface smooth? Have you measured the current on the stator, it should be balanced on all 3 phases - need CT's for this. Are they pedestal bearings? roller or sleeve?


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