Old English DC excited generator commutator brushes wont stop arcing

[Toeknee101]

we have tried setting the neutral offline on this old english DC excited generator several times, the lowest we have achieved is 26MV on the readings. the commutator brushes continue arcing everytime we change the neutral setting.

any ideas as to what could be the problem, maybe a deeper issue within the gen itself ? Is there another method we should use to set the neutral ?

 

[waross]

Been there, done that.
You may have shorted commutatorr windings.
To check:
Apply 110 or 120 VAC to the exciter field..
Connect your voltmeter to the brushes.
Rotate the exciter armature slowly.
A fluctuating voltage is an indication of unbalanced armature windings.
That is, probably turn to turn shorts.
We had an exciter arcing when it warmed up.
This test showed a fault that a growler could not find on a cold armature, or an armature that had been heated in an oven.
In operation the windings heat up and then transfer heat to the iron.
The differential expansion was causing the fault.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

By the way I may have wrongly assumed that the exciter was arcing.
The same test may be applied to a DC generator.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

DC machine's magnetic neutral axis is permanent (and usually factory set) and does not require frequent adjustments. Faults in armature and field windings including reversed interpole, bad commutator surface, unsuitable brushes etc. can also cause sparking.

Muthu

http://www.edison.co.in

 

[waross]

Actually the magnetic neutral shifts under load.
Thee purpose of the commutating poles or interpoles is to oppose this shift to lessen brush arcing undrr load.
The magnetic neutral of a DC machine at rest is an effect of the field excitation.
The armature current of a loaded machine creates another magnetic field.
The nagnetic neutral of a working machine is determined by the interaction between these two magnetic fields.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

Bill

Yes, the mna shifts from no load to load but the mna is fixed in the factory load test (type test) and is permanently marked on the brush rig for black commutation at all load currents. It is rarely altered at site regardless of the load current since any % of armature current distortion of the field is countered by the interpole carrying the same % current.

Muthu

http://www.edison.co.in

 

[Gr8blu]

Sparking at brush-commutator can be a result of a LOT of things. Some are electrical, some mechanical, and others environmental.

First question - what happened recently that made the sparking appear? Or why did you think you needed to adjust the neutral setting?

The following list is by no means exhaustive.
Electrical: excessive load (current or voltage), poor contact resistance (film), bad electrical connection (bolted joint), turn fault (interpole or main pole winding), ground fault (interpole, main pole, or armature winding), shorted bar(s) - at commutator surface or at riser, brush contact position incorrect with respect to machine electrical neutral, open circuit in one of the machine windings, etc.
Mechanical: brush arm circumferential spacing, brush arm skew (relative to commutator bar), high runout on commutator (bar-to-bar), weak brush spring(s), brush fit (to holder) incorrect/loose, brush fit (to commutator) incorrect, surface speed too high, commutator bar relative movement (loose commutator and/or excessive speed causes bars to "bow", possibly settling back to proper place when at rest), induced vibration (from process or alignment), incorrect circumferential spacing/alignment of main and/or interpoles, deformation of running surface (grooving, threading, high/low spots) etc.
Environmental/Other: Contaminant in film, presence of oil, abrasives, chemical reaction between atmosphere and brush compound resulting in increased brush wear/carbon dust formation, presence of silicone caulk (e.g. RTV) in air stream, incorrect ventilation (commutator surface too cold or hot), accumulation of conductive debris (carbon dust, etc.) in undesirable places - such as behind the risers which may result in shorted connection from bar to bar on the commutator, etc.

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