pin firing of a dc motor 1

[motorrepairshop]

I have a problem with a 800 hp dc motor. It seems to pin fire under the brushes. I have returned the comm to elimate any contamination. I have the correct grade of brush and it seems to be firing on the trailing edge of the brush.
Also the inerpoles were shimmed wrong (too strong). I am at my to the point of trying anything. Somebody said to me about platium coating the comm.The motor is used in a paper company. It only pin fires when the motor is on the test stand. At noload it is super

any help would be great

 

[waross]

By pin firing do you mean a line of small intense sparks on one edge of the brush? In a good motor it may be a sign of improper brush positioning. In a motor with a history of interpole issues it may be an interpole problem. Try shifting the brushes slightly in the direction to cover the sparks. If you can't eliminate the sparking but can change it from full load sparking at one side of the brushes to no load sparking at the other side of the brushes by shifting the brushes, then check the interpoles.
yours

 

[edison123]

To check Interpole Polarity

Mark the neutral position, then shift the brush-rigging assembly an inch or so. Apply low DC voltage to the A-leads (A1+ / A2-). Apply just enough voltage to turn the armature. The armature should rotate in the direction the brush-rigging was shifted. If the armature rotates the opposite direction, the polarity of the interpoles relative to the armature is wrong. Switch the leads at the brushholders and repeat this test.

Be sure to move all leads one position. If the motor has only 2 brush studs, interchange the leads. In a 4-pole machine with multiple circuits, the leads at all 4 brush studs must be moved (likewise for 6-pole machines, etc.).



To set neutral on a DC motor

The following method uses readily available household current and a voltmeter, so it can be done anywhere.

The AC method takes advantage of the transformer action between two windings in close proximity. The fields act as the primary with the armature as secondary.

Connect the leads of an AC volt-meter to two adjacent brushholders. Apply 120 volts AC to F1 & F2. Read the voltage across the brushholders. Shift the brushholder assembly to obtain the lowest possible reading. This is the neutral position.

A perfect zero reading is rare, but values below 0.010 volt AC should be expected.





* The shin is the device to find your furnitures in the dark *

 

[waross]

Hello edison123
Your method of setting the brushes with AC has a lot in common with a Growler. Once you have found the minimum volts position, with the current still applied, rotate the machine slowly. Any defects in the armature that would normally be found by a growler will show up as a varying voltage at the brushes.
yours

 

[UKpete]

Quite possibly on a machine of that size you physically can't get any of the connections wrong, but as posted above, they should be checked.
Quoting from my Morganite book "Carbon Brushes and Electrical Machines", on the causes of sparking:

Brush mounting and Maintenance:
sticking in holders, too loose in holders (normally about 0.2mm clearance in the brushbox),
clearance to commutator too large (esp. is comm has been frequently machined, typ. clearance should be 1.5 to 3.5mm), poor brush bedding, incorrect pressures.

Machine adjustment:
brush position, interpole strength

Mechanical fault:
commutator out of round (typ. max TIR say 0.001" on a newly
turned comm), lifted bar esp. during running (seen as unusual bar marking), machine out of balance, worn bearings, comm undercutting incorrect.

Electrical fault:
Comm riser joints, armature fault (again, seen as unusual bar marking).

Of course particular designs of dc motor never commutate particularly well, especially for the high speed (weak field) condition, but you can live with it. But if your sparking includes audible spitting from the trailing edge accompanied by fairly rapid deterioration in commutator condition and smoked brush faces/edges, then something is obviously wrong. Did this machine previously run without sparking? What is the commutator temperature after a test (shouldn't exceed 120°C, note that when measured after shutdown, it can go up a little before it starts to go down).

Why the interpole gap change? This is usually left unaltered as it is very difficult to set the interpoles by trial and error. The ideal interpole setting can be different for different operating points. When a new dc motor design is first tested the interpoles are set using black bands tests, using buck and boost of the interpole currents. Not a particularly easy test to set up (you need a large dc power supply) and it is time-consuming, but it is an effective way of arriving at the correct combination of interpole gap, tip shape and number of turns.

What is the commutator appearance?

Never heard of platinum coating a commutator, I wouldn't bother if I were you.

 

[aolalde]

Why you considered the interpoles were too strong? Did you make a black band test at full load armature current as UKpete explained?
The interpoles work to correct the armature reaction developed by the armature current increase due to load. The armature reaction distorts the field moving the neutral axis. Under no load condition the armature current is very low and the commutation appears to be good but is with load when the commutation is challenged.