DC Generator Brush Arcing

[Jacobs60bx]

I bench tested a 50 year old Robbins & Myers 250 rpm, 400 pound, 3000 watt, 120 vdc generator with a 188 segment commutator. These generators have 4 brushes. Three of the brushes, as originally equipped, are a soft carbon/graphite composition and the fourth brush, originally installed in the lower left position, is a hard carbon composition. I installed 4 of the original soft carbon/graphite brushes for bench testing. When the output rose above about 10 amperes, the lower left brush started arcing severely even though I had properly seated all the brushes. After I replaced the lower left brush with the correct original hard carbon brush, arcing was greatly reduced. I also tested another similar generator and had very similar results. I tried "lifting" each individual brush, one at a time, while the generators were running and there was very little difference in arcing of the remaining working brushes. I double checked the neutral setting of both brush rings with a 6 volt battery not trusting the factory marks...they checked good. I megged and growled both armatures and they checked good. The original manual states "There are two kinds of brushes used in the generator......the three brushes marked A-10 are to be installed in the two positive holders which are the upper and lower on the right side. Install the one brush marked 400NC in the other negative holder (lower left)."

These generators are shunt wound with simplex progressing wave windings.

Shouldn't all the brushes in a generator or motor conduct the same amount of current regardless of location? Robbins & Myers obviously corrected an arcing problem by using two different kinds of brushes. Does anyone know, at least in theory, why it's necessary other than to eliminate excessive brush arcing?

 

[waross]

I've never heard of this.
To set brush neutral, try 120 VAC.
Apply 120 VAC to the field. Shift the brush gear to zero volts across the brushes. It's a good idea to rotate the generator while the 120 VAC is connected to the field. If there are any turn to turn shorts in the armature, you will see a varying voltage on the brushes as the generator is rotated.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Strange. Never heard of it.

 

[starkopete]

Does this generator have interpoles? If yes, does it have as many interpoles as field poles?

 

[Jacobs60bx]

I checked the armature for shorts and opens by placing a growler on the armature and then measuring the voltage between commutator bars. I'm sure both armatures are ok.

No, there aren't any interpoles.

 

[Jacobs60bx]

I don't know if it's significant but this generator has 6 field coils but as stated earlier, only 4 brushes.

 

[edison123]

It's probably 4 main poles and 2 interpoles. Can you post some pics here ?

 

[Jacobs60bx]

All 6 field coils are identical. 1500 turns #19 wire. It definitely does not have any interpoles. I took photos of the armature but not the fields while it was disassembled.

 

[Jacobs60bx]

The field coils are all wired in series. I checked the polarity of the field coils while the armature was removed with a compass and they alternate north, south, north, south, north, south.

 

[erickench]

Okay, arcing becomes a problem if the motor is in a hazardous location. In a Class I location where you have flammable gases and vapors present you would have to use a totally enclosed type of motor or one which is identified for Class I Division I. The arc or spark can cause a fire in certain places.

Eric Kench, P.E.

 

[waross]

Arcing also becomes a problem when it damages commutators, brushes and/or brush holders.
Are all three brushes at the same angle to the commutator and do they all have the same spring tension?

Bill
--------------------
"Why not the best?"
Jimmy Carter