[ARC5] BC-454-B Dymamotor Question

[Kenneth G. Gordon]

On 18 Jan 2013 at 10:45, J. Forster wrote:

> Ken,
> 
> Agree on all points. However, I've never seen the commutator insulator
> segments get too high. It probably does happen as many manuals have
> directions for 'undercutting' the insulation after refinishing the
> commutators.

Right, John. 

I first learned about "undercutting" the insulation from an ancient (1920s ?) 
book on automobile maintenance. I used that info to fix both the generator 
and the starter motor in my first car, a 1930 Model "A" Ford. When I bought 
that car, for $50.00 in about 1960; it was much older than I was.

What happens is that since the metal that makes up the armature segments 
is actually somewhat softer, and a lot wider, than the insulator, the insulator 
wears down at a slower rate than the metal.

Or, when turning a very badly worn armature in a lathe, the insulator 
segments become equal in height. Those must be cut down a few 
thousandths to prevent brush bounce.

The first big indication of that sort of problem is excessive sparking between 
the armature and the brushes as the brush "jumps" from one metal segment 
to the next when it rides over the protruding insulator segment.

However, undercutting the armature insulation in a dynamotor would be 
much more difficult, since the dynamotor's armature so much smaller than 
an auto generator, and thus the insulator segments are much thinner and 
would require a very thin blade to do the job properly. For an auto generator 
or starter motor, we used a broken off hacksaw blade, which was almost 
exactly the correct thickness.

In the case under discussion, and after thinking about it, I am much more 
inclined to think that one or more of the old brushes is simply stuck. Since no 
armature is perfectly round, and no set of brush-holders is perfectly aligned, 
the brush is simply not touching the armature at one or more specific places 
as the armuature rotates.

I am also very suspicious of the bearings' condition: IMHO, no old dyamotor 
should ever be run before the bearings and brushes have been very 
thoroughly checked out and serviced, or replaced if necessary.

One other thing one should look for is the small wire that connects the brush 
to the end-clip. This wire runs through the tension spring. If that wire is 
broken or missing, the current must flow through the spring. These were 
never designed for this, and will usually overheat and lose their tension, 
allowing the brush to "bounce" or even in extreme cases, preventing it from 
making contact with the armature.

In my case, I chose out of my collection of DM-34s, the worst possible one I 
had. This one had been under water. After doing minimal work on it, 
including cleaning and lubricating the bearings, it works fine, except that it is 
very mechanically-noisy. Obviously at least one of the bearings has rusted 
balls in it. I am going to replace those asap. All four of the brushes were 
good, and were loose, although one had been improperly installed and the 
small clip on the end of the tension spring had been bent. 

One thing I found very interesting in my "research" on the DM-34s I have 
here is that the manual states unequivocally that experience has proven that 
replacing bearings in the field resulted in very much reduced operating life 
and it was stated flatly that if a DM-34 needed bearing-work, it was to be sent 
back to the factory.

I have been trying to figure out why this was the case.

vy 73,

Ken W7EKB