12V , 200W , DC motor

[N_Cook]

Needs new brushes as one had cracked along half the length but not broke
away, so forming a wedge in the slideway. Along with some conglomeration of
carbon etc in that part of the slideway and beyond into the gap and
presumably into the crack , until it jammed out of contact. Anything to be
aware of for the brush composition ? otherwise looks like a mains voltage
carbon/graphite brush. No name motor for spares. How to clean off the build
up of carbon on the commutator and is there an equivalent , for low V high
A motors, of bedding in with bedding stone ? as no aperature available to
poke any stone in there when assembled.

 

[N_Cook]

Clean the com with extra fine emery and stone with rotor in a lathe or
other apparatus. This is a scooter motor correct?

Actualy on a golf cart. A bugger to get at as one retaining bolt was seized,
steel bolt into tapped aluminium. Al had corroded rather than the steel on
our wet links. Is there a recognised way of chemically dissolving the
aluminium oxide for the next time of doing this. Luckily I could hack into
reinforced heavy duty structural plastic to release the mount , then undo
the bolt with molegrips, Impact driver after penetrating oil would not shift
it, only deforming the bolt head. Would grinding/drilling a well under the
head of such a bolt and a few drops of battery acid in there do anything. ?

 

[N_Cook]

golf trolley, not cart

 

[N_Cook]

I'd say that's a very unusual brush failure. Some general comments:

Brushes for DC motors typically have a copper and graphite content that
makes them feel slippery surface when rubbed between thumb/finger.
I've found that AC motor brushes just feel like plain carbon.

One of the guys at my local motor shop pointed out that it's important to
use brushes specifically for DC motors, as AC brushes can tear up a DC
commutator.

According to a technical course at a Reliance industrial drives training
school, the best surface contact that brushes can have, is the completely
smooth, slick black surface that brushes make after they're properly seated.

Any surface abnormalities need to be addressed before installing new
brushes, or the new brush life will be significantly reduced, while
aggravating and continuing the commutator damage.
Commutators are often recut to zero runout and a smooth surface by turning
them in a lathe, and copper isn't an easy material to get a very nice finish
with for a very light cut, for an inexperienced lathe user.

Over the years, I've used numerous easy procedures to "clean up"
commutators, and they have generally worked at least, well enough.
Different abrasives from ink erasers (with pumice imbedded in them) to
various ultra-fine sandpapers can sometimes smooth minor surface
irregularities, but don't do a good job of truing and smoothing such as on a
lathe.
One should be sure that any abrasive they might use doesn't imbed itelf into
the soft copper, and that the abrasive isn't electrically conductive.

In high powered, expensive industrial motors, the face of the brushes are
recut to match the lightly smaller diameter of the commutator, after
servicing.

In industrial motor applications, there are special brushes which are only
intalled temporarily at regular service intervals, which are used to refresh
and clean the commutator surface, then replaced with regular-duty brushes
for production runs.

Many low-priced motors aren't intended to be serviced, and any commutator
problems other than very minor scoring, indicate that it would be better to
replace the motor for reliable use.

Creating flat or low spots on a commutator needs to be avoided, as these
will lead to the brushes hopping and/or losing contact which will promote
arcing and damage to the brushes and commutator.

When brushes are removed for inspection, they should only be reinstalled in
their original orientation (not rotated 180 degrees).

It looks as though another feature of low V / high A motors is the brushes
have copper wire tails melded into the graphite , wheras mains ones can
often get away with end of conducting phosphor-bronze spring just resting
against end of brush and no copper braid.

 

[N_Cook]

I use a variable DC power supply and a gray ink eraser to polish the
commutator, then a modified Exacto knife blade to undercut the mica
insulation between segments. I run the motor at 100 to 200 RPM, and use
a light touch with the eraser so the dust isn't pulled between the brush
and commutator. I polished hundreds of commutators that way.

Unfortunately no access to the commutator when assembled. I'd have to mount
a pair of magnets near the rotor and bodge up some brushes. I will adopt the
compromise of spinning between centres on a lathe and use non-metallic
abbrasion/honing

 

[N_Cook]

Some Bosch 12V,250W automotive fuel pump brushes are about right, needing
cutting down half a mm W and H to fit, side entry copper wire tails as in
this use.

 

[N_Cook]

Well that's 2 impossible jobs done reassembling the motor
1/ Tying back the brushes to get them over the armature and then removing
the cord when in place.
2/ How to replace the 5 inch long steel screws , without any built-in
guides, passing between 2 powerful magnets. They go where the magnets want
them, not some midway path. Obviously made scratch marks before
disassempling but required the head of the screw held in molegrips until you
can feel it is in the tapped hole, screwdriver placed in head still in the
molegrips, pushing down, while you release the molegrips.

 

[N_Cook]

The 2 paralelled output TO220 thyristors had their identities ground off
before insertion. 200W, 12V motor so 17 amp so would they be say 30V 8amp,
10amp or 15 amp rating each? No fuse in the control anywhere but there is a
main relay and more electronics than just for controlling speed so could
there be an overload sensing cct that drops out the relay ? While at it
there is an off board loop of copper coloured wire, perhaps microbore copper
tube could that be a 20 or 500 amp fuse or just a dropper element for
overload sensing ?

 

[N_Cook]

50 amp not 500

 

[N_Cook]

I don't know about the semiconductors, but the loop of wire may be a braking
"resistor".
Battery powered power tools with permanent magnet fields sometimes have a
loop of wire to dump the spinning motor current into, when the trigger is
released. This causes the cutting blade, in a saw for example, to stop
sooner than just letting the armature to coast to a stop.

You may know a procedure for testing the LRA locked rotor amperage of the
motor, which may be a useful indicator for selecting the semiconductors.

Metal current feedback circuit "resistors" that I've seen in DC motor drives
and treadmill drive circuits sometimes look like strips of bare sheetmetal,
or just a half-circle of very heavy gage copper wire.
I guess the type of material used will depend upon the circuit designer's
background and material/part cost.

Taking a closer look it is probably solid 1.7mm copper wire in a loop above
the board of about 1 inch diameter. It is in the supply line to the power
devices (maybe powfets rather than thyristors) not the power to the
electronics. But operation is via rotary pot with switch so have to go throu
gh minimum revs (pulses) before switching off. So still a mystery, no way a
fuse , the cross section of the tracks, wide yes, must be less than 1.7mm
solid wire. Incidently now all in working order on the bench, not
reassembled yet onto the caddy a G Caddy TEDC 12201 that I find no www ref
to

 

[N_Cook]

Perhaps it is just a wire link, it does cross over some tracks, perhaps
someone decided 1mm was too small a section for copper wire to reliably hold
up in a vibrational environment. Still I cannot see any obvious track
knecking or other possible weak link/fuse possibility

 

[N_Cook]

maybe it's a current probe test point?
a loop you can hook a current probe over,without breaking the circuit.

--
Jim Yanik
jyanik
at
localnet
dot com

I'd not thought of that possibility
The thick wires to the battery are widely separated as it is a cuboid SLA,
so plenty of run to put a current probe around at that point. At the moment
I'm struggling to get the tubular framework back together. I assume when
they assemble it, it is done under force so each corner joint keeps
everything else in place, the screws must be for show or they do not hold it
all together as far as i can see. It is made to be foldable like a folding
bike and triangular sections and one triangle locked and braced against
another

 

[N_Cook]

If there is a next time I'd reverse the jaws of a sash cramp to force the
tubes apart, the 2 inches or so to release the cross tie tube. I wonder if
the chassis of those disability/mobility scooters are built the same way.

 

[N_Cook]

Now road tested I see how the traction works. Gearbox will spin backwards
because of course pitch and angle of the worm / spur gears, I hope there is
protection against the reverse emf generated as off on the control is not
fully off, 5volt measured just by leasurely hand spinning.. Then presumably
"spring" clutches in the wheels one L and opposite sense in the R one to
again allow for total freewheeling in one sense and somewhat braked in the
other direction.