Old electrical motor help

Hope so - sorry I didn't see this sooner. ;D



Not immediately helpful, but it all looks to be in very good nick for a 61-year-old motor: plenty of meat on the brushes, nice clean relatively unscored commutator, good-looking insulation on the windings, and a distinct lack of corrosion. ;D



It seems that the upper brush in this picure is earthed to the case of the motor without insulation. The lower one appears to be mounted on a peice of insulation material: Tufnol type stuff probably. If so, don't disturb it as it's liable to contain asbestos.

I'm going to guess that the live terminal from the switch connects to one of the rivets holding the brush carrier in?



Yup, sounds right - positive feed to both the armature (brush) and the field.



As I mentioned earlier, the top brush appears to be earthed. It also appears that the other side of the field winding is earthed. In fact, I'm quite sure that this is the case, but I'm putting in a note of caution as I don't have the thing in front of me.



It is, through the casing of the motor. Just make sure that the mating faces are nice and clean to ensure a good connection.

This figure from Wikipedia might help:


The round bit with the M in it represents the part of the motor that turns - the armature (windings) and commutator (segmented switch) - an assembly normally referred to as the rotor. The squares abutting the rotor symbol are the brushes. The oblong(s) to the right of the armature represent a resistance: the field windings. The blobs just show that the wires are connected to each other.

What you have here is a 'shunt' or parallel wound motor - 'A' in the figure - where the earth return from each winding is passed separately into the motor casing.

Finally - as a bit of an aside - if you wanted to make the wipers two speed, a resistor in the wire from the brush to the field would make it spin faster. This is a procedure known as field weakening if you felt like looking up some of the theory. If you have any questions do shout.

It's hard to say what value you'd need, though something around a third of the resistance of the field winding should work. A switch to short-circuit the resistor would bring the speed back down to stock. Of course, you'd have to drill a hole in the motor casing an fit a grommet to it to lead out the field wire (though you'd only need one - it doesn't matter if the wire isn't directly connected to the brush, as long as it gets it's feed from the same place - after the park switch probably).