S3 variable speed wiper motor

[ttaunton]

My 1968 S3 has a DR3A wiper motor. This has 2 spade terminals labelled 1 and 2.
"2" will be +12v, "1" will be a ground that bridges the self parking contact, and will be switched via the dashboard switch to make the motor run. Also an earth terminal to the body. So far so good. The wiper switch assembly incorporates a rotary rheostat, and 3 wires connected to it. According to theory, the way to vary the speed of a shunt wound motor is to vary the current through the field winding, using a resistor. However the DR3A does not have the field windings accessible, there aren't enough connections. Is this rheostat just crudely in series with the whole motor? None of my circuit diagrams show how the motor and the rheostat are connected, as they only show the 2 speed version of the motor which uses a different switch with no rheostat. Any info would be appreciated.

Tony Taunton

[billwill]

>Is this rheostat just crudely in series with the whole motor?

Yes I think so.


Its almost certainly a permanent magnet motor so the series/shunt basic theory is probably not relevant.

[ttaunton]

No, it's a shunt wound motor with a field winding and two brushes. Putting a rheostat in series with it will certainly slow it down........
Tony Taunton

[billwill]

You might be interested to know that the slow wipe electronic circuits that you can get (not specific to Lotus Elan) work by switching on full power for about a second every N seconds where you set N with a knob or switch.

The 1 second pulse is enough to start the motor & then the parking circuit forces it to complete one wipe cycle. Then it rests until the electronics pulse it again.

[prloz]

Tony,
I have a diagram, motor, and rheostat in front of me. The diagram shows four connections to the DR3A motor.
1. A green wire to the 12v. supply.
2. Terminal "A" (armature ?) with a red/green wire.
3. Terminal "F" (field ?) with a yellow/green wire.
4. A black ground wire to the motor housing.
The red/green wire on terminal "A" connects to the red/green wire on the rheostat.
The yellow/green wire on terminal "F" connects to the yellow/green wire on the rheostat.
The remaining black wire on the rheostat connects to ground.

My motor is part no. 75501E, date code 7/66. It does not have the terminals "A" or "F".
Instead, three wires exit the motor. A green wire, a red/green wire, and a yellow/green wire.
Seems there were many variations of DR3A motors. Some had terminals, others had flying leads, etc. Suggest you remove the end cover from the motor and check the internal wiring. Seems you don't have enough wires.
Hope this helps.
Regards,
Peter.