2 speed wipers for MM ~ Part 2

[Bally]

PART 2
The first step is to remove the Bakelite end cover and the pole piece to expose the innards. The heavy copper wires are doubled over where they are under the grub screws, and need to be straightened out to enable disassembly. Before doing this the wires should be annealed to prevent them from breaking. Do this by heating to red hot very quickly before the heat transfers too far then dunk them in cold water. Removing the end cover you will be able to remove the brush arms and brushes. Mark the brushes so that they can be reassembled the same way as found. Remove the two screws holding the insulation plate that the brush holders fit to. The next operation is probably the most difficult – the solder holding the heavy wires to the brush holder pivots has to be removed to enable the insulated plate and brush holder pivots to be removed. Best done with de-solder braid to get rid of as much solder as possible then working each side up a bit at a time whilst heating with an iron. When removed you will be able to see how the ends of the field winding are twisted around the heavy leads. Heat the negative lead and unwind the field wire. It needs to be shortened and a length of wire joined to it by soldering and insulating with heat shrink tube. The wire is then passed through a hole in the field coil former. This acts as a strain relief so as not to put too much strain on the fine field wire. The attached pics show how the field wire is connected and passed through the hole in the former. If there is no hole one will have to be drilled without damaging the field winding. Other pics show the new 3 mm. brass screw terminal and a view of the finished mod. When reassembling re anneal the heavy wires again and it may be necessary to clean excess solder from inside the pivot holes with a drill to enable the insulating plate/pivots to be passed over the heavy wires. I have slipped PTFE sleeving over the new field lead and the main leads. Make sure all is OK and cleaned properly for soldering before screwing the insulated plate back and re-soldering the heavy wires to the brush holder pivots. The rest is just reassembly. The original terminal I have marked with a red + connects to the 12 volt +ve wire (#17) from the fuel gauge. There is an alternative configuration shown in the wiring diagram ‘variable speed wipers’. Because I had a 25 ohm 50 watt wire wound potentiometer I used this circuit because it retained the original switch and by varying the pot gave an infinitely variable speed from 36 to 58 CPM. HAPPY WIPING.

[Paul M]

Hi Bally,

From your research, do you know if this will work on a later MM? I believe they have the same motor as my Wolseley 24/80, and I threw this cct below onto the Farina Forum.

I believe this cct came from an American MGB forum, but am keen to adapt it
Edit *** This is where I got it http://www.britishcarforum.com/bcf/content.php?156-Lucas-DR3A-Two-Speed-Wiper-motors
This forum has a lot of good info
Regards, Paul

[DON58VAN]

As I understand it, a downside of using this approach to get a slower speed is that the power of the motor is significantly reduced and it may struggle to drag the wipers across the screen.

Any thoughts on this?

Don

[Bally]

Hi Paul and Don Hi Paul
The circuit you have shown is electrically identical to the MM motor I have modified except for the park circuit. I believe this series resistor will work on any shunt connected motor. I have no understanding as to why weakening the field results in an increase of speed whilst maintaining full torque and nor do specialist motor winder/ repairers here in the ACT that I have spoken to. Whilst researching the matter I discovered that Lucas actually used this system in their later model DR wipers. I have attached pics from a Lucas doc. for you to see. Let me know how you get on with it. Don - Strangely the motor does not loose power. The difference between the no load speed and the speed wiping a wet screen is one cycle per minute. Cheers Bob

[Bally]

Hi Guys
It has come to my attention that I have specified a resistor of considerably
higher rating than actually required. I have attached a copy of test data from a Lucas Equipment Workshop Instruction buletin which shows the field current for a Model CR wiper motor at 0.8 amps. To be on the safe side make this 1.0 amps at 14 volts which is 14 watts so go for 15 watts or maybe a bit more for safety. I have actually measured field and armature currents at considerably less than this but with no load on the motor. Hope this helps.
Cheers Bob

[Paul M]

Thanks for adding more info Bally, I recall the mod in my diagram was based on a later two speed wiper motor.

From my basic motor theory many years ago, increase field resistance decreases field current, which reduces back emf and increases speed.

I think that is right, even if oversimplified and someone who is either younger or smarter (both?) can correct me

Cheers, Paul