A friend asked me to pose this question;
Does anyone know what the Watt/Amp rating is on a 6V Model A wiper motor?
He is being told he needs an 100W / 1.5OHM resistor to run it on his 12V system to bring it down to the proper speed without over heating by an automotive electrical shop that has been around for more then a few years. The small round ceramic voltage drops sold by such as Snyders get very hot very fast. So far the only resistors he can find with that rating are military and over $100 each.
Thanks
He may have to sneak in two old style chysler (Chrysler) resistors (the ceramic bar type) in series to drop it down. They used to use ONE to drop the 12 VDC to 9 VDC on the ignition coil. Cheap enough to use for a bench test. ws
To operate a 6v device you need to get a voltage dropping resistor that drops 6V off so that you have 6V left. If the shop said 1.5 ohms is correct then it would mean that the wiper motor is drawing 6/1.5= 4 amps. The wattage on that resistor would be dissipating (P=E*I) then 6*4=24 watts. Rule of thumb says you need to use a resistor about twice the wattage being dissipated. That means you need about a 50 watt resistor rather than 100 and there is typically a big difference in size between 50 watt resistor and a 100 watt resistor. Now this is making the assumption that 1.5 ohms is correct value but since they got the wattage wrong - I have doubts about the wiper motor amps. Since this is a Model T Forum I would assume you know someone with a Model T. Ask them if it is OK to hook up your wiper motor to their 6V system. On a T it is easy. Without the car running just hook the wiper motor hot lead to the T engine ground somewhere and the wiper motor ground to the Yellow/black wire connected to the cutout that is mounted on the generator. When you touch the wire to that connection the wiper motor should spin and the dash ammeter will tell you the amount of discharge since that yellow black wire goes directly to the ammeter and then on to the battery positive. If the motor has any type of speed control on it then move that around but since it is a Model A device you will want to play it safe and hook the wiper motor "HOT" lead to the model T ground and the wiper motor ground to the yel/black wire and hold the motor in your hand so that nothing else touches. Model A is positive ground so hooking it up that way will make it work like its in a model A except not mounted permanently. The gauge should register 4 amps discharge if all calculations are correct. If they are then go to NAPA and find an ignition ballast resistor that is about .75 ohms and buy 2 of them to hook in series since they typically can handle about 25 watts each. That will get you the 1.5 ohms total and 50 watts total. The resistors you want are usually a ceramic resistor block with resistance wire stretched across it to make up the resistance. Be careful where you mount the resistors since they are gonna get hot.
A second alternative is if the wiper motor is easily removable you might be able to find a 12V version of the motor to retrofit and that would be a way way better approach since that 12V motor would draw about 2 amps and there would be no need for a resistor nor its large amount of heat.
A third alternative is to use 2 of 6V batteries in series to make up 12V battery for the car and charging system. Then connect the wiper motor to the junction between the 2 batteries to thus tap half the total voltage. In days of old the battery cell straps were on the outside of the battery so it was possible to drill and thread a small hole in the middle ******** a 12V battery to thus tap "half the battery" to run radios and the like off 6V as needed. There is no way to do that with a modern battery so don't try it since it was dangerous in days of old and impossible but still dangerous to try it now.
