Windscreen wiper motors

Rich Wightman's article in MEW 225/226 uses a car windscreen wiper motor for a lathe power feed. Some time ago I tried using such a motor in a REMAP job; it had to be used in both directions. It worked fine for a while, but then mysteriously lost power. Initially I suspected the power supply, but that was OK. The problem turned out to be with the motor itself, which turned out to be intrinsic in its design and ruled out its use in this application.

Modern motors are very powerful and compact and, although they can be run in reverse, they not designed for it. In order to make them compact the brushes are arranged to be tangential (or nearly so) to the commutator, instead of radial. This way there is a larger area of contact on the end of each brush, thus allowing more current to flow. When the motor is run in reverse, the commutator drags the brushes a little in the opposite direction and wears a new arc on the end. After a a number of runs in each direction, the brushes end up with two arcs; each one about 1/2 the length of the original, thus reducing the current that gets into the windings, hence the lower power.

Apparently, the older motors didn't have this problem but I wasn't able to locate one. So I had to abandon the motor and use a different power source.

How about a cordless drill - has fwd and reverse, plenty of torque. Even has seed control on some of them.

Good alternative - it's designed for two way running and probably about the same power.I didn't think of that when I did the REMAP job.

Whether or not there's any merit in the theory about some windscreen wiper motors being unsuited for reverse operation, you could safely get around the problem by buying a "window winder" motor rather than a "windscreen wiper" motor. They are pretty much the same, comprising a motor and worm reduction but are undeniably designed for bidirectional operation. I bought a 24V window winder for peanuts from a surplus shop and it was fine. 0-30V variable power supply from Rapid Electronics gave a variable speed.

I've used a up/down car window motor for the past 12 or more years, I did know of the windscreen wiper motors problem. There was a good article in model engineer or model engineer workshop regard making your own speed controller using Thyristor and a few other bits, basically the same a 240 volt drill speed controller. I made a dog clutch to dis engage the motor for hand control, as required. I'm not sure about the wheel chair motors as they have a lot of grunt, maybe to much if things got a bit wrong and physically a bit big. I'm sure Neil will have the issue number. John

Muzzer, there is no 'theory' about some windscreen wiper motors not being suitable for running in both directions, 'some' are definitely not. Apart from the brushes mentioned, armature shaft endthrust is usually only catered for in one direction. I have also seen worm and pinion tooth profiles optimised for unidirectional operation.

In general window motors from older cars (say 1960 to 1990) are more amenable to being run in reverse, they are also probably more likely to be able to be dismantled to add extra thrust provisions. All wiper motors though will have blade 'parking' switches and they can slightly complicate the wiring.

Motors designed for electric windows are theoretically and practically, more suitable for our purposes, apart from the bidirectional rotation they will not have any parking switches. One caveat though is that they are not rated for continuous running. The motor only sees power for a few seconds at a time so the car manufacturer can overdrive the smallest and lightest (and cheapest) motor to do the job.

Ian P

Edited By Ian Phillips on 26/02/2015 19:11:54

I've got a good speed control article coming up soon.

Neil

On a similar note, I have a small Emco Unimat lathe. Its motor is getting distinctly tired, and I wonder if anybody out there has found a suitable substitute, both in power and size? I am thinking of using an old 12V drill as a donor, but there may be better solutions out there.

Regards, Chris

I'v used an OLD Lucas windscreen wiper motor on the table of my vertical milling machine for a good many years(pos 15), no troubles yet. I run it on a 17V supply with 3 speeds controlled by switched resistors.

Ian S C

I've just splashed out a couple of quid on a used Peugot 307 window motor and PWM controller similar to George's (Mechman48 on this forum). This will be for a motorised X axis. I'm hoping to also include a self latching relay linked into the motor controller circuit just to supply the coil voltage.

Ed.