[Newbie] DC motor education

Bi11Hudson said:
One major point that the discussion has (conveniently?) overlooked in the use of a small gear reduction motor such as a winshield wiper or electric window motor or a reduction drive from Amazon. A D-C motor is easily reversed. However, the motor cannot normally be overhauled in reverse.

In most cases of the automotive motors, the reduction is accomplished with a "worm" drive that is not reversible. The motor can be reversed, but the drive cannot be overhauled from a hand wheel or the like. The other type of reduction is with a "planetary" gear system. They can be overhauled but at a high cost. Basically a battery powered drill, there is no need to order a motor from Amazon, just locally acquire a used (even dead) battery drill and disassemble it.

If I were facing the particular situation described above, I think he is describing running the motor in reverse. That is easy, the exact method being a function of the method, not the motor. As also is speed control. On the other hand, using the handwheel manually and overhauling the motor is mostly out of the question. The D-C motor will require a lot of reduction to be truely functional, reducing or eliminating such overhauling. That eliminates most of the possibilities.

Were I doing the process, I would use a stepper with a reduction of, maximum, say 4:1. That would reduce the "poling" of a stepper to a usable level. While still allowing it, the stepper, to be overhauled. There would be resistance at the start, but once the leadscrew was moving it would be fairly easy to continue.

Looking at the problem from the view of industrial operation, possibly with complete automation, the use of a mechanically reduced "servo" motor makes plenty of sense. However, from the view of a hobbyist, there is still need for manual control on a (regular) basis.

I've lectured enough, you need to look into every angle of the problem.

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Bill, I'm not sure fully understand what you mean.
Are you saying that a wiper motor cannot be run in reverse?
 
A wiper motor, actually any brush type motor (AC or DC) can be run in reverse. Sometimes the "neutral plane needs to be adjusted to reduce sparking, but that's a whole 'nuther subject. The sparking just indicates the neutral plane needs fine tuning. Just swapping the "A" (armature) leads relative to the field, be it series, shunt, compound or permanant magnet, makes the motor run the other way. Even a sewing machine motor can be run in reverse. The connections that need to be swapped are internal, but it can be (relatively) easily done.

What I was expounding on was "overhauling" the motor by driving the load backward. Picture in your mind going down a hill in an automobile. When you release the accelerator, the car(load) overhauls the engine(motor). Called "engine braking" for automotive applications, that is what I am refering to as "overhauling". There may well be a more descriptive term, that's just what the "old timers" called it in my day.

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To quote Cool Hand Luke, "what we have heah is a failuh to communicate." :)

I'm pretty sure what Bill means is that, with the motor attached to the leadscrew, it will be difficult or impossible to back-drive it (he uses the term "overhaul"). So you'll probably have to include some kind of clutch mechanism in the linkage to the leadscrew. The motor can be run in either direction.

EDIT - Bill responded while I was typing, so listen to him, not me.
 
Bi11Hudson said:
A wiper motor, actually any brush type motor (AC or DC) can be run in reverse. Sometimes the "neutral plane needs to be adjusted to reduce sparking, but that's a whole 'nuther subject. The sparking just indicates the neutral plane needs fine tuning. Just swapping the "A" (armature) leads relative to the field, be it series, shunt, compound or permanant magnet, makes the motor run the other way. Even a sewing machine motor can be run in reverse. The connections that need to be swapped are internal, but it can be (relatively) easily done.

What I was expounding on was "overhauling" the motor by driving the load backward. Picture in your mind going down a hill in an automobile. When you release the accelerator, the car(load) overhauls the engine(motor). Called "engine braking" for automotive applications, that is what I am refering to as "overhauling". There may well be a more descriptive term, that's just what the "old timers" called it in my day.

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Gotcha, that makes complete sense.
I plan to build a clutch into the system as Stefan Gotteswinter did in his build.
 
Bi11Hudson said:
One major point that the discussion has (conveniently?) overlooked in the use of a small gear reduction motor such as a winshield wiper or electric window motor or a reduction drive from Amazon. A D-C motor is easily reversed. However, the motor cannot normally be overhauled in reverse.
Click to expand...
That sounds like one needs a way of uncoupling the motor. Some sort of dog clutch in a steel sleeve perhaps.
[Edit - Oops, the clutch suggestion got there while I was typing]
 
A dog clutch wouldn't really fit the bill. The leadscrew would need to be turned or the motor would need to run a fuzz before engaging. Either one is not acceptable. It would need to be a "friction" clutch of some sort that does not move anything when it first engages. It needn't be as complex as an automotive clutch, simply a grinder wheel rubbing against a piece of rubber gasket would suffice for a low torque use such as this.

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Thought this might be good food for thought, David. I like his solution, it's quite simple.

 
Lo-Fi said:
Thought this might be good food for thought, David. I like his solution, it's quite simple.

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Thanks for that. Stefan's approach is as expected, very elegant but not a small undertaking.
 
Yes back-driving I think is the more well-understood term. A worm gear drive usually cannot be driven backwards in either direction. A real advantage for things like winches, but sometimes a disadvantage
Sometimes a worm drive can be forced to drive backwards but only if they have fairly mild ratios and low friction. Exceptional case.
-M
 
DavidR8 said:
Gotcha, that makes complete sense.
I plan to build a clutch into the system as Stefan Gotteswinter did in his build.
Click to expand...
I built feeds for my mill drill X Y Z axis using DC motors worm gear drives
I coupled them to a couple of spur gears about 8 to 1 ratio
Use a slide mechanism with a lever to disconnect them
Use SCR variable supply no surging or slowdown
Has worked good for 20 years
Had a lot of free parts to make it work
I tend to use whats on hand
The motor you were looking at is reverseable this might be another option
 
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