[Newbie] DC motor education

DavidR8

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I'm contemplating building a power feed for my RF mill. Initally I looked at a stepper motor (primarily because I saw @myfordboy use one on YT). I've come to learn that a stepper motor might not be a good choice because even when not powered they have rotational 'steps'.
So looking at geared DC motors. I found this one:
What I can't figure out is if the speed can be varied.
 
It's quite easy to vary the speed on a brush type d.c. motor. Either a pot to vary the voltage or a rheostat the control the currnt or pulse width modulation. Here is one example of a low cost PWM control.
 
Thanks for that!
(I'm a stranger in a strange land when it comes to all things electrical)
 
If you decide to go the dc motor route, you will want to add a reversing switch. This is a simple double pole double throw toggle switch that is cross coupled. If you use a center off switch it can serve as on/off as well.
 
If you decide to go the dc motor route, you will want to add a reversing switch. This is a simple double pole double throw toggle switch that is cross coupled. If you use a center off switch it can serve as on/off as well.
Thanks, I wired in that type of switch for my VFD conversion for exactly that reason. Rev/off/fwd, works like a charm.
 
Most DC motors of that "windscreen wiper" sort are brush motors. I am not sure about the one in the Amazon link. It is true that stepper motors have distinct "steps" between magnet poles when you turn the shaft by hand. Electronic driven, a common step was 1.8°, If delivered fast enough, and the motor geared down, it looks like a smooth movement.

The guys who do CNC conversions will, by now, have a huge store of knowledge about using small motors to drive power feeds. There is a class of small DC servo-motors that have a tiny tacho-feedback generator on the back, for speed control with full torque.

This is where we get to the way motors work. If you limit the voltage to the motor, it will go slower, whether the method is to lose the volts as heat in a resistance, or to (more efficiently), feed it a pulsed voltage with variable "on" durations with a PWM speed controller. The other reason it would go slower is when the load varies. The way you get a constant set speed feed is to measure the speed, with feedback, and have electronics to respond appropriately.

Now, with small AC motors, they can be driven by servo electronics clever enough to sense the coil currents, and figure out the speed. We call them VFD's, although now, what they do has come a long way from simplistic frequency variation. For all this, I do allow the motor costs some, and additionally, the controller costs some more.

If the torque from the Makermotor is enough to turn your mill feed, and you can contrive a 12V supply for 5Amps, then what is left is feed rate control. The dilemma is that if you turn down the speed with your non-servo voltage control, you also turn down the torque. At zero speed, it acts like a stalled motor, and will take a very high current until the rotor moves. You get the "lurch" as it gets going, then it revs up to way beyond the speed you would need, and you have to dial it back. Only with a very geared down drive, can you just switch on, and have it come up to a preset speed.

I think you might ask among the CNC guys, what is a satisfactory nice straightforward controlled motor arrangement to turn a mill feed. By now, they will already have made all the mistakes, and will likely know.
 
Uh-oh! The motor David is looking at draws 5 amps, and the speed control RJ cited is only rated for 2 amps. I'd suggest something like this (rated for 10 amps):
If you look on Banggood's site,on eBay, or Amazon, there is a multitude of low cost PWM controllers that will handle higher currents.
 
I've used those little pwm controllers- they work well. Aliexpress has them
-Mark
 
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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