# Can I reduce rpm further of a gear motor by using PWM?

I am working on a project where I need to move the object from one point to another in a dead slow speed , The object doesnt weight more than 5 pounds . I found a DC gear motor with has a RPM of 0.5 but I thinks its still very fast for my requirement . This is the motor : http://www.servocity.com/html/0_5_rpm_gear_motor.html

This is the PWM : http://www.electronickits.com/kit/complete/motor/CKMX033.htm

Please suggest a way where I can reduce the speed drastically (Either simply using a potentionmeter or any variable device) . motor needs to be small the rest doesnt matter .Or you can propose me a entirely new solution .

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How are you using the motor to move the object? – Toby Lawrence Dec 12 '12 at 18:11
aren't there mechanical solutions to this like gear ratios? – vicatcu Dec 12 '12 at 18:22
Just use a smaller gear/pulley/wheel on the motor shaft. – Dave Tweed Dec 12 '12 at 19:32
@TobyLawrence Motor will be attached to a platform and will be glided on a rail track , I will use motor to drive the platform on a toothed rack. – Fenomatik Dec 12 '12 at 20:05
@vicatcu The gear motor already has a gear box which is yeilding an rpm of 0.5 – Fenomatik Dec 12 '12 at 20:06

## 4 Answers

What about a leadscrew and stepper motor? A stepper with microstepping will smoothly move an object down the leadscrew as slow as you'd like, with no discernable backlash, and you can get leadscrews up to 3 meters long pretty easily and cheaply. If a leadscrew is too pricey, what about a bog-standard threaded rod?

edit: Since you don't want a leadscrew, how about a stepper and a friction-fit drive wheel?

Drive the stepper with a Pololu stepper driver, a pulse generator (555, PIC10F, whatever). For a stepper, any 4-lead bipolar stepper will work. A sample one can be found at Allelectronics.

The Pololu drivers are easy to use. A hookup diagram is provided on their site.

Its worth noting if you do end up using the leadscrew, the lower torque from the stepper will be amplified oodles. The same mechanism can work for the gearmotor as well, just use a coupler. 1/4-20 requires 20 revolutions to move an inch, 3/8-12 requires 12 revolutions to move an inch. 0.5 RPM on the leadscrew means you'll be moving a whole inch every 6 hours :)

================== parts: ===================

As for where to get parts: Mcmaster has 12ft 3/8" ACME leadscrew (true leadscrew) for $16 ... search for 98935A622. A corresponding nut is$2.28: 94815A106

1/4-20 threaded rod is as cheap as \$8: 98957A029 and the nut is probably in your garage somewhere.

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I am trying to make a very light weight rail system , A leadscrew would add more weight to the system . Can I achieve the same goal using stepper motor only to move the same amount of load across the rail ? – Fenomatik Dec 12 '12 at 21:47
Sure, if you put a frictiony drive wheel on the stepper shaft. A 1.8*/step motor with 1/16th microstepping gives you 3200 steps per revolution, and if you do 1 step per second with a 3" drive wheel you're looking at 8 inches per hour travel. – insta Dec 12 '12 at 23:06
That is awesome , I have played around with basic DC and gear motors but not stepper .Can you recommend me any basic low voltage stepper motor with variable manual controller (to regulate speed upto requirement) . Please provide the URL if there is one I can buy online. – Fenomatik Dec 13 '12 at 0:14
@Fenomatik Controlling a stepper will require a stepper controller or a microcontroller + output driver. You will also be limited bt load carrying capacity when using stepper compared to DC motor. – Chetan Bhargava Dec 13 '12 at 23:11
Leadscrew solution looks interesting. Although not a fully electronic solution, I give +1. – Al Kepp Dec 13 '12 at 23:59

Using PWM controller will help you vary the motor speed.

Simple potentiometer won't help as it won't be able to handle the power unless it is a heavy wire-wound one.

You can also try various fixed wire-wound resistors to reduce the speed but it will be fixed.

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There are two ways one can PWM an ordinary permanent magnet motor: one can switch the motor between being connected to voltage and being open-circuit, or one can switch it between being connected to voltage and being short-circuited. The former approach will almost generally cause the motor to end up using less overall supply current, but will not offer very good speed control; speed control is more precise at lower PWM rates. The latter approach may work well, and offer both better control and better energy efficiency if the PWM rate is high enough relative to motor inductance, but not so high to cause significant switching losses. Energy efficiency of the latter approach, however, may be absolutely abysmal at moderately low-PWM rates.

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Just feed the motor less volts. Six or three. Try a bunch of different wall warts. Use the one that gives you the speed you want.

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