1
\$\begingroup\$

I want to get a fine speed regulation of Pittman DC 24 V brushed motor with 10,000 RPM for a hobby project. The voltage I will use is battery 12 V DC, so does that mean the speed will be 5,000 RPM?

I will try to find ready gearbox combination with 400 : 1 ratio because I want to get around 12.5 RPM at the shaft. The load will be pretty constant during the use of the motor. Motor will work continuously just rotating the load at very slow speed, so in several hours the load will only rotate 90 degrees. Mow I want to FINE control the speed of this gearmotor with PWM around 0.0133 RPM! Is this possible to achieve at all and is this possible to achieve with PWM?

I never did any electronic project in my life, so here my knowledge stops. From reading the internet, I suppose I need to use PWM. How I will regulate the speed with the PWM? Does PWM offer some sort of potentiometer or something else?

I see PWM images on Amazon and I can only see input and output connectors. I don't want to regulate and command the PWM with a computer or something similar to computer or computer program or something complex, just as simple as possible, just PWM and I suppose potentiometer.

If using a potentiometer, it is obviously that regulation needs to be logarithmic, because I want a lot of potentiometer movement around 0.0133 speed let's say 0.010 - 0.015 RPM, and a bit at 0.10 and not at all at 1 and 10 RPM. But I want also to push the potentiometer or whatever will regulate the speed to maximum from time to time, let's say every 10 or 15 minutes so to use that maximum speed at the shaft of 12.5 RPM. That maximum speed can be 10 or 15 RPM, it is not at all important. I just want this maximum speed to be also available for several seconds or a minute, nothing else.

\$\endgroup\$
11
  • 4
    \$\begingroup\$ You got bigger problems. No encoder = no speed regulation. Your second big problem is that if the motor doesn't already come with a gearbox, you should probably scrap it and find one that does. You will save money in the end. \$\endgroup\$
    – DKNguyen
    May 29, 2021 at 2:14
  • 3
    \$\begingroup\$ You will not get good control open loop, you need feedback so you can control the motor. There are many ways of doing this, a tachometer, optical sensor are some of the many possible ways of sensing.. \$\endgroup\$
    – Gil
    May 29, 2021 at 2:19
  • 1
    \$\begingroup\$ Half the voltage doesn't mean half the speed. It will be less than half the speed. The speed is dependent on the voltage, the load connected and the friction in the motor and gearbox as well as other inefficiencies in the setup. \$\endgroup\$
    – AJN
    May 29, 2021 at 2:22
  • 1
    \$\begingroup\$ You imply it running for hours in which case brushed motors might not be the best thing, but brushless motors increase complexity of the driver. \$\endgroup\$
    – DKNguyen
    May 29, 2021 at 2:25
  • 1
    \$\begingroup\$ how accurate do you need it? is this for aiming telescopes? \$\endgroup\$
    – Jasen
    May 29, 2021 at 2:58

1 Answer 1

4
\$\begingroup\$

Pittman DC 24 V brushed motor with 10,000 RPM for a hobby project. The voltage I will use is battery 12 V DC, so does that mean the speed will be 5,000 RPM?

About right, unloaded. If there is a mechanical load on the motor shaft it will be slower.

I will try to find ready gearbox combination with 400 : 1 ratio because I want to get around 12.5 RPM at the shaft. (...) I want to FINE control the speed of this gearmotor with PWM around 0.0133 RPM! Is this possible to achieve at all and is this possible to achieve with PWM?

No.

You can't have good control on a motor at 1% of its rated rpm. It's designed to work in a range say 10-100% of its rated RMP. At this low rpm it will have no torque, among other problems.

If you want accurate speed control without headaches, get a stepper motor with a controller, and a gearbox. If the torque ripple of a stepper is a problem, a controller with microstepping can help.

The nice thing about a stepper motor is it's synchronous. Unless your load is heavy enough to make it slip, it will go at exactly the same speed as the control signal, so you don't need to measure the rpm and use feedback to adjust the control voltage. It can also control speed over a wider range of rpm than a brushed motor.

You can also use a cheap brushed motor, with a flywheel if torque ripple is a problem, and a gearbox/pulleys/belts to make sure the maximum desired output rpm corresponds to a bit below the motor's nominal rpm (not 1%). But in this case you will need a circuit to measure the rotation speed and feedback to adjust the PWM to keep the speed constant.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.