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I need to lower the RPM of a simple DC motor to almost 20 - 30 RPM for some reasons. I cannot use PWM signals or gears to lower the RPM of the motor (I know how to use them but I want a simple solution.)

Is there any other way that I could slow down this motor? I tried to lower the voltage going to the motor using resistors but it didn't work (for me at least, I connected the resistor between the motor's one wire and the positive terminal of the battery supplying 3 volts. Let me know if I did something wrong.)

I don't care about torque that much so it's not a problem if I lose some torque in the process.

Hoping for a trick that could slow down this motor without using PWM or gears.

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  • \$\begingroup\$ A correctly sized resistor will indeed do it, albeit by wasting power. You can get an idea of how big a resistor you need by measuring the current the motor draws at 3V. If the resistor drops too much voltage the motor will be unable to start. Trying to get such a low RPM is unlikely without a specially designed motor. 20 RPM is very very slow for a small motor, so for most 3v motors, you will basically have no torque left at all at that speed. With a resistor you can probably get down to half the motor's rated speed, and with PWM maybe 10 or 20%. \$\endgroup\$
    – K H
    Jan 22, 2021 at 7:58
  • \$\begingroup\$ The gears and PWM are the cheapest simplest options unless you're willing to accept 50% speed as minimum RPM for your motor. \$\endgroup\$
    – K H
    Jan 22, 2021 at 8:07
  • \$\begingroup\$ A gearbox IS simple; it's a no-brainer. On the other hand, a control loop with speed feedback to avoid the problems of intermittent cogging torque at low speeds seems much more complex i.e. not simple. \$\endgroup\$
    – Andy aka
    Jan 22, 2021 at 10:01
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    \$\begingroup\$ timing belt, with reduction ... even easier than gears \$\endgroup\$
    – Pete W
    Jan 22, 2021 at 14:59
  • \$\begingroup\$ @PeteW, what speed would a DC motor run at? OP didn't say. Even if it was only 1000 RPM that would be 50:1 reduction. \$\endgroup\$
    – Transistor
    Jan 22, 2021 at 17:23

2 Answers 2

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The speed of DC motors depends on voltage, the current is proportional to the torque. If your mechanical load is very low, you will have low current, and the voltage drop in a resistor will be also low, so no change in speed. You can try with diodes, if the current is not very low, you will get moreless 0.5 volts with each one. Other trial should be to use a resistor voltage divider. Any how the speed control will be very poor.

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Motors are like capacitors, you can't use a single resistor to reduce the voltage on a capacitor, likewise a single resistor won't work well to reduce the voltage to a motor.

you could use two resistors to make a divider and have more success slowing the motor. the resistors will make lots of heat, but maybe you can use the heat for something.

getting less than 100RPM from a PMDC motor will be very difficult.

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