0
\$\begingroup\$

I am planning to use this motor for a project but I am not sure if I can control the speed of this motor.

This is 100rpm motor and I would like to use a potentiometer to control speed.

\$\endgroup\$
1
  • \$\begingroup\$ Not with a pot you can't. \$\endgroup\$
    – DKNguyen
    May 16, 2020 at 2:23

4 Answers 4

1
\$\begingroup\$

As its a small motor, you could control its speed with a "Rheostat" essentially a higher wattage potentiometer,

However the proper way to do it would be with a PWM controller, it switches on the supply voltage X% of the time, proportional to some analog input, say a potentiometer, this keeps the motors speed more consistent over varying loads.

Edit: Tinkercad exploration image attached. enter image description here

\$\endgroup\$
8
  • \$\begingroup\$ Thanks a lot so maybe I should use Arduino?would that be a good option? \$\endgroup\$
    – ssdesign
    May 16, 2020 at 2:31
  • \$\begingroup\$ an arduino is fine for creating a PWM signal, likely using something such as a mosfet to switch the motor, other options could be something like a H-bridge, just for things that will have a large number of online examples and tutorials for your. \$\endgroup\$
    – Reroute
    May 16, 2020 at 2:34
  • \$\begingroup\$ Whatever you do, DO NOT buy the L293 or L298 type of Bipolar Darlington H bridge sold in the Arduino realm. They are simply horrible ancient parts. Look for something FET based only, or you will regret it. \$\endgroup\$ May 16, 2020 at 3:04
  • \$\begingroup\$ Thanks for all the help. Do you have a suggestion for a specific H-Bridge for the motor I specified above? Does this look good? amazon.com/NOYITO-Transistor-High-Power-Electronic-Controller/… \$\endgroup\$
    – ssdesign
    May 16, 2020 at 4:45
  • 1
    \$\begingroup\$ @Drew there are numerous questions here documenting the horrid losses of the L298. \$\endgroup\$ May 16, 2020 at 9:57
0
\$\begingroup\$

Assuming you don't want to make a whole project out of this speed control, I would consider some all-in-one solutions.

  1. There are many off the shelf motor speed controllers sold on websites catering to hobbyists, for example Amazon, Sparkfun, Pololu and Adafruit. Here's an example:

Onyehn-Controller-Control-Switch-Dimmer

  1. You could use an adjustable voltage regulator to change the speed of the motor. I would highly recommend using a switching regulator, because it will be much more efficient, and the whole thing won't get crazy hot. Because your motor is only 3v this might give you the most options. Some examples:

LM2596s Buck Converter

Another LM2596 Buck Converter

\$\endgroup\$
0
\$\begingroup\$

You could use a potentiometer to control the speed. However the potentiometer track must be rated to handle the current, and to get good control at lower speeds it will have to waste a lot of power.

The rated current draw of your motor is 0.08 A. At light load it will be less, perhaps ~0.04 A, and at stall 3-5 times higher, perhaps ~0.3 A. The pot must be rated to handle the stall current on any part of the track. \$P = I^2 * R\$, so a 5 watt 25 ohm pot should handle up to ~0.4 A.

The next question is, how much voltage variation can you expect with different loading? This depends on the potentiometer's output resistance, which is highest at the midpoint. The equivalent circuit might look like this:-

schematic

simulate this circuit – Schematic created using CircuitLab

If the motor drew no current then the potentiometer would output 5 V / 2 = 2.5 V, and it would draw 5 V / 25 Ω = 0.2 A and waste 5 V * 0.2 A = 1 watt.

The Thevenin equivalent resistance is 12.5 / 2 = 6.25 Ω, so if the motor draws 40 mA the voltage will drop by 0.04 * 6.25 = 0.25 V and it will actually only get 2.25 V. At 80 mA it will get 2.0 V, causing it to run ~20% slower than expected. This may be acceptable, but the pot will still dissipate ~1 W, 6 times more than the motor!

You could use a higher resistance pot to reduce quiescent power consumption, but then the voltage variation with different loading will be higher. If it is too high the motor might not even start up due to not getting enough current at stall. A higher resistance pot will also have a lower current rating for the same power rating.

\$\endgroup\$
0
\$\begingroup\$

It would be possible to control the speed of the motor, powered by a LM317-based 0-3V adjustable power supply, using the potentiometer in the power supply itself.

A flyback diode would be required across the motor.

Here's a reference for a 0-3V adjustable LM317 power supply without the minimum 1.2 - 1.3V output voltage limitation.

https://www.edn.com/use-an-lm317-as-0-to-3v-adjustable-regulator/#id2782607-48-a

enter image description here

\$\endgroup\$
4
  • \$\begingroup\$ This will be extremely inefficient. And the LM317 will get very hot. \$\endgroup\$
    – Drew
    May 16, 2020 at 6:04
  • 1
    \$\begingroup\$ Hi Drew, The dissipation would be around 0.2W only. \$\endgroup\$
    – vu2nan
    May 16, 2020 at 6:23
  • 1
    \$\begingroup\$ We'd have to make some assumptions about the motor curve, but lets say op sets the voltage to 1.5v and the motor draws 50ma (stall current at 3v is 100mA). The dissipation would be .075W. That's not too bad. I was incorrect, my apologies. \$\endgroup\$
    – Drew
    May 16, 2020 at 6:57
  • \$\begingroup\$ Hi Drew, It would be higher, with the input to the LM317 being 5V. Many thanks for your response. \$\endgroup\$
    – vu2nan
    May 16, 2020 at 7:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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