# speed of dc motor

#### slimnick

Joined Nov 25, 2007
21
i am wondering it there is anyway to slow a small dc motor the can be bought to slow its RPM to only make 2 revolutions per second.
input voltage = 12Vdc.

i dont know if its feasible but just thought i would ask.

#### thingmaker3

Joined May 16, 2005
5,083
What is the unmodified motor speed? And what kind of torque do you need?

#### Voltboy

Joined Jan 10, 2007
197
If you calculate the speed then you could use gears or even PWM, but it would be kinda hard to determine the right values.
I think it would be easier using a modified servo or a stepper motor.

#### eeboy

Joined Sep 27, 2007
90
I tend to agree with Yoda. If you don't need a large torque then a stepper motor would be the easiest method to achieve exactly 2 RPM.

If the rated RPM is on the lower end (say 100 RPM or lower) then you may be able to achieve reasonable (good enough) results by trial and error with PWM (or a similar method to control the current).

Otherwise you are probably going to need to develop a full blown control system.

#### beenthere

Joined Apr 20, 2004
15,819
2 RPS is 120 RPM - should be no problem with PWM drive. It might be easier to find a gearhead motor that delivers about that output without needing a speed control. You get torque multiplication as an added benefit.

#### Voltboy

Joined Jan 10, 2007
197
but he wants 2 RPM not 2 RPS

#### beenthere

Joined Apr 20, 2004
15,819

#### Voltboy

Joined Jan 10, 2007
197
oops.. sorry i only did saw the RPM and stop reading

#### slimnick

Joined Nov 25, 2007
21
ok i found a schematic for a PWM ill give it a look over and if any problems ill write again. thanx

#### Voltboy

Joined Jan 10, 2007
197
It will be kinda hard to get 2 RPS with PWM.
Why do you need it at exactly 2 RPS?

#### beenthere

Joined Apr 20, 2004
15,819
That's why I suggested a geared-down motor. Much easier to use PWM successfully.