Hi guys

A friend of mine asked me, he wants a QUICK and DIRTY way to control a 12V DC motor between 90RPM to 100RPM (+/- 10%)at a constant load, what is the easier way to do it?

Thanks.

 

Pwm .

 

how many amps is the motor rated? how many RPM is it rated? give all the info you have on the motor.

 

how many amps is the motor rated? how many RPM is it rated? give all the info you have on the motor.

Good question, my friend didn't give me tho info, but he said something about a DC motor uses in a car's wiper. (I assume that's the one he has)

 

Pwm .

Is there a dirty and easy way to get to 90RPM to 100RPM +/- 10%? like some sort of estimate of a good thumb of rule or something?

 

Here's a way to measure motor speed:
http://www.ncwoodworker.net/forums/showthread.php?t=48714

 

The quick and dirty answer could be as simple as putting a rheostat in series with the motor, but not knowing anything about the motor, this could be a terrible suggestion.

 

A regulated and adjustable voltage supply will run a DC motor at a more-or-less constant speed. So something like a bench power supply or a LM317 adjustable voltage regulator will work.

The big problem is your speed range. To reliably get 90 RPM you need a geared DC motor so the motor can be going many times faster RPM but the output shaft is going 90 RPM. Otherwise if you try to run the motor too slow it will have no effective torque and be very unstable in RPM.

 

Here's a way to measure motor speed:
http://www.ncwoodworker.net/forums/showthread.php?t=48714

ha, that looks like the quick and easy way I want, thanks tubeguy!!

 

You can set or provide the voltage at a fixed value sufficient to provide the torque need for the particular load, this is only good for constant torque loads however.
Anything else and some kind of feedback, current sense resistor etc is needed.
Max.

 

You can set or provide the voltage at a fixed value sufficient to provide the torque need for the particular load, this is only good for constant torque loads however.
...

That's true with higher RPMs. At very low RPM a DC motor will be near or below it's no-load stall speed, so will stop and start or behave erratically, and even the smallest change in load will stall the motor. You also have detent load-torque pulses caused by the motor poles and commutation.

90 RPM is well below the stable open-loop running speed of most small DC motors.

If Bug13 could provide some motor specs that would help a lot, particularly the RPM per volt value (sometimes called k/V). A photo of the motor would help a lot too.

 

That really depends a lot on the technology of the motor, how many poles, skewed rotor etc.
Most of the T.M. motors, which are not generally Hi-Tech, can perform below 90rpm with simple SCR type control.
Max.

 

I will get more info of my friend when I see him next time, I don't have any more info at the moment

 

That really depends a lot on the technology of the motor, how many poles, skewed rotor etc.
Most of the T.M. motors, which are not generally Hi-Tech, can perform below 90rpm with simple SCR type control.
Max.

Sorry but I've never seen a typical "12v DC motor" that was good at 90 RPM. Most have a top speed of a few thousand RPM and 90 RPM performance is either very poor or simply below the stall speed.

I'm not sure what a T.M. motor is.