OT: DC Motor Speed Control

I'm playing with a small DC motor, similar in size to a windshield wiper motor and was curious what it would take to change the speed.

Can I do it with resistors? Possibly energizing different circuits supplying power to the motor depending on what speed I would like to run?

Thanks,

Originally posted by cntrlfrk:

Can I do it with resistors?

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That will give you really poor speed regulation. An unloaded DC motor will draw very little current. You will drop very little voltage across your resistor. As the load increases and the current draw increases the resistor will drop more and more voltage. This is voltage not available to the motor to overcome the motor back EMF. This will cause the motor to slow down significantly as it is loaded.

If you are just playing you could probably build yourself a transistor based Class A amplifier and drive the motor with that. It will be pretty easy if you keep it bare bones (no current limit, etc.). Just control the output voltage. The transistor will regulate the current required to get the desired voltage. This still won't be as good as a full blown drive but it will get you in the ballpark.

Keith

Look at a LM317 (1.5 amp) or LM350 (3 amp) voltage regulator. I've used these before as cheap & dirty speed regulators when the load is fairly constant.

Originally posted by keithkyll:

Look at a LM317 (1.5 amp) or LM350 (3 amp) voltage regulator. I've used these before as cheap & dirty speed regulators when the load is fairly constant.

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Now that I like. Good one.

Sometimes I just ain't sure about y'all, y'sll sure do want to make things complicated.

Seems to me that for many, many, years cars used DC motors for windshield wipers and have different (at least 2) settings. How do y'all reckon this was done, many cases it was a voltage divider.

What y'all have to worry about in this case is wattage, ex: 12v motor @ 2A will be 24 watts. You can use a potentiometer OR resistors as a voltage divider to easily control a DC motor depending on your needs. You vary the voltage applied to the motor to obtain different speeds.

If you have a problem with voltage dividers etc then check here:
http://www.patchn.com/DC/DC_6.html You did not offer full details of the motor but make sure the potentiometer or resistors are capable of full wattage for the motor.

Who would have thunk it. I guess you really can find anything on the web.

Take a look at this page:
http://www.scary-terry.com/wipmtr/wipmtr2.htm

Keith

I don't know if you want it simple. But if you want to use some electronics, make yourself a PWM.

cntrlfrk said:

Can I do it with resistors? Possibly energizing different circuits supplying power to the motor depending on what speed I would like to run?

Thanks,

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Yes,you can.First alternative,on the field line a variable resistor (reosta) that may be, for instance, 33 ohm might be placed.But this resistor must be in high power.And you normally have to adjust it while there is NO power on the motor.But as you have dc motor with small power, you can adjust the resistor with comfort.

And second alternative that you may change the speed on the motor is to play tachogenerator feedback to control card.You can do it with a small amount power consumption with a threepot that may be 5 kohm.And you can adjust it while there is power on the motor.

Thanks for the responses everyone. This ought to get me going!