View Full Version : DC Motor Current Draw Question

09-21-2006, 08:28 PM
I have ordered a couple of 12V DC motors for propulsion for my robot project.· The documentation for the motors says they draw 1.5 amps under no load.· Is there a rule of thumb or approximation for how much current they'll draw under load?· I need to order a motor controller for them, and I'm looking at the MotorMind C which can supply 2.25 amps when cooled.··Does·this·sound adequate·to move·a 15 to 25 pound robot?

Also, I noticed Parallax has discontinued the MotorMind C.· Was there a problem with it?· For anyone who has used this controller -- what is your opinion of it?

Rich Mays

Paul Baker
09-22-2006, 02:58 AM
The important figure of a motor when choosing a controller is the stall current. This is the current draw of the motor when the shaft is stuck and also the condition when the current will be greatest. The current drawn at load will be between the no-load current and the stall current. I haven't worked with the motor controllers, but there is always the HB-25 if you want to play it safe.

Paul Baker (mailto:pbaker@parallax.com)
Propeller Applications Engineer
[/url][url=http://www.parallax.com] (http://www.parallax.com)
Parallax, Inc. (http://www.parallax.com)

09-22-2006, 07:43 PM
Depending on the motor, you may be faced with 10 times the no-load current at full load, maybe more at stall. You really should try to get the specs on the motor.

The trick sometimes used is to figure out the maximum current the controller can put out and figure out the resistance that will limit the current to that value. Say you have a 25 amp controller, at 12 volts. The resistance would be 12v/25a= 0.48 ohms.

Put that resistance in series with the motor leads.

Your motor will operate at slightly less than full performance, but you will not ruin the controller during testing. As you get more comfortable with what it takes to move the load, you can decrease the resistance.


Tom Sisk