Why do I need a motor controller?

[JohnBot]

Hi all,

Ok, this may be a basic question, but I’m not totally sure why I need a motor controller on a geared DC motor. i.e:

Let’s say I have a 18 volt DC motor that’s designed to run at 22 amps max. Why not just connect the 18-volt battery, with switch, to the motor. Turn on the switch, start motor, turn off, etc. I understand that if I needed to reduce speed, or needed any sort of variable speed control, that a controller is necessary. But if it’s as simple as on/off, do I still need one? Also, if I wanted it to run slower couldn’t I just hook up a potentiometer or a smaller battery?

Thanks for the help.

John

[waltr]

You could just use a switch but how would a microcontroller throw the switch? A motor controller is intended to be processor interfaced. What about reverse? or a brake? What about smoothly accelerating the motor so the gears don't strip or the wheels spin. What about deaccelerating?

if I wanted it to run slower couldn’t I just hook up a potentiometer or a smaller battery?

This might work for a small motor with a very light load but not easily with a motor that draws 22Amps at 18V. It can be done with a huge potentiometer (lots of power turned into heat) but DC motors have much lower torque at lower voltage. A motor controller will use PWM to control the motor speed with produces much more torque at low speeds and is much more efficient (less power turned into heat) which is important when running from batteries.

[JohnBot]

Good points, thanks.

This design is not for a robot though, it's for a lead screw. So it does happen to be a case where I can throw the switch manually. I was thinking of jsut using a basic hbridge, assuming I can build one for that amount of power. Would that be sufficient?

Then, basically, it would turn the motor on to full-forward, or full-reverse, correct?

Smooth acceleration, and loss of torque seem to be a problem. Hadn't thought of that.

[waltr]

It all on the application.
My garage doors use just a switch to turn on the motor, works fine.

[nottoooily]

You can use a mechanical DPDT switch to do forward/reverse.

If sudden acceleration can break something, that suggests to me the motor is overpowered.

As Waltr says it really depends on how the everything's designed. A car starter motor has more power and is just whacked on without any kind of controller.