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Why do I need a motor controller? — Parallax Forums

Why do I need a motor controller?

JohnBot115JohnBot115 Posts: 13
edited 2010-03-13 13:49 in Robotics
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

Comments

  • Roy ElthamRoy Eltham Posts: 3,000
    edited 2010-03-10 21:03
    The most basic motor controller is a simple relay switch. Check out erco's Retrobot thread where he uses a handful of relays to get forward, reverse, and 2 power levels for 2 geared motors.

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  • Martin_HMartin_H Posts: 4,051
    edited 2010-03-10 21:05
    You don't want to use a potentiometer for speed control as that is wasteful of energy.

    If by switch you mean a relay which you trigger with a microcontroller I/O pin that will work if you take care to ensure you don't exceed the current requirements of the microcontroller. It is also possible to use relays in such a manner to increase the voltage to a motor and control its speed. Erco has a thread where he did exactly that.

    I have a personal preference for a transistor h bridge and pulse width modulation (PWM) for speed control. But directly controlling them with a microcontroller has two problems. The first is you will consume four or six I/O pins (depending upon the h bridge) to control two motors. The second is you need to keep signalling the motor just to get it to run at a constant speed. Now here's where a motor controller helps. It usually handles all these details for you and you signal it using serial I/O and two pins.
  • sohel2009sohel2009 Posts: 3
    edited 2010-03-13 13:49
    The vacuum is probably the reason for the climate control. As for the blower motor resistor being the other problem, not likely. You see what a blower motor resistor does is resist some current so that fan has variable speed, when the resistor dies it only works on high. I would say that the actual blower motor is the issue not the relay.

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