Controlling a DC motor

[JohnG]

I bought a tug to tow a helicopter. The tug has a 12-volt electric motor which runs off an automobile battery. The motor goes in forward and reverse, but only at one speed -- slow! So, I am putting a bigger sprocket on the motor shaft to get more speed, but I'd like to be able to slow the motor down for tight-space applications. The motor is rated at 22 amps at 12 volts D.C. Ideally, I would like to put a variable resistor in line with the motor. As an alternative, I would like to be able to switch in a fixed-value resistor to drop the voltage to the motor by half or so. I figure that the motor's resistance is 6/11 ohm, so the variable or fixed resistor would need to be in this ballpark.
My questions are: (1) Is this a viable solution? (2) If so, do you know a source for a variable or fixed resistor at these values and power capability? Thanks!

 

[ante]

Hi.
Are there open cell connectors on your battery?

Ante :roll:

 

[JohnG]

I don't know what an "open cell connector" is. It's just a regular car battery with a positive and negative terminal. By the way, I see you're from Sweden. So is my wife -- she was born and raised in Angelholm.

 

[ante]

Well, Angelholm is about 120 km north from were I live, it’s a lovely place especially in summertime. Say hallo to your wife, I’m sure she knows Trelleborg.
OK, open connectors is when the “jumpers” between the cells are on top of the battery. If they are you can get hold of 2, 4, 6, 8, 10, or 12Volts on a 12Volt battery. If not a variable resistor is possible but it will have to be 500 to 1000W rating. If you are in to electronics you can build a pwm circuit with some paralleled mosfets for step less controlling of the speed. Or you can by a complete unit. It all comes down to costs, time, and interests. I’m sure there are a few more alternatives to but it is possible.

Ante :roll:

 

[TillEulenspiegel]

Well Johng
The answer to Your questions is (1 no and (2 Yes.

The use of a variable or fixed resistor using the V-A figures You have given , is not really practical. What you have to remember is that a resistor is a passive component and basically puts a "drag" on the available supply to the load ( motor ). It radiates the energy it uses as heat , so even if your running the motor at %50 , the drain on the battery is the same ( or more ) then running the motor at %100. The best solution is to build or buy a PWM controller as Ante said.

The prospect of building from scratch would be fun if Your into electronics but as far as convenience and cost , you would be better off purchasing a device.
This place WWW.Grainger.com , is a good source for you . I bet when you get thru paying the price and the shipping You will be better off.

A 1hp controller should do the trick

 

[IRQ57]

Dc Motor

Hello,
Before to go to Graiger, I would suggest you to try first with your resistance (very simple) and then with a variable pulse width modulator drive, if the first does not satisfy you.

The resistor is of a very low resistance ( a fraction of a ohm), you may use an iron wire # 12 or so, put in series a lenght of a meter to try, and trim it to fit your needs. You will be wasting energy, but consuming less than with the full voltage applied to the motor.

For the more complicated controller (not so), see WWW.cpemma.co.uk/555pwm.html

With this simple circuit you can vary the motor speed in an ample range. You must use an IGBT for at least 100A, or MOSFETS to sum that current. For full power use a by pass contactor.