Best way to step-down DC voltage in high current app (100A)?

Question

I've converted my lawn mower to electric with a 12V winch motor and two 12V batteries, and have $300 into it at this point. It pulls around 70 - 120 Amps.

... problem is it gets too hot and spins almost scary fast - it doesn't just mulch, it vaporizes.

Anyway, it seems I should be able to solve both problems by dialing down the voltage to around 10V. I could just throw a big carbon rod in series ($10), or was thinking about a big diode array (like 40 3A diodes in parallel) though that would only drop 0.6V ($50), or get a switching DC-DC converter ($100+). I'd prefer to do something where I'm not wasting 20% of my power (carbon-rod method).

Is there another way that's cheaper and/or better, or which is the best approach? Would the diode array method just burn up voltage the same way a resistor would or do they actually save energy vs. using a resistor to achieve the same effect?

Answer 1

I think what you need is a variable speed and torque controller. The DC motor you have is more than likely designed for low speed high torque applications, a winch usually has a gear train to reduce the output speed and convert the power output into torque.

Because the motor is operating in a virtually no load situation, all the input energy is being converted to speed.

In a DC motor, the magnetic field is created by the current

through the field winding in the stator. This field is always at

right angles to the field created by the armature winding. This

condition, known as field orientation, is needed to generate

maximum torque. The commutator-brush assembly ensures

this condition is maintained regardless of the rotor position.

Once field orientation is achieved, the DC motor’s torque is

easily controlled by varying the armature current and by

keeping the magnetising current constant.

In your case you could try introducing some resistance to the armature current via the brush assembly, but you really need a meens of feeding back the shaft speed into a control loop which whill vary the armature current according to the shaft speed, so that as the motor is loaded, ie slows down, more current is fed to the armature to speed it up.  

Magnetic or optical sensor attached to the shaft and drive electronics to control the armature current. I don't kow how practical this would be for your machine.

Maybe a manual control using a pot and a couple of power transistors to form a kind of "throttle" arrangement to contol the armature current.

I'm sorry I can't offer a direct solution but perhaps this will help you in the right direction.

Answer 2

A resistor is the cheapest way to go.

It should be scary, the blades could shatter...

at 100 amps and 2 volts, that is 200 watts, 0.02 ohms. difficult to find in that range. You may be able to wind your own out of copper wire, but the problem is keeping it from melting. bare #6 wire, you would need 50 feet.

Resistor, transistor, diodes, you are still wasting 20% of your power. DC-DC converters at that power level will be expensive and still waste 10-20% of your power.

One interesting alternative is pulse width control. You need lots of big transistors and a heat sink, but you won't waste as much power because the transistors are switched between totally on and totally off. With a good design, when on they will drop half a volt, using 50 watts. Basically you feed the transistors a pulse train, with a variable duty cycle. 50% duty cycle, you get half the voltage to the motor. 90%, you get 90% to the motor, etc. You can dial it down to 10% or lower for low speed operation.

I don't know what frequency to run it at, depends on the motor. Start at about 20hz would be a guess. Expect more noise and vibration, as the motor is getting switched on and off 20 times per second.

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