Controller as 48v-12v reducer

[Jim C]

Has anyone done this and have details on it?
I need to power a winch and Do Not want and accsy battery?

Thanks

[JohnnieB]

Interesting idea, but it'll probably smoke the 12V winch motor since a motor speed controller's output is chopped DC whose voltage is equal to the input voltage.

The winch has a series motor and the output stage of the series drive controller is a set of MOSFETs that are turned completely on (48V output) and completely off (0V output) about 18,000 times per second and how long they stay on during each cycle determines how many amps the motor is allowed to draw. Therefore a controller is an amp limiting device, not a voltage limiting device.

When we measure the output voltage with a voltmeter, we read average voltage rather than peak voltage, so the output voltage of a controller appears to vary from 0V to whatever the battery pack voltage is, but if you look at the output with an oscilloscope, you will see the out voltage equals the input voltage and just isn't on all the time, so the average output voltage is less than the input voltage whenever the duty cycle is less than 100%.

It might work, but I'd definitely use OPM (Other People's Money) to test the theory.
Are you volunteering?

[Jim C]

Thanks Johnnie for the informative answer.
I was under the impression voltage varied along with amps.
Lol, I was going to find a cheap stock used controller for the purpose, but I'm thinking 48v will fry it. Even though it's a cheap HF winch, I still don't desire to waste it.

[cgtech]

So you are saying the dc motor in the winch is somehow going to act differently than the dc motor on the axle? I think if you devised a way to automatically throttle the controller to keep the output at what "feels like" 12v to the load, it would be fine. Im thinking a suitable resistor between the B+ & M- terminal to hold voltage down a little when there is no load from the winch. And maybe a zener diode and op-amp with something to set the throttle into the correct range (maybe a 4500 ohm resistor for a controller with 0-5k throttle). I think i might start trying to make one of these. The idea i laid out may not be perfect, but it also wasnt thought-out for any longer than it took to type this.

[JohnnieB]

Originally Posted by cgtech

So you are saying the dc motor in the winch is somehow going to act differently than the dc motor on the axle? I think if you devised a way to automatically throttle the controller to keep the output at what "feels like" 12v to the load, it would be fine. Im thinking a suitable resistor between the B+ & M- terminal to hold voltage down a little when there is no load from the winch. And maybe a zener diode and op-amp with something to set the throttle into the correct range (maybe a 4500 ohm resistor for a controller with 0-5k throttle). I think i might start trying to make one of these. The idea i laid out may not be perfect, but it also wasnt thought-out for any longer than it took to type this.

No, the motor will act the same, but it is a 12V motor that is being hit with 48V DC pulses and I'm not sure if it'll play well at 4 times its rated voltage. It would be akin to running a stock 36V cart motor with a 144V battery pack via a 144V controller.

The motor is an inductive load, so it will eventually average the voltage, but those first few pulses might be pretty rough on its health. On second thought, the impedance on the leading edge of the pulse will be very high and decrease as the magnetic field's expansion rate slows, so how hard the motor windings are hit depends on the TC time constant.

Maybe a BFC (Big F...ing Capacitor) might be the answer.

To get about 12V with a 48V input, you'd want about 1/4 throttle or about 1.25K with a 0-5K throttle.