OT: The magic of PWM for controlling motors

The electrified boatlift is working fine, but it wants 12 volts.
That's available if I just suck one tit of the 24 volt trolling motor
supply, but I'd rather balance the load -- so I'd like to drop 24DC
to 12DC. PWM can do that.
I'm guessing that the ATV winch motor draws 50 amps or less of 12
volts because the 10-gage wires don't get warm and it doesn't seem to
be straining any in spite of how it screams. It screams about the
same going up or down. Some of y'all may have known women like
that. Not me, I'm from Minnesota.
Anyway, my son the silicon peddler came over with a stick of sample
MOSFETS rated at 120 amps. They oughtta work. He left with my
Milwaukee portable bandsaw, but I'm happy to report that it has
returned after doing whatever he did with it. Blade seemed OK and
clean enough that I won't bother soaking it in bleach to remove DNA.
I cooked a little circuit to use the MOSFETs, based on a really cheap
fluorescent ballast chip from IR. Half bridge driver with
bootstrapped topside drive, what's not to like -- particularly since
I had samples. They're obsolete, but they work just fine.
The whole circuit, using dinosaur-age thru-hole parts and laid out on
a single-layer board by this dinosaur, is about 1-1/2" x 2". I
suppose I'll beef up the 50-amp traces with a bit of 14-gage house
wire soldered to the traces.
Funny how things work some days. Consider the size and weight of a
600 VA transformer vs this 24VDC to 12VDC converter that will cost me
about zip and will be half the size of a deck of smokes just because
it turned out that way. The stars and planets were all in alignment
on this one.
Smoke test in the next coupla days.
Reply to
Don Foreman
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Don, you better put in some sort of limit switch at both ends of travel, or at the very least, a manual "panic" switch in series with the motor.
Silicon; it treats us good. Then the smoke comes out of it, and it treats us bad.
Gives new meaning to "a hard on".
LLoyd
Reply to
Lloyd E. Sponenburgh
let me know if you want a few feet of 10 gauge aerospace hookup wire, 200C rated, for free (you gave me some good suggestions before)
i
Reply to
Ignoramus15692
It's manually operated with the pendant-mounted momentary switches that came with the winch motor. Those switches are considerably more robust than most available limit switches. If one of those switches fails short, then I can yank the big plug from the trolling motor socket that's right next to my foot when I'm operating the lift. All I'd need to do is give the pendant a hard yank. There are also 50-amp circuit breakers in the battery compartment. The MOSFET's are rated for 120 amps continuous, 480 amps surge (very short surge!). I doubt if the TM batteries with about 35 feet (round trip) of 8-gage wire between them and the socket and another 15 to 20 feet (round trip) of 10-gage from socket to pendant to motor will even deliver 120 amps. I know a deepcycle TM battery wouldn't jumpstart my truck once when I left the lights on at the boatlanch years ago.
Reply to
Don Foreman
50% on/off ratio or is it something different? I know if you double the voltage you quadruple the power. Just wondering if I am thinking right off the top of my head. Maybe a bit of extra on time to deal with drops across semi conductor device. What is that amount? .6 volt give or take a bit?
Wes S
Reply to
clutch
Have you tried to run the 12V motor straight of 24V? If it is always connected to the load it may do the job just fine and a little faster in the bargain. Its what I would try first before starting to mess about with silicon devices. I would be surprised if the winch motor could not handle twice the voltage with half the current for the same power demand. Klaus
D> The electrified boatlift is working fine, but it wants 12 volts.
Reply to
klaus
50% on/off ratio or is it something different? I know if you double the voltage you quadruple the power. Just wondering if I am thinking right off the top of my head. Maybe a bit of extra on time to deal with drops across semi conductor device. What is that amount? .6 volt give or take a bit?
Wes S
Reply to
clutch
Yes, 50% duty cycle.
Drops on the semiconductors will be about 0.16 volts at 50 amps. I'm using MOSFET's with Rds(on) of 0.0032 ohms. At a more likely 30 amps, the drop in the semiconductors will be about 0.096 volts.
Reply to
Don Foreman
That occurred to me too, Klaus. Frankly, I'm chicken to try it. The motor already runs at over 4000 RPM upstream of the 157:1 gear reduction. I'm not sure it would tolerate 2X speed -- and at 2X voltage it would run at real close to 2X speed.
It's a whole lot easier to replace silicon than it is to replace the motor, and I have lots of silicon spares. Messing about with silicon devices is what I did for a living for many years.
Twice the voltage does not imply half the current. The current would not drop, would probably rise a bit. Torque is what lifts a load; the power it takes depends on how fast it moves at required torque. Lifing it twice as fast would take at least twice the power. Torque in a DCPM motor is proportional to current, so the current would stay about the same, plus some additional current to deal with increased friction countertorque at higher speed.
Reply to
Don Foreman

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