Thanks for the confirmation.
more like 1700 watts.

1700 watts is what the MOTOR will use at full load. If there is some resistor to drop 3 volts with the motor running NO LOAD that will be a lot less power, First, because the resistor has only THREE VOLTS across it, and second, because the motor draws less current with no load. CERTAINLY the resistor will still dissipate a lot of power, and it will get hot. But nowhere near 1700 watts.

My Apology! Not trying to jump all over, just that I sometimes get excited when I see a misunderstanding.

 

Doing the math I would need a .021 ohm resistor and a switch capable of handling the current . Not something that pops out when doing an internet search. The PWM control sounds like the way to go because they are readily available but why limit the current , wouldn't that limit the power. And why have it on a separate circuit , can you not just dial in the power with the potentiometer? And what range of speed control is it capable of, is a reduction gear even required?

 

The reason that I suggest a separate circuit for the free-running portion of the cycle is that the main circuit will need to be able to carry the full load current, which could be 142 amps, or more if the load becomes excessive. The free-running portion of the cycle would not draw nearly that much current. Probably the free-running portion of the cycle will be less than thirty amps, since the only load will be a bit of friction. So why have the speed limiting circuit be able to handle the high current load for a much longer time?? That is a lot of extra expense for not much benefit.
Of course, it may also be possible to add a level-winding function to the anchor winch mechanism. I have seen that function on a long-pull winch, and it appeared to work quite well.

 

I'm beginning to think the solution is mechanical rather than electrical. A proper brake would slow the winch down when free running and is easily adjusted to load. The brake is already there but needs a good lining for the band. It's handy and weather resistant.

 

I'm beginning to think the solution is mechanical rather than electrical. A proper brake would slow the winch down when free running and is easily adjusted to load. The brake is already there but needs a good lining for the band. It's handy and weather resistant.

Rather than brake the winch drum, why not a friction brake for the cable?? It seems that the problem is that when the slack cable is wound rapidly it does not wind flat and neatly on the drum. Some sort of wiper arrangement could also remove water from the cale surface, which would extend the cable life and improve the system reliability.

 

The concept of using PWM is fine. It essentially turning the power on and off really often, so the motor never has time to get up to full speed. You could put a speed control on it and adjust the speed however you like. The practical challenge is that PWM outputs usually use MOSFETs and they can't take much abuse. So if something happened that stalled the motor, they might overheat and die in a fraction of a second. Also, they will likely be mounted on a circuit board which won't like getting wet. But it probably will need lots of ventilation to keep the MOSFETs from getting too hot, or really big heat sinks on the outside of a sealed system.

All of those things can be dealt with. In lower-current applications (I have only dealt with maybe 15 to 30 amps), I am a huge fan of the Infineon smart high-side switches and H-bridges (if you need to run the motor both directions, you are probably looking at H-bridges, which are made with internal MOSFETs). Those Infineon parts automatically turn themselves off if they get too hot or if the current exceeds some limit specified in the datasheet. In my experience, they have always exceeded the datasheet specs, unlike four other brands I tested. But if you are buying a PWM driver rather than designing one from scratch, you are stuck with whatever they used for MOSFETs. If they provided similar protections, it might be pretty tolerant to abuse. If not, you have zero margin for error and a single problem will likely smoke it. Getting one spec'd for more current than you will typically need is a good start, but no guarantee. One reason for that is that the stall current for motors can be several times higher than the max current they are rated to use at their max (sustainable) load and buying a PWM driver that large is likely cost prohibitive.

I'm guessing that those practical challenges are why most of the replies you have received on a "circuits" forum are pointing you toward other solutions. But if you can find a good pre-made PWM driver that addresses those challenges, it might very well be the best solution. Let us know what you find!

 

Thank you for that explanation of PWM controllers and their limitations. It's good to know it is a doable solution if you can find the right equipment. I think such a unit could be mounted close to the motor but on the interior of the boat and out of the weather with plenty of air circulation. The winch free wheels out so no requirement for change of rotation. Would a circuit breaker before the PWM controller not prevent burnouts?
For now I am going to install the unit as is and in the spring when I relaunch test it under actual working conditions instead of bench tests.

 

Would a circuit breaker before the PWM controller not prevent burnouts?

A circuit breaker is good for keeping wires from overheating over a matter of seconds, but responds way too slowly to prevent a MOSFET from burning out if the current gets really high. The tiny piece of silicon inside has very little thermal mass, so it can get too hot very quickly.

 

ONE THING CERTAIN is that the pass Mosfets for up to 142 amps will need to be mounted on heat sinks, and that there will be more than one device. Also, that is the full load current without any overloads.
So now there appears another question: How much power does it actually require to raise the anchor??? And how often will the anchor be caught in seaweed? The big questions are just how much power does it actually take to raise the anchor?? AND is the present motor selection correct, or even close to correct??
Possibly a slower motor that produces more torque would be a better choice. OR possibly a different gear reduction package??

 

My design criteria for the winch was based what was available on the internet and a lot of ignorance. I chose an aftermarket Ramsay winch motor because 1700 W was more than most of the quality windlasses used. It was already a winch motor so had the duty cycle I required. It was listed as having 2100 rpm output. From this I was able to find a planetary gear reduction that I could couple to the motor to get the final drum speed. The ignorance was not knowing it was series wound and the speed characteristics of that type of motor.
How much power does it take to raise an anchor? I spoke with a diver who lived near a small harbour with pinnacle rocks that regularly snag anchors and is called out at least once a season to free anchors, so I would say enough power to stall the motor.

 

1700W ÷ 12V (Assuming DC) = 141 2/3 Amps. That's 3HP.

8:1 means 3HP (whatever the torque is) equates to 24HP. Further "4.16 to 1 reduction' changes 24HP to the equivalent of nearly 100HP. There's not much meaning in those numbers without knowing the torque of the motor. Whatever the torque is you're talking a torque reading of 33 1/4 times the motor torque.

Congratulations, you have just created the first multiplying perpetual energy machine!
Seriously, a gearbox changes the torque of the shaft by the gear ration. The power (less losses) remains exactly the same.

 

Congratulations, you have just created the first multiplying perpetual energy machine!

Really ? ? ?

a gearbox changes the torque of the shaft by the gear ration.

That was kind of the point I was making.

The power (less losses) remains exactly the same.

Since work is a function of accomplishment over time, the same power can move 100 pounds 10 feet in 10 seconds as it can moving 10 pounds 100 feet in 10 seconds. And as you mentioned, "Less losses". At some point there comes a trade-off where the losses exceed the gains. IN THEORY - you can use a tiny alarm clock motor to move a freight train. You won't get very far and you'll wait a lifetime to see any progress. "In Theory". But due to losses of friction and windage, and other parasitic losses, there comes a point where the gear reduction can be too much for the clock motor to even turn. But I'm talking about "Theory". In life nothing works the way theory would suggest.

Archimedes: 'Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.' In theory that is.

 

OK, so that size motor is needed, AND it may be asked to run even beyond the rated power for a short time. THAT means that it would draw a lot of current. IF a full current PWM system is used for the speed control, those transistors will need to carry that whole current, probably for several seconds. THAT will be a whole lot of heat in a small volume very rapidly.

THAT is the reason that I suggested a separate circuit to run the motor at a slower speed to take up the slack. Of course, depending on the physical push-button for the "raise anchor" command, that could be as simple as only pressing the button half way down to take up the slack, and then full down to give the motor full power.