Hi Orest,
You asked "Hp at motor x 3.6 = Hp at the shaft?" The answer is no. You can't created more power through gearing. The formula is power = force times speed (or time). Stated better for our puposes is Watts (HP) = torque * RPM. So the power is constant, you can trade speed for torque or torque for speed though gearing.
If you put 3000W in (180V * 17A) gearing won't change that. What the gearing will do is trade speed to torque. Your 3hp motor isn't stong enough to spin at full speed with the load that your prop generates, so you need to increase the gearing advantage to let the motor spin up to speed.
Given your current gear ratio, it looks like your motor only has enough power to spin the prop at about 300RPM so you should aim for a ratio that matches the motor to the prop. Given your current data, that calcs out to 5.8:1. But with the torque multiplied up, the motor might be able to spin the prop a bit faster. If this was my project I would try a ratio around 5:1, tending towards the 5.8:1 if the specific 5:1 ratio pulleys aren't readily available.
I would also try to make sure that you can't run too far past the 14A rating on your motor, even while cruising at full throttle. That should keep the components happier and reduce the smell of baked plastic. Keep in mind that your amps will go higher if your tied to the dock or under acceleration, once your speed stabilizes, the current should stabilize too.
My target for my conversion has been 95% hull speed at full throttle pulling 5-5.5kW and my system delivers exactly that (you all have seen the performance data enough times already). But if I'm tied to the dock, the motor pulls more than 7.5kW at full throttle as the prop works against the additional load. If I had geared my system to max out at 5.5kW at the dock, then I would be giving up speed on the open water. Since I have been able to load test my system at the dock by running more power through my drive and the motor and controller temps stayed well within spec, I could problably make my gearing a little taller.
My current gear ratio is 2.55:1, my target motor RPM is 1900, and my prop is a 4 blade 13x15.5 (photos of the prop are here -
http://groups.yahoo.com/group/electricboats/photos/album/1967401930/pic/list see the pic labelled old and new props) The next gear ratio available in my gearbox model is 2.21:1, 15% taller.
I guess that it would be an interesting experiment to swap the gearbox and do a round of performance testing to see if the numbers change much, though I would predict that my max throttle setting would be closer to 6kW underway. Would that generate more speed? Since I'm already at 95% of hull speed at 5kW, probably not. This would slow my motor down for a given speed at low throttle settings, which would probably cost me in efficiency. Alternatively the next ratio up is 2.77:1, a 9% change. This would probably lower my top speed just a little, but the efficiency at lower speeds could go up as the motor is allowed to spin a little more freely.
As you can see, I don't believe that there is a way to optimize the drive ratios on paper, there are just too many variables. I know that I'm very close to optimum right now (beginner's luck?) and any theoretical improvements like I just described would need to be proven in the real world before I gave them any credibility.
Jumping back to your conversion; I think that you should switch your ratio to 5:1 and see what happens, just like others have suggested. That ratio should be much closer to optimum than what you have now. It may still not be perfect, but it should be much better.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Orest Iwaszko <orestyko@...> wrote:
>
> Here are the results for my 180 volt 3 Hp system .
>
>
> I'm trying to determine the correct pulley size for the shaft . This is the challenge right now
>
> Since the motor should spin at 1750 at 100% full power but with the present (2.5 inch on the motor and 9 inch on the shaft ) it bogs down at 1100 rpm / 65% power setting. Here the controller ( the weak link ) starts warming up anything over 65 really heats up the controller in 15 min to plastic smelling levels. The dc motor remains cool .
>
>
> I did tests and here are the results
>
>
>
> power motor amp dc shaft Hp developed HP developed boat ( I DIDNT DEVOTE ENERGY TO BOAT SPEED BECAUSE IT WAS THE MOST UNRELIABLE MEASUREMENT OF THE LOT )
> level rpm draw voltage rpm at motor at shaft speed IM CONFIDENT THAT 60% POWER WAS AROUND 2.3 -2.5 AND TOP SPEED OVER 3 MPH )
> amps X volts
> 750
>
> 10 120 2.3 18 30 .05
> 20 310 4.6 38 77 .23 In my case my pulley ratio is 2.5 : 9 or 3.6 : 1
> 30 490 7.0 56 122 .52 So Hp at motor x 3.6 = Hp at the shaft ?
> 40 650 9.8 74 162 .96
> 50 790 12.5 92 197 1.53
> 60 910 15.1 108 225 2.17 2.5
> 70 1005 17.1 120 251 2.73
> 80 1180 18.1 128 270 3.08
> 90 1100 18.6 128 275 3.17 3.3
> 100 1100 18.8 128 275 3.2
>
>
> The question is how much larger should the shaft pulley be and what benefits can I expect with an optimized pulley.
>
>
>
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