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Old 07-21-10, 11:57 PM   #1
toolingjim
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Default Another electric riding mower

I've been kicking around the idea of electrifying my riding mower in a manner similar to the one Daox has done. Rather than hijack his thread, I decided to start this one to put my ideas out there and get advice as to their practicality.

Here's the concept:
I plan to build this mower based on my old but sound Toro 8-32 which is (surprise, surprise) an 8 horse gas mower with a 32 inch cut. As many parts are no longer available, but the suspension, chassis, and transaxle are sound, and it is a relatively small rider by modern standards, I believe it is a good candidate for conversion.
I am basing this conversion on two principles:
1.)The often quoted rule of thumb that says 1 horse of electric motor is equivalent to 4 horse of gas motor due, I guess, to the ability of an electric motor to pull from 0 rpm.
2.) A rotary tip speed on the mower blade of about 15000 feet per minute. I am using that figure to back engineer the drive for the deck.

The plan:
The drive I am considering is a bit unusual. I have the room, so I am going with two motors, one for the deck and one for the drive. Each one would be powered by it's own battery. The drive motor would power the mower through the original transaxle and clutch.
Here's where I am seeking advice. I have tentatively settled on two 3/4 horse, 1750 rpm, 12 volt motors. These draw 58 amps at full load. Using two 115 ah deep cycle batteries, and following the general rule of discharging the battery only to 50% of full charge, I should get about 1 to 1 1/2 hours of running time. This is about what I need for my 1/2 acre lot.
The total horsepower equivalent is lower that the original motor rating, but I don't think that will be a problem. I have owned this mower a long time, and driven it with the engine worn and producing far less than its rated horsepower. Before I overhauled the motor, it was probably producing about 6hp. It mowed just fine, just wouldn't start when it was hot due to ring wear.
The biggest difference from the original configuration of the power system is the replacement of the 3600 rpm powerplant with 1750 rpm motors. I plan to deal with this in the drive by juggling pulley diameters to partially compensate for the lower rpms, and running in a higher gear to make up the rest. For the deck, I haven't done the math yet, but I think I can get the target tip speed of 15000 fpm with new pulley ratios.

I would like those of you who know something about electrically driven transport to tell me if this concept sounds practical. I am especially interested in knowing if the 1 horse electric = 4 horse gas rule is accurate, and if the 15000 fpm blade tip speed is a sensible target for the deck drive. I found both numbers on the web and want to verify them with people who have experience in the real world.
I have sources for all the major components, which I plan to buy new. This is a hobby build, but I want to build a solid working machine, not a tinkertoy (emphasis on tinker). There are many details I haven't discussed here, but I need to know if the concept appears viable.

So tell me folks, how does it sound?

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Last edited by toolingjim; 07-22-10 at 12:04 AM..
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