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Old 07-21-10, 10:57 PM   #1
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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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Old 07-22-10, 06:52 AM   #2
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Woo, good to see you going forward with this.

Do you have a link to the specs on the motor? If not can you give a little more info on them. Are they series wound or permanent magnet? Anyway, in most cases you can just bump the voltage and get more rpms out of the motors.
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Old 07-22-10, 09:59 PM   #3
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These motors seem to be readily available from Surplus Center (Burden Sales Co.) at half the price of other sellers and $50+ cheaper than even Ebay sellers.

My post count does not allow me to post the link. Google Surplus Center. Check DC motors, base mount, reversible, and scroll down to the 3/4 horse motor, #10-2424

I had favorable dealings with these folks some years ago.

I am trying to avoid overvoltage as it tends to overheat the motor, causing premature failure. I have looked for higher rpm motors with similar specs to no avail. If anyone knows of similar motors at 3600 with similar amp draw I would like to hear about them. The current drive has pulley ratios reducing the rpms by a factor of approximately 2.5:1, so I have room to tune both systems. I am just hoping the forum members can point out any big glitches in the scenario I have put forth.
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Old 07-23-10, 11:03 AM   #4
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the 3/4 horse power for the drive system will work but with no controller you will be at the mercy of your terrain, so if you have any inclines on you property your motor will strain real bad and pull more amps then the motor is rated for and heat up real fast and fry.. where with a controller you can control the way your motor acts and the amp draw.. also you should have a motor the spins at a min. of 2500 rpm.. with 1750rpm you will be lucky if you hit 2 mph..for the deck you will need at least a 3hp motor.. the cutting cuts best at 3400 rpm but will get by with 2400 rpm with sharp blades..
i use a 1.5hp 24 volt motor on the deck of my MINI. the deck is 22" and it gets pretty warm after an hour of cutting.. i use a 500 watt motor for the drive system with a controller and it runs hot i run it 1 hour then give it a 10 minute cooling down period.. i put a blower on the drive motor. after the 10 minutes of cool i run it for another hour.. after that the batteries are reading 11.5 volts and i try not to go any lower then that..
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Old 07-23-10, 05:54 PM   #5
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Quote:
Originally Posted by aproldan View Post
the 3/4 horse power for the drive system will work but with no controller you will be at the mercy of your terrain, so if you have any inclines on you property your motor will strain real bad and pull more amps then the motor is rated for and heat up real fast and fry.. where with a controller you can control the way your motor acts and the amp draw.. also you should have a motor the spins at a min. of 2500 rpm.. with 1750rpm you will be lucky if you hit 2 mph..for the deck you will need at least a 3hp motor.. the cutting cuts best at 3400 rpm but will get by with 2400 rpm with sharp blades..
i use a 1.5hp 24 volt motor on the deck of my MINI. the deck is 22" and it gets pretty warm after an hour of cutting.. i use a 500 watt motor for the drive system with a controller and it runs hot i run it 1 hour then give it a 10 minute cooling down period.. i put a blower on the drive motor. after the 10 minutes of cool i run it for another hour.. after that the batteries are reading 11.5 volts and i try not to go any lower then that..
Can you explain how the controller you are talking about works? The only motor control systems I am familiar with are stepper motor controls and variable frequency drives for three phase ac. Is this a current limiter like stepper controls? Thanks in advance for any info you can offer.
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Old 07-23-10, 09:05 PM   #6
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toolingjim here is a simple way to understand the electric components,, just think of a gas vehicle
electric motor = gas engine
controller = carburetor
electric throttle = gas pedal
batteries(volts) = gas(fuel)
amps = octane
so the controller works with your throttle.. and controls how your motor responds and acts.. they also regulate the output voltage and amps(fuel/octane) going to the motor..
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Old 07-23-10, 10:01 PM   #7
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Just a thought - assuming your deck has two cutting blades, would it be possible to mount a motor on each blade? The Etrack front mounted deck has three blades with a motor on each.
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Old 07-24-10, 02:54 PM   #8
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I think you have some things mixed up. Running at a higher voltage does not increase heat. Amperage is what creates heat. Going to a higher voltage allows more power at the same amperage. So, you can either get more power for the same heat generated, or the same power for less heat generated. Going too high over the rated voltage can be a problem and your brushes may spark and wear out faster. Of course, that all depends on the design of the motor. Many of the electric car guys are running their 24-36V forklift motors at 72, 96, even 144 volts without problems.
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Old 07-24-10, 03:32 PM   #9
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Quote:
Originally Posted by nibs View Post
Just a thought - assuming your deck has two cutting blades, would it be possible to mount a motor on each blade? The Etrack front mounted deck has three blades with a motor on each.
No joy. This is a one blader. I think 32" or thereabouts is where the single blade maxes out. I would guess centrifugal force and tip speed starts getting a bit high at that point. I know the Toro Wheelhorse 38" is a two blade deck. I have not seen a consumer riding mower with a single blade above 32"-36" though they might be out there. Thanks for trying though.
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Old 07-24-10, 04:43 PM   #10
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Quote:
Originally Posted by Daox View Post
I think you have some things mixed up. Running at a higher voltage does not increase heat. Amperage is what creates heat. Going to a higher voltage allows more power at the same amperage. So, you can either get more power for the same heat generated, or the same power for less heat generated. Going too high over the rated voltage can be a problem and your brushes may spark and wear out faster. Of course, that all depends on the design of the motor. Many of the electric car guys are running their 24-36V forklift motors at 72, 96, even 144 volts without problems.
I'm not mixed up, just ignorant (grin). I was analogizing from ac motors, where doubling the voltage will cause much heat and rapid loss of the magic smoke. I know volts x amps = watts = horsepower, so I can see where for instance 12 volts at 30 amps would equal 24 volts at 15 amps, but I've never thought about why there are more rpms at 24 volts than 12. Does anyone have an explanation of why this works?
I have a dim notion regarding the relationship between rpms and torque, so I guess I need to go back to school on this part of the plan. I have been trying to go with 12 volts because the paper amp hours matches what I need, and I can fit two batteries easily, but three or more gets dicey. I would have thought there was some downside to higher voltages, or we all would be running RC car motors at 150 volts. This may be a very large question, but what am I missing?

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