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A few years ago, upon my mother-in-law's passing I acquired her electric "Mobility Scooter". It was equipped with 2, 12-volt batteries, an electronic speed and direction control, a built-in battery charger, and an electric motor drive with magnetic brake. I decided that I could salvage some of these parts and convert an old lawn tractor hulk into an electric tractor.

Nine Inch Drive Wheels, Not Cool

My first chore was to get rid of the small 9-inch drive wheels and replace them with something larger. I found a couple surplus wheels with 18-inch tires and a ¾ inch axle hub, (the size if the axle on the electric motor drive). My theory told me that with double the wheel diameter the speed would be double the original. After purchasing and receiving the wheels I found that they fit the axles and the axle keys were perfect. Only problem was I needed to extend the axle about 2 inches on each side to get a place to secure the wheel. The original wheels were secured with a 5/16-inch bolt in the center of each axle. I fabricated extensions for each axle using ¾-inch bolts with the head ends cut off to achieve the required length. I drilled and tapped the cut ends about ¾-inches deep. Using a 5/16-inch stud, I joined the extensions to the axle, giving me a threaded ¾-inch axle to secure the wheels with a nut on each side.

Electric Motor on Mobility Scooter with motor, transmission, differential and 9-inch wheels

The target vehicle - A 1999 Montgomery Wards18/38

Motor/Transmission Attachment

This turned out to be easier than I expected. With a little drilling and hacksawing I was able to attach the electric drive structure in the same chassis location as the original transaxle.

Chassis Clean-up

I removed the running 18hp engine and installed it on a recently purchased, 1999 Craftsman 19hp lawn tractor with a busted engine. I took out all the belts, pulleys, springs, levers and metal parts that were required to make the old tractor drive and mow. This left me with the front wheel steering structure and the new electric rear wheel drive.

The Initial Test Drive

It was time to find out if all this and future work would be worth all my time and effort. I placed two 12-volt batteries in the old engine space and built a crude dashboard with the necessary switches and the electronic speed/direction controller so that it could operate. On level ground it performed adequately but came to a halt when it encountered a small slope. On level ground, with "full throttle", the maximum speed was a little scary since I had no operating brakes. The overall performance rating was poor.

Redesign Time

My conclusion was that the electronic Speed/Direction controller was not giving me full available battery power. To test this theory I decided to remove the controller and drive the electric motor directly from the batteries. This would give me an idea of the motor/transmission maximum capability. Since I had two 12-volt batteries I would have two speeds, slow by applying 12 volts and fast by applying 24 volts. Before placing my body in jeopardy, I jacked up the rear wheels and did a static test of the wheel rotation. At 12 volts the wheels turned at a good speed. At 24 volts, as expected, they turned faster. I took it off the jacks and put my body on the board that served as my temporary seat. I clipped the wire to the single 12-volt battery and off I went at a pretty good clip. The hill climbing performance was better but not great. I then clipped onto the 24-volt battery configuration and almost flipped the tractor when I put it into a turn. Remember I had no brakes connected. Level speed was great and hill performance seemed adequate. I was inspired to continue the project.

The Final Design

In order to have a controllable vehicle I needed to design some high current switch circuitry to select the two speeds, a brake circuit, a dead man stopping control, and a modification of the old electronics so that the battery charger would function. I tried to measure the motor operating current with a dashboard battery-charge/discharge ammeter, scaled to 50 amps maximum current. It would not register so I assumed the operating current was over 50 amps. My only readily available source of close to reasonably priced 100 amp switches was power circuit breakers and these are not easily configured to do voltage reversal for direction control. Another possibility was to use engine starter relays but these are only rated for intermittent operation. After a lot of Internet searching I found a source of starter type relays rated at 100 amps continuous, 150 amps surge, with 12-volt coils, for only cost $12 each. Now, I was in business.

I started with a block Diagram of the required and desired operating features.

The two critical functions, the "Speed Selector, and the "Direction Selector" were designed using six of the newly purchased, high current, relays. The "Speed Selector" has to connect either 12 or 24 volts to the motor. To accomplish this I use two relays, and a double pole, double throw, with center open, switch. The switch applies coil voltage to the relay that needs to be closed.

The "Direction Selector" is a bit more complicated. It must apply the selected speed voltage to the motor and be able to switch the polarity of voltage being applied. To accomplish this I used 4 relays in a bridge circuit. The "Direction Selector Switch" activates the two "Fwd Relays" when switched in the "Fwd" position, connecting the selected positive voltage to the positive side of the motor and the battery minus to the negative side of the motor. When switched to the "Rev" position the two "Rev Relays" are activated reversing the voltage polarity connected to the motor. The switch's "center OFF" position prevents the four relays from simultaneously connecting during the switching time of the relays.

We can now go forward and reverse, fast or slow. Now we need a brake capability. The motor is equipped with a magnetic brake that locks the motor when it is deactivated. It must have voltage applied to free the motor for operation of the vehicle. I originally planned to replace this brake with a mechanical, pedal operated disk or band brake attached to one of the rear wheels. Again I had trouble locating a reasonably priced disk or band brake that would fit my ¾ inch axle.

After evaluating the dynamic brakes characteristics I decided to use it for my braking system. My design also gave me a "Dead Man Switch" capability to prevent a driverless vehicle run away. My original tractor had a "Brake/Clutch" pedal set up for mechanical braking and engine to transmission belt drive deactivation. I rummaged through the parts I had removed from the original tractor and found the "Brake/Clutch" pedal and it's connecting hardware. With this and the fender mounted speed shift lever I went to the drawing board and came up with a working solution. The safety switch, which was part of the brake/clutch set-up is normally open and is closed when the brake/clutch pedal is depressed. The original speed control lever will lock the clutch-brake pedal in the depressed position if it is moved up to the top position.

If I connect the relay coil activating voltage for the "Speed Selection Switch", the "Direction Selection Switch" and the Magnetic Brake they will all be active if I hold down the Clutch-Brake pedal while driving. Releasing the pedal will open the switch causing all selection relays to open and activate the brake. If I put the original speed control lever into the top position, locking down the clutch-brake pedal, I can live dangerously and take my foot off the pedal and keep driving. The "Speed Selection Switch" also acts as a brake disable when the tractor needs to manually pushed or pulled.

In the Full Circuit Schematic Diagram, below. You will notice that I have added a few non-critical functions; a light switch, to connect the front and rear lights, a battery voltage monitor with a switch so each battery can be checked individually, (actually all I could find was an inexpensive 12 volt meter), and a battery charge current, ammeter, (this was part of the original scooter circuitry).

Circuit Fabrication

With the circuit design and the critical functions tested it was time to build the complete circuit into the tractor chassis. I placed the two batteries in the space where the engine was originally. I used the original battery platform, located behind the engine, as the mounting platform or the six high current relays. Since there was only room for four relays on the platform, I mounted two relays on the underside of the platform. I rebuilt the existing dashboard so all the dashboard stuff, (switches, meters and charging receptacle), mounted into existing holes or modified existing holes. It turned out pretty good. I used #10 wire for the high current functions and #12 and #14 wire for the lower current, control functions.

Painting and Tractor Completion

I have selected a "Claret Wine" color for the tractor, thus the name,

The "KW", (kilowatt abbreviation), is an electrical term conjuring up a feeling of power and of course "Crush" for the grape crush that makes wine or the athletic term inferring the ultimate defeat of the opponent. The Claret Wine paint color can be seen on the the photos below.

Project Cost

If I did not have Mobility Scooter sitting around I probably would not have attempted this project. The free scooter parts and the availability of an inexpensive lawn tractor made this a cost effective build. I have tried to determine the actual "out of pocket" costs for both the total project and the "only necessary parts". The total cost includes the parts to make it work as well as the added "nice to have" features, like paint, lights, meters, new seat, etc. For information purposes I have also tried to include the source of the parts.

Originally posted on January 2011.
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