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     I converted this 1976 Simplicity tractor to a generator powered, variable speed drive controlled electric tractor - and it works great!   I have 5 acres and use it all of the time to haul brush, firewood, and  tools.  It's also a portable power source with 110, 220 VAC and 30 amps  12VDC on tap.  It even has air brakes like the big rigs!
    Mostly the reason I did this was to demonstrate the usefulness of  new variable speed technology and the computer controls we make.  I'm  the owner of Integral Controls and we make computer controls for water  pumps.  We're just starting out and wanted something for demonstration purposes so this is what we came up with.
    Please don't hesitate to contact me with questions.

Best regards,

Britt Storkson
Integral Controls
1-800-760-6874

Here are the pictures of my computer controlled generator powered tractor.  The pictures are numbered and the numbers correspond to the descriptions below. 

This all started because I had an old garden tractor with a 3-speed manual transmission and a power-take-off that the kids enjoyed driving around the 5 acres that I own here in The Dalles.  I soon discovered that it was difficult if not impossible for the average kid to push down the stiff spring clutch that it had in order to start and stop the thing.  Not being one to accept less than ideal equipment for the kids and being pretty good at making computer controls this is what I came up with: 

To start the project I had a 4400 watt generator left over from a display trailer I built to demonstrate our product, Integral Controls Pump Station.  The display contains two of our control units, a small water tank, a ½ horsepower water pump and variable speed drive so the customers could see variable speed pumping, on a miniature scale, in actual operation.  The generator worked fine but it was simply too noisy and the clients and myself couldn’t carry on a normal conversation while that generator was screaming. 

To solve the noise problem I powered the display with two 6-volt gel cell batteries and a true sinewave DC to AC inverter connected to a 500 watt 2:1 step-up transformer to make the 240 volts AC needed to operate the Variable Frequency Drive for this display.  With this setup I also have about 1000 watts of 120 Volts AC and 12 volts DC on tap.  Now the only thing heard when the display is in operation is almost undetectable cooling fan noise from the DC-AC inverter.

I also built a battery charging system so I can charge the batteries either by plugging the display trailer into a regular 120 volt AC receptacle or plugging into my pickup truck charging system so it can charge the batteries while I’m driving down the road from client to client.  The state-of-the-art battery charging system includes a newly developed DC to DC converter that is capable of taking a 10-20 volts DC at the inputs and converting it into a tightly regulated 13.5-16.5 VDC at the outputs.  The output voltage is adjustable and determined by the computer control I built based on temperature and other battery charging criteria.

The charging rate from my pickup truck is very limited (about 60 watts) because the pickup has a relatively small 37 amp original equipment alternator and by the time one adds up all the other electrical energy demands such as vehicle and trailer lights there isn’t much left.  When the system is charging and I’m driving along often I’ll only register 10-10.5 volts from that alternator when the norm is 13.8 volts.  That’s because I’m overloading alternator somewhat and it can’t produce the all of the energy I’m demanding from it.  So it responds by reducing the voltage (in accordance with ohm’s law).  I can get away with this because most of my travel with the display is during cooler weather and the heat generated by the charging system overload can be quickly dissipated without causing harm. 

I could install a much larger alternator but then the alternator would be worth more than the entire pickup (it’s a 1974 Chevrolet ½ ton model with nearly 500,000 miles on it) and other modifications would be required.  I would probably need to install a double pulley arrangement to handle the extra torque needed to turn the larger alternator at high current demands and I would need to run much larger cables back to the trailer plus make several changes to the display charging system.  It certainly could be done but would be much work and expense for very limited benefits.

1. View of entire tractor with hood and headlight.

2. Front view of tractor.

3. Throttle lever.  This tractor is truly “drive by wire”.  There is a notch in the throttle lever that defines neutral/stop.  Moving the lever up commands a forward direction and how far up the lever is moved defines the motor speed.  The farther up the lever is moved the faster the tractor will go.  Pushing the throttle down defines reverse which is a fixed speed but can be adjusted in the control unit.  This throttle lever will soon have a “joystick” mounted on it for ease of use.

To implement all of this the throttle lever moves a 10K trim pot (or variable resistor…see photo #9) which produces a variable voltage ranging from about 0.8 volts to 2.4 volts.  The computer control “reads” this voltage and decides what the speed and direction of the electric motor is supposed to be.  By prior measurement it was determined that the forward command would be defined as 0.8-1.7 volts, Neutral/stop would be defined as 1.7–2.1 volts and reverse would be defined as 2.1-2.3 volts.  This corresponds to where the operator would normally assume that forward, neutral and reverse would be on the throttle.

For the Forward direction 1.7 volts was to be the slowest speed and 0.8 volts was to be the fastest speed.  This number is “crunched” in the computer control to come up with a target shaft speed of about 1-58 revolutions per second that converts to about 1-3450 RPM…3450 RPM being the maximum speed of the 240 volt 3- phase electric motor I used.

4.  I’ve installed a much larger muffler on the Briggs and Stratton 8 HP air cooled engine for quieter operation.

5. Key Start operation.  The original generator had a pull starter that failed shortly after I bought it.  Briggs & Stratton makes an electric start kit that can be retrofitted to this model which includes the ring gear on the flywheel and the starter motor.  Pictured is the key switch.

6.  Several 12 volt DC relays are used for this tractor.  Their functions are listed below.

A. One DPDT relay is used to short the ignition to ground when stopping the engine and routing voltage to the starter relay when starting.

B. A second DPDT relay is used to enable operation.  A password is required to be able to start the tractor.  If the password isn’t correct the tractor won’t start.  A master password is stored in a secret location it too can be changed to prevent unauthorized operation.

C. A SPDT relay is used to enable/disable motor operation through the VFD.  When the throttle lever is in the Neutral/Stop position this relay opens and stops the electric motor.

D. A SPDT relay is used for reverse operation.  When the relay is in the normally closed position the motor will go forward.  When reverse is selected this relay coil is energized and the motor reverses.

E. A SPDT relay operates the headlight.  The computer control implements a toggle function.  Push the button once and the light is on.  Push it again and the light is off.

7.  Braking Resistors.  These are two 150 Ohm 100 watt resistors connected in parallel for an effective resistance of 75 Ohms and 200 watt power dissipation rating.  These resistors are used by the Variable Frequency Drive to “dump” energy during motor braking.  With a 3-Phase AC motor electric braking can be achieved by injecting DC current into the AC motor.  There is also a “back feeding” of electrical energy because an electric motor acts as a crude generator when driven by an external rotational force – in this case the tractor braking to a stop.  This “back feeding” is sensed by the VFD and would cause a shutdown (the VFD interprets this as a problem condition) but this energy is instead harmlessly “dumped” or dissipated as heat in these resistors.

8.   12 Volt DC 30 amp power supply.  This takes 120 volts AC from the generator and provides 12 Volts DC for charging the battery and other tasks such as headlight operation.  The input voltage on the transformer has multiple taps for variable voltage input.  The input is set at 120 volts but can be changed to 108 volts as a simple way to increase the output voltage.  It’s possible to increase the output voltage to about 13.5 volts by changing the taps on the input - something I’ll probably do if the charging voltage to the battery isn’t sufficient.  I could put together a computer controlled DC-DC converter or some similar contraption to get the job done but it’s usually best to go with the simplest plan of attack as long as it gets the job done adequately.  In this case changing the taps on the transformer would be the simplest way to increase the voltage to the battery.

9.  Throttle variable resistor (potentiometer).  See picture #3 for details.

10, 11.       Electric motor shaft coupling, optical sensor and reflector.  The optical sensor consists of two components…an emitter and a detector mounted side by side.  The emitter is a light emitting diode (LED) that shoots a light beam to a reflector – that you can barely see in the picture as a shiny, reflective spot.  The beam is then reflected back to the detector which consists of a photo transistor that is optimized to respond to whatever wavelength light is emitted by LED.  Such matching of emitter and detector helps greatly with white light rejection so the sensor is not confused by ambient light. 

As the shaft coupling rotates the light beam is alternately passed through the space in the shaft coupling or blocked by the two shaft coupling bolts.  When the light beam goes from on to off (as sensed by the detector) that is counted as one “pulse”.  The computer I built simply counts all of the pulses and logs the total count into memory each second.  That’s how we determine the ground speed of the tractor.  The throttle position tells the computer how fast the operator wants to go and that value is compared with how fast the tractor is presently traveling.  If the tractor pulses per second is slower than desired the voltage output to the VFD is increased and the motor turns faster and vice versa. 

12. Another addition to this tractor is a 13 gallon fuel tank.  At a fuel consumption rate of about 1 per hour this fuel tank will allow the tractor to go about ½ day without refueling.  The computer I made also includes a fuel gauge that is implemented simply by weighing the fuel at the bottom of the tank.  This gauge can be calibrated in the computer so a dash-mounted analog meter reads full scale for full tank and low scale for an empty tank.  A standard electric fuel pump is fitted to pump fuel to the carburetor.

13.  Another view of the steel fuel tank. 

14.  Briggs & Stratton OEM starter motor.  The generator originally came with a spring loaded pull starter which soon failed so I installed an electric starter.

15.  Sealed gel-cell battery.  No worries about refilling or spilling battery acid.  At the top of the picture there’s a aluminum junction box so where I can tap in to 120 or 240 VAC from the portable power source (my tractor).

16.  Motortronics 3 HP Variable Frequency Drive (VFD).  This VFD accepts 240VAC single phase and converts it to 240 VAC 3 phase to drive a 3 HP inverter duty electric motor.  This VFD adjusts the output frequency and voltage to adjust the speed of the motor on command from the computer control I built. This VFD has held up quite well in spite of the unconventional application.

17.  I’ll keep the present fuel tank as a vent for the big fuel tank for now.

18.  Left side view of the tractor.

 

 

 

 

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