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.
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.
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
|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
|4. I’ve installed a much
larger muffler on the Briggs and Stratton 8 HP air cooled engine for
|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.
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
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.
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.
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.
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.
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
view of the steel fuel tank.
Briggs & Stratton OEM starter motor. The generator originally came with a
spring loaded pull starter which soon failed so I installed an electric
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).
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.
I’ll keep the present fuel tank as a vent for
the big fuel tank for now.
side view of the tractor.