Wire heads unite!
I guess I should first mention that this is a rough combination of schematic and block
diagram which leaves out many of the smaller, but still important details. Please don't
go and build this, it is mostly for demonstration of concept and/or philosophical pondering. I've based
it on a couple of diagrams at my disposal (Bob Brant's book has one and Bob Batson provided
is otherwise my own doing: good or bad. Some of my limitations stem from file and image
size, others from pains in the wrist from all of this mousing.
Red is 144vdc positive, purple is 12vdc
positive, while black is ground (the 144vdc and 12vdc grounds ARE separate). I've indicated
the heavy, main current flow by the wider lines.
Also, wires which cross each other are not connected unless you see a junction at the crossing.
We'll read this like a book, starting from left to right and then jumping all over the place
after the cat jumps in our lap.
- 144 volt DC battery bank
- As mentioned before this will consist of eight batteries in the rear compartment and
four up front. I've simplified it into a nice box here with safety fuses (the squiggly things)
on each output. In fact there will probably be a couple of these in the back and another
one or two in the front. I think the fuses will be rated at 400-500 amps.
- The EV will have at least one, maybe two ammeters to measure the current flow. This one
will measure current to and from the batteries, so we can see how much they are losing
while driving, the rate of recovery during charging, and maybe even the regeneration level.
Another common place for an ammeter is between the motor and the controller. The resistor
above the ammeter is called a shunt and is what really allows a dinky little meter to read
such high levels of current. The shunt is a big chunk of metal with a very small resistance
which translates into a small voltage drop across it. The ammeter is calibrated such that
a given voltage drop across the fixed shunt translates into a meter reading of
amperage. With typical shunts 50 millivolts equals 500 amps.
- I didn't draw it, but the charger has a power cord running into it. It converts normal
110 volt AC into a high enough voltage to charge the batteries. Some EV owners have
bigger chargers that run off of 240vac and can charge the batteries faster. These don't
typically live in the EV due to weight. The 110vac model stays in the car since it is
pretty easy to find a place to plug in, no matter where you go.
- Volt Meter
- This is pretty generic. In reality I'm planning on installing an E-Meter, which is a compact
digital metering unit that measures voltage, current, kilowatt-hours, amp-hours, time remaining
(fuel gauge), and historical battery information. I don't have the wiring diagram for this
yet, but it will need a connection much like I've drawn and will probably hook up to the shunt
- DC-DC Converter
- Think of this device as an alternator for the EV. When you drive a combustion engine the
alternator recharges the battery and kicks in extra current when you have lots of electrical
items going (fan, lights, winch, 8-track). The DC-DC takes power from our big bank of batteries
and gives some to the auxiliary battery if needed.
- 12 Volt Auxiliary Battery
- Well, the good news is that this battery doesn't need any cold-cranking-amps for those
cold winter mornings. It provides twelve volts for all of the other electronics
still left in the car. We could get rid of it and just use a DC-DC converter, but it is
comforting to have both of them around, just in case. What I haven't shown are all of the
existing and perhaps one additional fuse related to this circuit.
- Ignition Switch
- The ignition switch has lost some importance. The whole concept of "starting"
is no longer relevant. The EV merely needs an on/off switch and the ignition
switch is as good of candidate as any. Some of the newer, commercial EVs (like
the GM Impact) don't even have an ignition switch. They use a little electronic
keypad for entering in a code to unlock and turn on the EV. Maybe if I get some
spare brain cells I'll make one of these too.
- Circuit Breaker
- Not sure what I'm going to use yet, but this is a master on/off switch for all of the EV's
drive circuitry. That way you can completely turn off the circuit when you are doing work,
or can use it to quickly shutdown in an emergency.
- PotBox (Accelerator)
- This is what the accelerator cable hooks to now. The box translates foot pressure into
a variable resistance to
let the controller know how fast you want to go. In addition there is a safety switch inside
that helps prevent starting the EV with the gas pedal floored. Really: you don't have to
give the gas pedal a couple pumps to get an EV started.
- Just to the right of the circuit breaker are two contactors. These are specialized relays with
the ability to handle much heavier voltage and current loads. I show two of them for safety
reasons. In the EV both contactors must be closed before current will flow. Having
two contactors ensures that you can still shut things off if one should get stuck. An EV
with regeneration may have one or two additional contactors.
- There is one small relay between the PotBox and the Controller. A relay (and contactors) are
used to have a small voltage/current turning on and off a much larger one. The relay shown
is for providing the controller with a "sense" voltage. This just means that the
controller has an interlock pin on which it wants to see the full supply voltage before it
will work. This line doesn't carry heavy current and thus a smaller relay can be used. What
isn't shown is an additional fuse to protect this circuit path.
- A Pulse Width Modulation (PWM) controller is typical in most EVs. I haven't pinned mine down
yet, but the basic concept is the same. The controller sits like a switch between the negative
side of the batteries and the negative contact of the motor. The positive cable is hooked to
the controller as well, but for reasons other than direct control (spike suppression, etc..).
If you think of the controller as a switch, then it is easy to see that if you repeatedly
closed the switch for a second and then opened it for half a minute that the motor would
be off most of the time and you wouldn't go very far (let's assume we are in Kansas).
If you started closing it longer, say five seconds, then you'd start making some
progress, jerky, but progress. Well the controller does this, but at a much faster rate and
the ON time is much smaller. The controller also has some other smarts built into it; how
much depends on the make. Most controllers will monitor themselves for overcurrent and
overheating conditions, shutting down temporarily if needed. There are also safety
interlocks to make sure everything is hooked up right, and some controllers will even
monitor other aspects of the motor. One thing I haven't shown is a cooling fan that
will probably be mounted below the controller to help keep it cool.
- DC Motor
- The motor in my EV is a series wound DC motor. What that means is that there aren't any
magnets in this motor, but instead a field winding of wire that becomes an electro magnet
when current flows through it. A series motor means that the same current that flows
through the rest of the motor (armature) also flows through the field windings. This means
heavy duty wire and connections for every aspect of the motor to withstand the high current,
but there are also some benefits concerning torque which I won't go into. The motor is
wired in the diagram above so that the negative output side of the controller goes into
the field winding(S1), the field winding output is shorted to the armature(S2-A2), and the other
armature connection (A1) is hooked to the positive side of our 144 volts.
- And of course there are all kinds of wires, connectors, screws, nuts, bolts, heat shrink
tubing, and various chunks of knuckles.
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