Wire heads unite!

11/3/95

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 one) but 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.

Ammeter

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.

Charger

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 as well.

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.

Contactors

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.

Relays

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.

Controller

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.

Miscellaneous

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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