Stuff I’ve Learned: Controllers, Contactors and Converters (oh my!)

“What’s under the tank?” seems to be the second most common question after people get “How far does it go?” out of their system…  my answer is usually “BRAINS!  The BRAINS are under the tank.”  Every Zombie Fembot wants brains, right?

The Controller

It’s the truth, though.  The controller is the brains of the system- they don’t call it a controller for nothin’.  Essentially the controller is for determining the motor speed.  As a byproduct, you get all sorts of other features, but that’s the root purpose of the device.  Rather than simply acting as a rheostat, changing the voltage, and thus the RPM of the motor, it does this by giving the motor pulses of full voltage- the more frequent the pulses, the faster the motor will spin.  You feed the controller with a small throttle control, usually a simple potentiometer or Hall-effect control working in the 0-5000 ohm range, and the controller takes that information and converts it into pulses along the entire RPM range.

Since it’s a signal-response type of control, you get the benefit of dialing in the curve of the response.  You can be very specific about how fast, and how much the throttle input will affect the resulting RPM of the motor, from the very start- 0 RPM, to the top of the range- usually around 3500 RPM.  This is one of the unique things about electric drive.  On a gas motor, you control this, or try to, with various injection, valve and ignition timing settings.  With an electric motor you just program the curve into the controller.

Here, for example, is what the programming on my Alltrax controller looks like- a very simple WinXP application you hook up to the controller with a serial cable.

Choosing the right controller is a little tricky.  Naturally, since the controller and the motor have to play nice with each other, you need to pick a controller that is designed for the type of motor you’re using.  Any of the controller product listings will separate their products into types for each type of motor, as well as the current ratings and voltage of the various applications of every motor they support.  I’ve seen several builders have less than satisfying performance, and it often is because of a poor matchup between the controller and the motor.

My answer, as you probably can predict, is to go with a proven combination.  I decided on the standard Mars ME0709 motor, so my next step was to find out what people were using, and what seemed to work best.  The controller I chose was the Alltrax  AXE 7245.  All the programming and support information is right on the Alltrax website, and their tech support has been really fast to respond to questions- another reason to pick a reputable and common model. The key here is to decide on the motor, then pick a controller that works well with that motor.

The gravy- that is, the stuff you get that’s extra- with controllers is stuff like data logging and regenerative braking.  If you’re running a motor type and model that can do regen well, you can get a controller that will  allow you to program your regen performance.  If you need data logging- that is, the ability to record and download the performance and control that your controller has fed the motor over a period of time- you can do that too, with certain models of controllers.

The Contactor

The contactor is your safety valve- not so much yours, but your controller’s.  You can wire in a main power switch, but the contactor allows the controller to shut itself off from the main battery pack if something catastrophic is happening.

It’s a big relay that’s usually wired to the pack voltage, turning it on or off.  The contactor has a threshold voltage usually, a voltage it needs to hold the contacts in position, so if the pack voltage drops, the contactors will pull apart.  It’s usually set up on it’s own switch- a “kill” switch that has to be able to handle the high voltage of the pack (usually not the key switch that is on the bike- they’re not able to handle much more than the basic 12V system) and then wired to the controller so if the controller needs to shut off the incoming high pack power, it can.

You may be tempted to run the system without a contactor.  It’s a bad idea.

The Converter

OK, so you’re running your pack voltage, what, 72V maybe?  How are you going to power your horn and lights?  This is where a lot of people use what’s called a DC to DC converter.  I don’t, actually, I just use a separate 12V battery, because I had some extras and the room to mount them, but the DC/DC converter just takes your pack voltage and steps it down to 12V.  It’s probably a good idea- certainly it simplifies the charging (you only have one pack, and one voltage to charge), but it does, however remotely, link the high-voltage pack to your low-voltage systems.  OK, I’m superstitious, but that makes me squidgy.  (A technical term…)  Plus it costs a little more, and I’m cheap too.

Wiring Diagram

The way you put this whole thing together depends not only on the type of motor you use and the controller you’ve chosen, but also the decisions you’ve  made about chargers and convertors, fuses and safety switches.  If you’re unsure about it, absolutely look for some advice on the various online groups, and test wire the system with small test leads that you don’t mind melting.  Also, I found it incredibly helpful to label my connections, however obvious their place was…  but that didn’t keep me from making some mistakes resulting in some fairly dramatic, uh, effects.

Here’s a basic wiring diagram for the Alltrax I have with the Mars (and the old brushed ETEK) motor.  (Use at your own risk.)

Let me underscore this point.

Every motor and controller combination is going to need a very specific wiring strategy, and it’s not something that you should leave to trial and error…  making mistakes putting this part of the system together can be expensive.  Even fatal.  Seriously.

To repeat the advice of Ed “Juiced” Fargo, “…make if fast, make it fun- but make it safe!”



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