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The Open ReVolt EV controller inspires a lot of questions. Here's the answers to the most popular ones.
What is an EV controller?
The three main parts to an electric vehicle drivetrain are the batteries, motor, and the controller. The controller acts like a valve, regulating the amount of electricity that goes from the batteries to the motor. It does this mainly in response to the pedal position, but it can also depend on vehicle speed, component temperatures, and battery charge.
What is the Open ReVolt project?
Spearheaded by ecomodder.com user MPaulHolmes, Open ReVolt is a project to make a do-it-yourself, open source EV controller. Since they aren't mass-produced yet, the controller is often the most expensive single component for homebuilt EVs. Since they're so specialized, most people can't simply scavenge controllers like they can motors, batteries, and gliders (aka vehicle bodies).
How does an EV controller work?
The simplest EV controller would just be a switch. Turn it on and the car goes, turn it off and it coasts. This isn't very practical for automobiles, but some toy cars use this system.
Conceptually automotive controllers are almost as simple – they just turn the switch on and off really fast using electronic switches; typically power semiconductors called MOSFETs. By varying how long they're on versus off, the speed of the motor can be turned up and down (a technique called pulse width modulation or PWM). Other techniques of varying the speed and power of electric motors exist, but have fallen from favor with today's technology.
Of course, nitpicky details creep in. The jerky voltage of PWM can be smoothed out by putting large capacitors after the switches. The current induced by the motor windings can flow around the MOSFETs without burning them up by adding diodes to the circuit. A MOSFET that's only partway on creates a lot of heat, so the switching needs to happen as fast as possible. Diodes create even more heat, so they're sometimes replaced with another set of MOSFETs in a trick known as synchronous rectification. The MOSFETs need to turn on simultaneously so the early bird doesn't get fried. EMI can screw with the control board. Etc…
I'm tech savvy. How does an EV controller work?
Oh, sorry about that. An EV controller uses a feedback loop to regulate the current going to the motor by adjusting the voltage. To avoid oscillations, a PI controller (or sometimes weighted averages) is used. Most feature thermal cutoffs and hardware current limiters, but some have watchdog timers, high-pedal lockouts, polarity reversal protection, MOSFET short protection, RS-232, motor speed sensors, vehicle speed sensors, traction battery monitors, and even integrated charge controllers. See Open ReVolt's Current Features.
EVs are battery powered ...so what's with the AC controller?
Until 2011 or so most DIY EVs used DC motors for various reasons, but mostly because of the complexity of the controllers. The great debate about Edison's DC versus Tesla and Westinghouses AC may never end, because of their differences. Presently the advantages of an AC "squirrel cage induction motor" over certain characteristics of DC motors seem to outweigh the controller complexity issue. With today's electronics technology, and availability of semiconductor devices, AC controllers are efficient, affordable and robust enough to be used. This improves our options as we make the engineering decisions which will give us the vehicle most suitable for our own applications.
What's so great about the Open ReVolt?
Several things, it's open, modular, inexpensive, and free. No, that's not a typo.
As the name would imply, Open ReVolt is open, which means its development happens online for everyone to see. Want to know what a certain part does and why? It's all in the design thread. This way many, many people with different backgrounds are watching for mistakes and problems, but also opportunities for improvement. This includes you!
The Open ReVolt is designed to be as modular as possible. A control board houses all the low-voltage electronics like microcontrollers and data and throttle cables. A power board contains the MOSFETs, capacitors, and diodes, as well as the bus bars which connect to the motor and batteries. To protect the control board, it is connected to the power board with an optoisolator. The control board uses its own separate battery for power
Since the two boards are modular, you can size the power board for your vehicle without affecting the control board. An electric bike and an electric lorry could use the same control board (simply by attaching it to a higher-current power board). The power board can accept any battery voltage from 0 to 144 V.
A big driver in the design process is to make the finished controller inexpensive. This means being smart with what components are called for, group buying to drive down cost, and taking advantage of generously donated parts, time, and resources.
Sadly we can't give away the actual electronic parts, but the schematics, software, and blueprints are all 100% free. MPaulHolmes can't break down your door for copying the Open ReVolt design, because you already own it! (strictly speaking everyone does) This is a big deal for EV controllers, since recouping development costs is a big part of why small-volume products like them are so expensive.
Sounds cool. Where do I start?
If you're looking to help out/contribute to the advance of the project, there are several ways to dive in. Editing this wiki is one. You can offer anything else on the design thread. Things that have been offered include:
- beta testing time (use one of these controllers in their already-running EV)
- machine shop time (to make parts for the controllers)
- business discounts (when placing orders for components specifically related to making an Open ReVolt controller)
- components (to be used to build test systems)
- financial donations (to buy parts)
- encouragement (Thanks to all!)