How does this thing work, anyway?
Electric cars have been becoming much more exciting recently, which has gone a long way to changing the minds of gearheads. The problem that many of us have found is that electric cars present a whole host of unique challenges. Right from the start, you should be thinking about some of these things…
- What components should you select to get the performance you want – acceleration, top speed, and range?
- Where do you put the batteries without ruining the handling?
- Do I need to improve the suspension?
- How does this thing work, anyway?!
Right now, I just want to address the basics. I’m not an electrical engineer, but I’ve picked up enough information to build my own car, and I’ll share that with you.
What Parts do I need?
Everyone knows that you need a motor and batteries, but there’s a lot more to a build than that. At this point it is worth noting that a DC (direct current) build and an AC (alternating current) build are different. I won’t weigh the pros and cons right now other than to say that a DC build is cheaper and simpler. That is also how I converted my MG, so we will start there. AC builds are excellent, but I will save that discussion for when I add some information about my new project.
Here is a short list of the big things that you will need:
- Series Wound DC Motor
- Series Wound DC Motor Controller
- Battery Pack
- DC-DC Converter
- All of the goodies to connect everything…
I specify a series wound DC motor because there are a couple of other DC motors, including SepEx (Separately Excited), and BLDC (Brushless DC). SepEx motors are harder to get suitable controllers for, and BLDC motors tend to be more expensive setups.
What does series wound mean, then? First, let’s talk about what it isn’t. If you’re messed around with electric motors in the past, you’re probably familiar with hooking up a small motor to a battery. One wire to one terminal, the other wire to the other terminal. If you flip the connections, the motor will spin in the other direction. These types of motors have permanent magnets inside of the housing, and the armature spins inside of them. This is not a series wound motor.
In a series wound motor, the permanent magnets on the outside of the armature are replaced with another set of coils – the field coils. These coils generate the magnetic field that the magnets normally would in the smaller motor. Power flows into the field coils, then out of the field coils into the armature coils, and from the armature coils out of the motor.
The motor I’m using in my MG is a Kostov 11 HV, which is an 11″ Series Wound DC motor. You can click on “Motor” at the top of the page to see some more pictures. It’s a hefty motor that is suitable for moving a truck. Since I wanted high performance with no transmission, I opted for a larger motor. More on that later.
For each type of DC motor, there is a controller to match. So, for our series wound DC motor, we need a matching series wound DC motor controller. Think of the motor controller as the gas pedal for your car. You tell it how fast you want to go, and it takes you there.
First note – the RPM of an electric motor like this is related to the voltage you run it at. The higher the voltage, the faster it spins.
Second note – the torque of an electric motor is related to the current you run through it. More amps, more torque.
Controlling the voltage and the amperage sent to the motor controls it’s speed and torque. In the end, these controllers tend to use torque control for that natural driving feel. They measure the current being sent to the motor, and turn a switch on and off to make the current going to the motor match what you request with the throttle pedal.
These controllers use large, solid state switches which can turn on and off thousands of times per second. The longer they are on, the more juice they let the motor have. At full throttle, they might be on 100% of the time, while cruising around town they might only be on 1%-4% of the time. This concept is referred to as the duty cycle.
I chose a DIY controller largely due to budgetary constraints. That being said, it’s no slouch and can hold it’s own against $2,000 controllers. It is rated for about 340 volts and 1200 amps. In the MG, I’m running a peak of 200 volts and 1000 amps.
The standard for EVs now is lithium. In the past, lead acid batteries were used, and they were heavy, expensive, and frankly pretty lame. Lithium batteries are used in almost every mobile device these days (laptops, tablets, phones, hospital equipment, etc.) They’re comparatively light and energy dense, and they’re becoming cheaper every day. As we mentioned earlier, higher voltage will make a motor spin faster. Since we want to go fast, these lithium packs are usually at least 144 volts. Mine is 200, and some OEM EVs use battery packs in the 360 volt range or more.
Lithium is also very volatile, so we have to be careful with these batteries when building a car and when using them (either driving or charging). It’s easy to destroy the batteries or set something on fire, so systems must be put into place to monitor the batteries.
What I didn’t mention earlier is that the current you give the motor determines it’s torque. The battery pack needs both a high voltage and a lot of power to really get this thing moving. The battery is actually the biggest, most expensive component of an electric car.
This isn’t something you may have given a lot of thought to, but we have to get power back into the batteries some how. That is the job of the charger.
The charger takes whatever input voltage you have and converts it to the voltage that you need for the battery pack. Most chargers have some level of intelligence, and they will vary the voltage and current to make sure that the batteries are getting just what they need. Usually this involves increasing the voltage as the charge of the pack increases up to it’s maximum. At that point, the charger reduces the current until it finishes charging.
The charger will often interface with the BMS (Battery Management System) for advanced protection and control.
The output of a charger is measure in Kw (Kilowatts). Your wall outlet is usually 110 volts on a 15 amp circuit. You multiply volts by amps to get kW, so your outlet can put out 110Vx15A=1,650kW. If you were to plug your car into that outlet, you could add 1,650kW of juice to your battery pack per hour. Fast charging stations may be able to supply upwards of 10kW to a charger, however. That would charge the same car in 1/6th of the time.
A car with a gasoline engine has an alternator to recharge the 12v battery and run all of the electrical goodies in the car. An electric car needs the same thing, and that is the job of the DC-DC converter. The converter is a switch-mode power supply.
The converter will take the voltage of the battery pack as an input, and it will usually output about 13.7 volts to keep the 12v battery happy. The only job of the 12V battery is to close the main contactor, essentially turning on the electric car. They come in many forms with varying input and output ratings.
The converter works by rapidly triggering a switch inside of the converter (over 15,000 times per second!) to dump power from the battery pack into a transformer. The transformer steps the voltage down to something usable for the 12V system, and it is filtered by some additional circuitry.
All of the goodies to connect everything…
There is too much to list off in this section, but it would include things like connectors, throttle sensors, instrumentation, adapter plates, etc. It’s the rest of the stuff needed to actually put these parts in a car and make it move. I’ll get into all of that fun stuff another time.