The Electric Motor
To power your motorcycle, you’re going to need a motor. But what type, what size, and where do you get it from!? This project used a Briggs & Stratton Etek. It’s a DC (Direct Current), brushed, pancake motor, rated at up to 48V and 150 amps continuous. I got it used, through Craigslist, from a college student who built those robots that battled each other. He was using this motor to swing a hammer, but it was too powerful, and he kept breaking hammer handles! So why this motor?
DC – Direct Current Direct current motors are very straight forward. They are easy to control the speed of. Also, batteries use direct current. By using a DC motor, there’s no intermediate step of converting DC battery power to AC power to run the motor.
Face Mount The Briggs motor has eight holes on the end (the “face”) of the motor to make it easy to mount to a piece of flat steel or aluminum. Some motors have a “foot” on the bottom of them for mounting, which wouldn’t have been as easy to use in this situation.
Permanent Magnet Permanent Magnet motors tend to be very compact. They create rotational energy (torque) by pushing two magnetic fields against each other. The one magnetic field is produced by current from the batteries (an electro-magnet). The other magnetic field is from mineral permanent magnets. These magnets are much more compact than a second electro-magnet would be, allowing for an overall powerful, yet small motor. The limiting factor in the design is the strength of the permanent magnetic field. Many permanent magnet motors spin equally well in either direction. Just swap the positive and negative battery cables for it to spin the other way. The permanent magnets are ALWAYS magnetic! So don’t drop a washer near one of the vent slots, or it will get sucked in and you have to take the whole thing apart to get it out! Since then, I made sure ALL washers are stainless steel (not only are they corrosion-resistant, but they are non-magnetic as well.) I chose this motor knowing that many other people had used the same one in their electric motorcycle designs. http://www.evalbum.com/mtrbr/BRIG Permanent magnet motors are generally designed to spin equally well in either direction. If the motor spins the opposite direction of what you intended, all you have to do is swap the two cables. On a large motorcycle, you could take advantage of this with a reversing contactor to have a reverse gear.
It’s not all about horsepower….
Electric motors are rated differently than gas engines are in terms of their power. A gas engine is rated in horsepower with the engine running at nearly maximum speed and fuel consumption (full-out!) An electric motor is rated at how much power is can put out continuously – for hours at a time. So, a horsepower rating between an engine and an electric motor is not apples to apples. More and more engines are also now being rated in Watts. A watt is a unit of power used. Most people understand watts, as in that a 100-watt light bulb uses more power than a 75-watt lightbulb. It puts out more power (as light and heat) but also costs more on your electric bill. In electric vehicle design, keep in mind that volts x amps = watts. Also, 1 Horsepower is roughly 746 watts. So, it’s pretty easy to do some simple math to figure out the power of our motor. By being connected to four 12V batteries in series, the system nominal voltage is 48V. The motor is rated at 150 amps continuous. 48 x 150 = 7,200 watts. Divide that by 746 (watts to horsepower) and you get about 9.6 horsepower. That doesn’t sound like a lot.
However, you can pull much higher amperage briefly through the motor – typically three or four times as much. My system amperage is limited by the fact that the motor controller maxes out at 300 amps. That still means we can get DOUBLE the power out of the motor compared to what you might think it can produce, just based on the numbers stamped on it. Combine that with increased efficiency (by completely losing the transmission) and the fact that you have FULL TORQUE right off the line (a gas engine has to rev up to several thousand RPM to get into it’s best power band) and even a compact electric motor has far better acceleration than you think it might. I later had my cycle tested on a dynometer at a large Harley-Davidson gathering. The cycle “officially” clocked-in as 12hp. But when the guy first went to ride the cycle up to the dyno, he almost threw himself off with how quick it accelerated!
What other motors might you use in your electric motorcycle? Besides permanent magnet DC motors there are also Series-Wound and Brushless DC motors as well as some new AC motors. Series-wound motors are similar to permanent magnet DC motors. They are bulkier, but produce fantastic torque! You could use a series-wound drive motor out of a junked electric forklift. Do not use a pump motor. Those typically do not have a male driveshaft. Same goes for electric golf cart motors. They may otherwise sound like a good motor for a cycle, but unless you have a way to easily connect a standard sprocket to the motor, they will be a lot of tinkering to make work for your project. ( A friend of mine is currently working on designing a kit with a specialty part allowing anyone to build their own electric motorcycle using an off-the-shelf golf cart motor. Look for that in the future.) Brushless DC and AC motors are very similar. They require dedicated controllers designed specifically for them. If you go that route, buy your motor and controller as a matched set through a reputable dealer. in general, all these motors are air-cooled, so you don’t need a motorcycle with a radiator on it. For planning purposes, you want to know that your motor will FIT in the motorcycle before you buy it! Made sure to measure the space you have and the physical size of the motor before you buy. If the motor is not in front of you in person, don’t worry, most mainstream manufactured motors have diagrams that you can download, that include the physical dimensions. Besides the diagram showing physical dimensions, it also lists important information on torque, voltage, RPM, etc. That helps you plan out your cycle design as well. LINK TO ETEK DIAGRAM