by Steve Meyer, Contributing Editor
Mechatronic challenges come in all shapes and sizes. In the field of motorcycle racing, electric superbikes represent a whole new set of challenges for motor and drive technology.
In the industrial arena, electric motor and drive technology has dramatically advanced machinery performance in many applications. High speed assembly of printed circuit boards would simply not be economical without some fairly sophisticated servomotors and drives.
In the field of electric transportation, focusing on electric motors and drive technology provides the critically needed solution for pushing the performance envelope. In competition motorcycle racing, electric superbikes have established the technology as a serious challenge to gasoline powered vehicles.
Once the exclusive domain of small gasoline engines that were pushed to the limits, electric motorcycles are now holding their own with their fuel powered brethren. Coincidentally, both variations share the need for motors with speeds in excess of 10,000 rpm and extremely high performance cooling systems to keep the motors from burning up. The high speed is a way to greatly increase the power density of the motor because horsepower is a speed-dependent measure of work, and the same principle holds true for both technologies.
Unfortunately, electric powered vehicles have always suffered from the perception of being sluggish and having low performance. This is strictly due to the limitations of lead acid battery technology. All portable electric systems are limited by the storage technology, which either limits range or the ability to accelerate. For those who remember the early days of high-end car stereos, the same problem existed. Lead acid batteries had a tough time giving up enough power to make you really feel the bass lines. Boost capacitors that were the size of a giant soft drink can were added to increase fast reacting power so bass frequencies were fully produced. Similarly, as lead acid technology is giving way to the various lithium chemistries, higher internal voltage has helped this storage technology achieve much better discharge rates and life expectancy.
With all-electric racing bikes that run at 170+ miles per hour and production bikes that get 100 miles to a charge, Brammo is making a great case for the idea that electric powered vehicles can be as high performing as their gas-powered counterparts. It is very gratifying to see electric motorcycles competing with gas powered superbikes to the point that the racing associations are re-formulating the competition guidelines so that gas and electrics can compete head to head. It is an amazing transformation as fans get to see and don’t hear the almost silent E-bikes race as they consistently show in the top 10.
Mastering the challenge
In any power conversion process, the ultimate limit is the amount of heat generated. All three of the basic sub-systems of the bike, motor, inverter and battery pack have thermal management issues that must be solved in order to reach the goal of winning. Parker’s proprietary cooling technology is employed to help shed the thermal load of the motor and inverter and extend system performance.
In the inverter, power semiconductors are used to convert single-phase direct current battery power to 3-phase excitation currents in the motor. The transistor power curve is a line based on the derivative of current over time. The rate at which current can be provided through the inverter will also determine the limit of the acceleration available in the drive motor. In order to achieve the performance requirement, a lot of ‘overdesign’ in the inverter is required.
While Parker makes matched inverters for the GVM motor, the small size and light weight required for motorcycle racing requires a special inverter. High efficiency liquid cooling is required to achieve the size reduction in the inverter as well as increase the power available from the motor. But more is needed. Brammo uses an inverter from Rinehart Motion Systems that uses embedded microcontrollers to permit extremely precise control of the power and timing relative to the motor phases. That timing and control are also required to get the best performance from the unique magnetic geometry of the enhanced permanent magnet rotor and stator design. Vector algorithms operate like the distributor in an internal combustion engine. By changing the timing of the current in the three phases, the power curve and response of the motor can be dramatically impacted. The same motor can produce the higher torque at low speed needed for acceleration, and low torque at high speed that is needed for high speed in the straightaway.
The translation of current in the motor and inverter is also impacted by the battery pack. In bulk battery technology the ability to release current at a high rate is a function of the electrochemistry and mechanical construction of the battery. Traditional lead plates in acid have very limited ability to release large amounts of current. A major current surge like a short can distort the shape of the plates causing internal shorts and battery failure.
In battery packs with large numbers of smaller cells, as we see in today’s lithium packs, it becomes a simple matter to “tune” the assembly of cells by configuring different number of cells in parallel versus cells in series. Thus the same number of batteries can produce a pack with very high current capability and short life, or lower current and longer charge life. Because the lithium cell has higher internal voltage to begin with, the energy density is about four times that of the traditional lead acid battery which leads to smaller packs for electric vehicles. The higher voltage also makes possible higher voltage inverters with smaller, lower current devices.
To master the challenge, a lot of performance data must be gathered during racing. Brammo engineers created a “lab on wheels” approach to the instrumentation of their bikes so that all aspects of battery, inverter and motor performance could be logged and analyzed, each race creating more information about how to make each system perform ideally.
Beyond the racing, Brammo engineers try to use everything learned during racing to improve their production motorcycle. As Brian Wismann puts it, “Everything we do in racing is focused on producing the world’s fastest production motorcycle.” This was a great part of tradition that has been lost in automobile racing as the engineering to meet competitive speed in modern racing at over 200 mph doesn’t typically translate well into features on production cars. It is great to see that focus restored in the motorcycle manufacturing arena, especially electric motorcycles.
Creating the ultimate electric motor for racing
The grueling conditions of competition racing will drive any hardware system to the limit. So every limit that is identified becomes a step toward higher reliability systems that can not only handle the severe demands of the race environment, but translate into a much improved production motor and drive.
In 2010, using Parker’s standard MPP 1904 PMAC traction motor as a starting point, Brammo engineers integrated a frameless version to allow tighter packaging of the motor with the bike. Over time, successive iterations of the windings were created and tested to find the best match of peak torque production.
The collaboration required a learning curve of more than a year for Parker to better understand the requirements of electric motorcycle racing. Ultimately, the effort has resulted in improvements to the current GVM product line. The GVM traction motor gets its increased power density over standard motors by using ultra thin laminations that reduce eddy current losses and segmented stator construction that permits fully automated coil winding. The distributed winding also produces extremely short end turns, which reduces copper losses—particularly at high speeds and/or high current.
Add to the list of improvements Parker’s patented cooling technology and the GVM traction system is able to achieve 2.3 kW/kg continuous power density, just over 1 hp/lb and almost twice the power at peak. These features are exactly what are needed when considering a high performance electric drivetrain.
Even more amazing is the wide range of motor configurations based on two diameters, six stack lengths and a wide array of windings to match motors to applications with optimum base speed. Any of these unique combinations can be manufactured in typically three-to-five weeks because of the manufacturing strategy that Parker uses to support the GVM line.
Parker continues to support the racing effort of Team Parker Brammo and the two companies continue to expand the capabilities of electric drivetrain performance. Here’s a video of the bikes in action:
Parker Hannifin Corp., Electromechanical Automation (Motion Control Systems)