Electric-motor design for automobiles kicks into overdrive
When electric motors were used mostly in factories, plant managers could control variables such as temperature, dust, moisture, vibration and lubrication. Put electric motors in an automobile, however, and all of those factors become subject to wide variation. Compass looks at how a few of the industry’s top engineers are designing electric motors for transportation.
Electric motors – the workhorses of power generation in factories since the mid-1800s – have operated for generations in stationary, controlled environments. Today, however, these machines are in demand for use in all types of hybrid and electric vehicles – and the fact that the electric motor is going places is bringing big changes.
Ward’s Automotive estimates that manufacturers will increase the number of battery-powered vehicle models they produce from just 18 in 2018 to 85 by 2025. Putting motors on the road requires them to operate in rapidly changing conditions; motors also need to be smaller, more powerful and more flexible.
The future of such motors can be glimpsed in Bletchley, Great Britain, at Integral Powertrain. One of its high-end motors can feed information on any deviation from the desired current level back to the motor controller 75,000 times a second, to help maintain output at desired levels.
“The controllability of the motors is staggering,” said Luke Barker, Integral Powertrain’s technical director. “In electric cars, it is well known that most braking can be accomplished not by the brakes, but by reversing the motor’s torque, which also charges the battery. We can control torque so rapidly and precisely that the motor can also be used, for example, to improve speed and refinement of gear changes. We routinely go from a maximum torque in one direction to maximum torque in the other in less than one-hundredth of a second. This is a level of controllability not really exhibited by other types of powertrain.”
Computerized-design systems contribute to the sophistication of today’s electric motors, enabling designers and engineers to create digital models that simulate how a motor will perform in a vehicle’s powertrain long before it is actually manufactured. Simulations can predict – and software can control – such factors as electromagnetic output, vibration and stress.
“THE ABILITY TO WORK OUT [WITH SIMULATION SOFTWARE] WHAT YOUR MOTOR WILL DO BEFORE YOU RUN IT IS VERY HIGH.”TECHNICAL DIRECTOR, INTEGRAL POWERTRAIN
“The ability to work out what your motor will do before you run it is very high,” Barker said. “We have simulation tools validated by correlation data from previous motors, which means we are pretty good predictively. This is partly achieved because the motors are quite scalable. You might be working on a motor five times bigger than a previous motor, but electromagnetically they can look very similar.”
Simulation software and digital mockups allow Integral Powertrain to design for manufacturing and identify which suppliers will be used. “It avoids having to go through the learning curve with every motor, because all of that knowledge is captured in the rules-based design,” Barker said.
Kreisel Electric, based in Austria, acquires electric motors from suppliers and then combines them with transmissions, batteries and battery management to create complete electric vehicle systems – including the world’s first electrified Hummer H1. Created in 2017 for actor and former California governor Arnold Schwarzenegger, the vehicle has 360 kW output, 100 kWh capacity and a range of nearly 300 km (186 miles).
Like Integral Powertrain, Kreisel Electric uses sophisticated modeling and simulation software to optimize a motor’s performance and balance all components of an electric powertrain, said Helmut Kastler, Kreisel Electric’s head of mechanical and electrical engineering.
“We can integrate parameters like the heat transfer from the motor,” Kastler said. “We can set temperature limits. We can basically simulate everything.”
“WE CAN INTEGRATE PARAMETERS LIKE THE HEAT TRANSFER FROM THE MOTOR. WE CAN SET TEMPERATURE LIMITS. WE CAN BASICALLY SIMULATE EVERYTHING.”HEAD OF MECHANICAL AND ELECTRICAL ENGINEERING, KREISEL ELECTRIC
One challenge is verifying that transmissions can keep pace with new, more powerful electric motors. In the early days of applying electric motors to transportation applications, a transmission had to handle 5,000 to 6,000 revolutions per minute (rpm); now, electric motors can generate 15,000 to 20,000 rpm. Electric sensors on the transmissions communicate information to the vehicle’s electronic control unit (ECU), which acts like a central nervous system, providing lubrication on demand to limit friction and heat.
“The software allows us to easily connect the characteristics of individual parts together and see the combined effect of the mechanical power, thermal power, electrical power, each to the other, so that we can create a whole system,” said Johannes Pumsleitner, a research engineer at Kreisel Electric. “We can start simulating systems by providing any parameter, and we can also simulate driving behavior.”
Noise management is another key issue. An electric motor can run almost silently. But the transmission, which has more mechanical components, generates noise if it is not properly synchronized with the motor. “We have to take care of the noise,” Kastler said. “You can simulate whatever you want, but you need to know which attribute has to be optimized.”
Design and simulation tools also helped pave the way for an entirely new motor topology that enables power-dense, smaller-sized, highefficiency electric motors.
In 2006, researchers at Oxford University in the UK succeeded in eliminating the large piece of formed iron, called the yoke, used in permanent magnet motor designs. The yoke, a heavy structural element, normally carries magnetic flux from the motor’s magnets to the current-carrying copper wires. In place of the yoke, Oxford researchers found that lesser quantities of specialized iron material yielded greater power and torque levels from significantly lower weight, smaller motors. The new topology is called YASA: Yokeless And Segmented Armature.
The breakthrough enables reduced vehicle weight and contributes to net vehicle efficiencies. For example, YASA motors used with two-speed gears reduce the battery size by more than 10% in some cases, compared to the radial motor and single-speed gear drivetrains common in first-generation Battery Electric Vehicles (BEVs), said Ajay Lukha, chief commercial officer for the YASA company, which spun off from the university’s research project in 2009.
YASA motor benefits have profound ramifications for the design and cost of an electric vehicle, Lukha said. The YASA motor eliminates about 25% of the electrical losses from a comparable machine, making it more efficient. The design also improves power/torque density by a factor of at least three, meaning the motors are much smaller and lighter. YASA also uses computerized design and simulation to predict how its motors will perform on specific vehicles.
“It all starts with the motors and running the motors,” Lukha said. “With having simulations of optimized power and torque along with vehicle dynamic parameters and anticipating how they interact with the performance of the machine.”
“WE CREATE MODELS THAT ALLOW US TO COMPARE A YASA MOTOR VERSUS OTHER MOTOR TYPES AT A VEHICLE SYSTEM LEVEL.”CHIEF COMMERCIAL OFFICER, YASA
Because every vehicle manufacturer has different performance objectives for drive dynamics, efficiency and speed, YASA uses the software to predict how entire vehicles will perform in respect to overall efficiency.
“Our simulations have now become so sophisticated, we create models that allow us to compare a YASA motor versus other motor types at a vehicle system level,” Lukha said. “You can take certain data from a particular vehicle and you can model the efficiency at the systems level to demonstrate the optimal motor choice.”Back to top
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