The challenges involved in maximizing electric vehicle (EV) traction inverter efficiency include using an optimal modulation strategy to maximize the efficiency of the traction motor while minimizing noise vibration harshness (NVH), which negatively impacts the experience of vehicle occupants, and efficiently driving the power transistors in the inverter.
The torque-speed-efficiency map is an essential tool for analyzing EV traction drive efficiency. It’s a plot of the motor efficiency along its operating region, including the nominal conditions and the maximum power envelope. It describes the maximum efficiency for any speed and torque combination (Figure 1). Better traction motor-control software and improved inverter designs can contribute to widening the operating range, delivering more power and better efficiency over a broader range of conditions.
Iron and copper losses
The primary sources of losses in a synchronous motor are iron losses (from the iron core) and copper losses (from the copper coils). Iron losses are highest at higher speeds and lower torque due to the Eddy currents induced in the iron, reducing efficiency and generating heat. Beyond a certain speed, harmonics and noise can be injected into the motor, further reducing its performance.
At higher torque and lower speeds, copper losses are highest. More current is needed in the coils to deliver the torque, and that increases the IR losses, reducing efficiency and generating heat.
Improved motor-drive software is needed to address the challenges of iron and copper losses and expand the efficient operating region. At each point in the torque-speed efficiency map, the mix of copper and iron losses changes. Standard approaches like sinusoidal pulse width modulation (SPWM) have limitations. A more complex method like space vector pulse-width modulation (SVPWM) can improve performance by up to 15%.
Other approaches include various continuous and discontinuous PWM techniques (CPWM and DPWM, respectively), which have different mixes of benefits and challenges related to the torque/speed operating set points. A common drawback is the inability to minimize NVH as much as desired. A new approach called adaptive PWM control (APC) has been developed.
APC uses voltage phase compensation (VPC) to continuously correct the magnetic flux and extend the motor operating range and field-oriented control (FoC) with a selective harmonic elimination (SHE) algorithm to shape the phase voltages from the inverter, removing harmonics and transistor switching noise, improving NVH across the torque-speed efficiency map. An overmodulation with noise reduction (ONR) algorithm minimizes copper losses in the “constant power” area of the motor torque-speed map (Figure 2).
Pre-switching power semiconductors
Maximizing motor efficiency is only one part of the puzzle for EV drivetrains. The inverter itself must also be highly efficient. A significant source of inverter inefficiency can be the power semiconductors. Power semiconductor losses consist of two elements: conduction losses and switching losses. Today’s power semiconductor devices are highly developed and are available with extremely low conduction losses. In most applications, switching losses dominate and need to be minimized.
Pre-switching is a technique for eliminating switching losses in wide bandgap, silicon-carbide, and gallium-nitride power transistors. It can also reduce switching losses in silicon IGBTs by up to 80%. Pre-switching uses an artificial intelligence algorithm to adjust the switching cycle by cycle to ensure high efficiency under varying input voltage, loading, temperature, and other factors.
An inverter using pre-switching can switch up to 10 times faster while maintaining high efficiency, resulting in smaller and lighter solutions. The inverter produces a low-distortion sine wave output, improving motor efficiency and minimizing NVH concerns.
Variable strength gate drive
Another way to improve inverter efficiency is to use variable-strength gate drives for the power switches, which effectively change the switching speed under varying operating conditions like variations in input voltage, loading, and temperature.
Table 1 illustrates the benefits of varying the strength of the gate drive from 5 to 20 A.
The parameters are:
- EON is the turn-on losses
- EOFF is the turn-off losses
- VDS, MAX is the maximum voltage overshoot
- TOFF is the voltage slew rate of the drain-to-source voltage on the power transistor during turn-off
- TON is the voltage slew rate of the drain-to-source voltage on the power transistor during turn-off
References
- A Comprehensive Review of Advanced Traction Motor Control Techniques Suitable for Electric Vehicle Applications, IEEE Access
- How to maximize SiC traction inverter efficiency with real-time variable gate drive strength, Texas Instruments
- Increasing the range of EV with the same battery and better software, Silicon Mobility
- Losses in Efficiency Maps of Electric Vehicles, MDPI energies
- Motor – Efficiency maps – Overview, Altair
- The Era of Forced Resonant Soft-Switching, Pre-Switch
Images
- Figure 1, Altair, near top of page
- Figure 2, Silicon Mobility, merger of Figures 2 & 3
- Table 1, Texas Instruments, halfway down the page
You may also like:
Filed Under: FAQs, Software