Understanding the Full Value of Inverters in Hybrid and Fully Electric Applications
Inverters are becoming increasingly prevalent in on- and off-road vehicles as the industry’s desire for improved energy efficiency and performance continues to grow, leading to a rise in development of electric vehicles. In fully electric and hybrid powertrains, inverters are essential because they convert direct current (DC) power from the battery packs into alternating current (AC) to drive the electric motors.
When electrifying hydraulics in applications with electric power take offs (ePTOs) for commercial vehicles, inverters are a key enabler for increasing efficiency by controlling hydraulic flow on demand. Flow on demand means the motor’s speed is adjusted according to the hydraulic system or user’s actual demand, effectively reducing energy waste and the heat created by continuously running fixed displacement pumps from an ICE engine.
Depending on the application, energy savings from electrified systems have been documented at 20-30%, and even higher. Other benefits include reduced mechanical wear and tear, quieter operation, and downsized thermal systems.
Due to the many benefits possible, adoption of hybrid and full-electric vehicles is increasing. Therefore, it is important for design teams to have an understanding of how inverters and other vital components used in these vehicles work and the role they play in overall vehicle performance.
How an Inverter Works
An inverter fundamentally enables the conversion of energy from one form to another. In hybrid- and full-electric vehicles, the inverter converts DC to AC power into torque. Energy stored in the battery supplies DC power. The inverter converts the DC power to AC for the electric motor, which converts the AC power into torque. An inverter can also convert in reverse.
At the heart of inverters are little electronic devices called MOSFETs or transistors acting as switches. Typically, the faster the switching, the better. Faster switching speeds when inverting energy from DC to AC mean less energy is lost as heat and more energy is used to generate torque.
Reducing heat minimizes the cooling required to maintain operating temperatures. When an inverter gets too hot it will derate performance to reduce life-limiting stress in the system.
A major role of the inverter is to control torque which causes the motor to turn and the vehicle's wheels to move. Precise current control enables torque control which is especially important to ensure safe longitudinal control of the vehicle to prevent unintended or undesirable movement of the vehicle like lurching forward when the vehicle is taking off or when decelerating.
The ability to absorb torque is also important for limiting speed. In addition, it helps to protect the drivetrain from excessive torque and the inverter from generating too much current. By adjusting the motor speed and torque in real-time, inverters improve the overall driver experience.
Inverters also regulate voltage, which is important when providing power back to the vehicle’s distribution system. Specifically, they use pulse width modulation (PWM) to control the frequency and voltage of the AC output. PWM works by rapidly switching the DC voltage on and off, creating a pulsed voltage waveform. The width of these pulses is adjusted to control the average output voltage and frequency. The resulting pulses are then filtered to produce a smoother, near-sine wave AC output.
Inverters typically incorporate feedback control systems to monitor the output voltage and current. These systems compare the actual output with the desired output and make adjustments to the PWM control to maintain stable voltage and frequency, even when there are variations in the input voltage or load.
The challenge in doing so is greater with electric motors in electric vehicles (EV) which possess a wider, more flexible speed range — as much as four times — compared to internal combustion engines (ICE), as well as wider torque ranges.
As previously noted, the inverter adjusts the frequency and amplitude of the AC current which directly controls the motor’s speed and torque. Electric motors can deliver instantaneous torque from a standstill (0 RPM) and maintain efficiency at high RPMs, unlike the narrow band of rotations per minute (RPM) ICEs are capable of maintaining.
This wide speed range enables smooth and continuous acceleration and deceleration. However, the motor’s efficiency isn’t constant across its entire speed range. It typically operates most efficiently within a specific RPM range.
Ultimately, the inverter plays a major role in drivetrain optimization. By regulating the motor speed and torque, the inverter can enable the motor to operate within its optimal RPM range. When specified correctly, the motor/inverter can be sized and tuned for peak efficiency at common or typical operating points like cruising speed or peak pump efficiency. This results in optimized energy consumption and extends the vehicle’s range or reduces downtime for charging.
The inverter is able to optimize for performance by ensuring that the appropriate level of torque and power is available at different speeds, maximizing the vehicle’s acceleration and responsiveness.
It would be misleading, though, to imply that inverters are the only crucial components in managing torque. Aside from vehicle control, sizing permanent magnet (PM) motors is crucial for efficient management of torque in electric vehicles. They offer high efficiency (meaning they convert a large percentage of electrical energy into mechanical energy) and precise torque control that contribute to the overall performance and range of EVs. The magnetic field generated by the permanent magnets in the rotor interacts with the magnetic field in the stator windings, creating torque that propels the vehicle.
The Role of Inverters in Regenerative Braking
Few people think about how much energy is lost during the typical braking event. Mechanical braking is extremely wasteful, causing most of the inertial energy to turn into heat. Regeneration (aka regenerative braking), in contrast, is up to 98% efficient.
Regenerative braking is a feature used in electric and hybrid vehicles that captures the kinetic energy of the vehicle when slowing down and converts it into DC power that can be used to recharge the battery. Instead of the energy being lost as heat, noise, and brake dust (particulate matter) through traditional braking methods, regenerative braking recovers much of that energy, improving overall efficiency, extending driving range, and reducing brake system maintenance.
Regenerative braking, however, is not possible in electric and hybrid vehicles without an inverter which acts as a bidirectional power converter to convert the generated mechanical energy back into electrical energy for storage in the battery. Without an inverter, it would not be possible to capture and reuse the energy generated during regenerative braking. It’s thanks to the inverter that hybrid and fully electric vehicles are as energy efficient as they are today.
Inverters are Significant to the Functionality of Hybrid and Fully Electric Vehicles
As a result of growing adoption rates of hybrid and electric technologies, the inverter has become a focal point in various technical articles and presentations across multiple industries because of its integral role in improving performance and energy efficiency.
Consider everything that an inverter is responsible for:
- Powering the main traction motor to enable precise speed and torque control based on demand.
- Managing energy generation and energy recuperation in the braking process (regenerative) in EV and hybrid applications.
- Powering electric hydraulic pumps to offer precise control and enhanced efficiency compared to traditional hydraulic systems.
- Enhancing control of steering and the ePTO in on-road vehicles.
- Controlling material handling and other auxiliary work functions in off-road applications.
- Enabling electronic control of hydraulic flow (previously handled by an analog system) to provide a smoother, enhanced driving experience.
- Enabling engine-off hydraulics operation.
- Enhancing electrohydraulic tuning (including efficiency, sizing, and circuit tuning).
- Mitigating energy loss.
The benefits of all these functions include emissions and noise reduction, as well as greater energy efficiency to meet today’s more stringent environmental standards.
Distributed Work Functions and Inverters Key to Efficiency Gains in Future Electric Vehicle Designs
Traditional ICEs are large central power sources with multiple pumps connected to them. In this centralized architecture, large electric motors, inverters and pumps are required to satisfy demand.
As electrification technologies continue to evolve, there is a dramatic impact on the design of the vehicle with virtually endless options available. For example, the motor, gear train and pump could be combined to create a single power unit. In the future, fans, compressors and winches, among other components, could also be electrified.
The distribution of work to smaller electrified systems will enable tighter control over the performance of specific circuits as power becomes more localized, improving the overall efficiency of mobile equipment.
The electrification of vehicles is becoming more popular as more OEMs and operators recognize the tremendous potential for gaining energy efficiency by electrifying powertrains. At the most basic level, the role of inverters in the electrification process is to enable electromagnetic force to generate torque.
Inverters are a key component in the electrification of off- and on-road vehicles and in realizing significantly greater energy efficiency. Among other things, they make possible regenerative braking, which helps to recapture a large percentage of all the energy that is currently wasted in mechanical braking.
Electrifying hydraulics increases pump placement flexibility as there is no longer a need to generate hydraulic flow from the drivetrain (ICE, transmission, and driveline) on equipment which opens the door to more decentralized architectures enabling distributed power and more precise control over specific components to gain greater performance and efficiency.
At the end of the day, it is ultimately efficiency that matters most, and inverters are vital components in any efficiency success story.
This article was written and contributed by Jonah Leason, Electrification Product Manager and Tim Dobbin, product sales manager, at Parker Hannifin.
