Approximately 253 million cars and trucks roam the roads in the United States. In 2015, electric-vehicle (EV) sales were estimated to have been 462,000. Earlier this year, a study by Bloomberg New Energy Finance stated that sales of EVs is expected to reach 41 million by 2040 or 35 percent of vehicle sales by 2040. And, according to market research from IDTechEx, EVs will represent a $400 billion opportunity for manufacturers by 2026.
These EVs are significantly different than their mechanical counterparts. At the forefront of electric and hybrid vehicles are the computerized systems that control them. As these e-vehicles are designed, unique challenges spring up in this ever-growing market.
Because of the unique design of EV powertrains and their impact on the vehicle’s power systems, design engineers must use components that are energy-efficient, lightweight and compact. They must address parasitic currents that can cause energy leakage, electromagnetic interference, and other issues. Components must be able to withstand vibration, impact, and wide temperature ranges while reducing electromagnetic interference, voltage spikes and ground currents. And, components need to consume energy as efficiently as possible to extend the life of the electric motor and battery systems.
In addition to a thorough procurement process, there are some best practices in EV design that revolve around electromagnetic-compatibility (EMC) filtering and powertrain design, which help to minimize these risks. EMC components and filters help provide the precise conditions for automotive communication, power systems, and networks to work properly.
Synchronous and Asynchronous Motor Optimization
The majority of electrical drives can be broken down into two different types of motors: asynchronous and synchronous. Light automobiles typically utilize synchronous motors with permanent magnets, whereas heavier industrial vehicles utilize asynchronous, or induction, motors where the torque is generated by electromagnetic induction.
Typically, the vehicle manufacturers for both synchronous and asynchronous motors limit them to a maximum permissible voltage rise (dV/dt) at the inverter terminals of about 5 kV/µs, as specified by IEC 60034-18-41. Without these limits, parasitic capacitance in the windings can combine with the dV/dt of the inverter, causing high earth leakage currents to occur, leading to sparking in the bearings and surface erosion because of the dielectric strength of the windings. As a result, the bearings are at a greater risk of failure over a shorter life span.
To achieve high energy efficiency, the power semiconductors within the inverters must be operated at switching frequencies in the 4- and 15-kHz range. However, this may result in high amplitude harmonics, which at 1 MHz can cause interference between the power drive and the medium wave band. This makes medium wave reception nearly impossible in the vehicle if it’s not planned for.
Thus, engineers need to use or create an inverter that’s electromagnetically compatible and gentle on the motor. Often, distributors will work with several component suppliers to create parts lists that are not only compatible, but optimized for such applications.
As an example, Mouser worked with Infineon and TDK to redevelop key components and match other existing ones to create the HybridPACK modules and EPCOS DC link capacitor. Combined, these solutions not only use the latest chips with a dielectric strength of 705 V, but cut the ESL in the dc link nearly in half from about 30 nH to about 15 nH. Regardless of what solution is selected, it’s essential to optimize components to achieve the best utilization possible, and often distributors and reference designs offered by manufacturers can help speed the process.
Infineon Technologies and TDK developed an integrated solution that enables inverters to be used in e-mobility powertrains and industrial applications.
Ferrite Cores Increase Motor Life
Voltage spikes caused by steep pulse edges from the inverter, when combined with parasitic inductances of the motor cables, is another item of concern. Not only are they exaggerated when combined, but they can lead to a higher parasitic capacitive load between the windings of the motor and its housing. This leakage of currents may result in the destruction of motor windings caused by arcing.
To combat this issue, ferrite ring cores should be used to route motor cables from the inverter. This significantly reduces common-mode interference and lowers leakage currents to non-critical levels due to the lower dV/dt, and ensures Classes I through III limits are properly followed.
This graph shows a significant reduction of the overvoltage. Due to lower voltage spikes when switching, the IGBT module and the motor are protected.
Not all ferrite ring cores are the same, so engineers should specify cores which are optimized for their particular application in terms of frequency ranges and temperatures.
Shielded Cables Aid Electromagnetic Compatibility
As mentioned, inverters operate with the pulse-width-modulation control of the motor, causing EMC issues for both the input and output sides of the inverters. The resulting conducted and radiated emissions can be minimized through encapsulation or shielding of cables.
Because motors are typically placed as close to the wheels as possible, and the battery and inverter are distributed in different places throughout the vehicle, the connection of the inverter and battery has to be made by a long shielded cable. This can cause interference in the low-voltage system of the vehicle due to the length of cables. It can also cause voltage spikes that are large enough to damage the inverter and battery. Finally, long cables can create high shield currents that contribute to emissions in the high-frequency range.
The use of shielded cables reduces the spikes, EMI, and other interference. The design of the connection from the cable shielding to the battery should have a tremendously low impedance that supports the shielding and minimizes EMC issues.
While shielded cables are the best solution at the time, they may be weakened over time due to vibration, impact, temperature, oxidation, and corrosion. These can weaken the shielding connection, which causes impedance to rise over time.
EMC Filters Reduce interference
High-voltage dc filters address many of the other potential EMC issues. For EVs, dc filters used should have a maximum voltage of 600 V dc, correlating to the standard voltages of the batteries used in the system.
The EMC filters selected should filter drive systems with power ratings in excess of 100 kW and cover a range between 150 A dc and 350 A dc. This results in a low dc resistance and avoids significant losses in the system.
The plots illustrate the emissions when using the EPCOS HV DC EMC filter. By using the filter between the battery and inverter, it was possible to significantly reduce the conducted interference despite using an unshielded cable.
EMC filters are so efficient that it’s no longer essential to use shielded cables between battery and inverter. This offers significant advantages in terms of cost and weight, as well as better long-term stability due to the weakening of the shielding over time.
In a real-world test with an unshielded cable, the conducted interference was reduced by approximately 70 dB (a factor of 3,000). While eliminating shielded cables isn’t recommended in every application, the use of EMC filters to save on weight, weight distribution, and space is a possibility with the use of EMC filters.
The use of power converters, ferrite cores, shielded cables, and EMC filters offer best practices to reduce energy leakage, electromagnetic interference, and preserve the life of an electric motor. This not only provides a better experience and longer lifespan of the vehicle, but saves space, reduces weight and minimizes costs.
With these savings, engineers can better optimize vehicles and innovate new features that will aid in the adoption of EVs as they overtake their mechanical counterparts by the middle of this century.