How Inverters and Converters Work in Hybrids and Electric Vehicles

Christine and Scott Gable are hybrid auto and alternative fuel experts who have brewed their own biodiesel and traveled 125,000 miles on waste vegetable oil.

In a hybrid and other electric vehicles (EVs), two key elements work together to manage power and recharge the circuits. Here is how these critical components—the inverter and converter—work in tandem.

Broadly speaking, an inverter is an electrical device that converts electricity derived from a DC (Direct Current) source to AC (Alternating Current) of the type that can be used to drive a device or appliance. In a solar power system, for example, the power stored by batteries charged by solar panels is converted to standard AC power by the inverter, which provides the power to plug-in outlets and other standard 120-volt devices.

An inverter serves the same kind of function in a hybrid or EV car, and the theory of operation is relatively simple. DC power, from a hybrid battery, for example, is fed to the primary winding in a transformer within the inverter housing. Through an electronic switch (generally a set of semiconductor transistors), the direction of the flow of current is continuously and regularly flip-flopped (the electrical charge travels into the primary winding, then abruptly reverses and flows back out). The in/outflow of electricity produces AC current in the transformer's secondary winding circuit. Ultimately, this induced alternating current electricity provides power for an AC load—for example, an electric vehicle's (EV) electric traction motor.

A rectifier is a similar device to an inverter except that it does the opposite, converting AC power to DC power.

More properly called a voltage converter, this electrical device actually changes the voltage (either AC or DC) of an electrical power source. There are two types of voltage converters: step up converters (which increases voltage) and step down converters (which decreases voltage). The most common use of a converter is to take a relatively low voltage source and step-it-up to high voltage for heavy-duty work in a high power consumption load, but they can also be used in reverse to reduce voltage for a light load source.

An inverter/converter is, as the name implies, one single unit that houses both an inverter and a converter. These are the devices that are used by both EVs and hybrids to manage their electric drive systems. Along with a built-in charge controller, the inverter/converter supplies current to the battery pack for recharging during regenerative braking, and it also provides electricity to the motor/generator for vehicle propulsion. Both hybrids and EVs use relatively low-voltage DC batteries (about 210 volts) to keep the physical size down, but they also generally use highly efficient high voltage (about 650 volts) AC motor/generators. The inverter/converter unit choreographs how these divergent voltages and current types work together.

Because of the use of transformers and semiconductors (and the accompanying resistance encountered), enormous amounts of heat are emitted by these devices. Adequate cooling and ventilation are paramount to keeping the components operational. For this reason, inverter/converter installations in hybrid vehicles have their own dedicated cooling systems, complete with pumps and radiators, that are entirely independent of the engine's cooling system.

"Solar Integration: Inverters and Grid Services Basics." U.S. Department of Energy.

"Converters, Inverters, and Controls." Center for Advanced Automotive Technology.

Chen, Hua, et al. "Electrified Automotive Powertrain Architecture Using Composite DC–DC Converters." IEEE Trans Power Electron, vol. 32, 2017, pp. 98-116., doi:10.1109/TPEL.2016.2533347