What Is a Traction Inverter? Evolution of Traction Inverters (Part 1 of 3)

As a North American technology company, Exro Technologies Inc. is at the forefront of developing a new class of power electronics and control technology that expands the capabilities of electric motors and batteries. Our company has developed the Coil Driver™, which is an adaptive traction inverter that replaces the standard traction inverter in electric vehicles (EVs). The Coil Driver™ offers system-level optimizations that can help reduce costs, improve performance, and simplify system design in electric vehicles.

To increase awareness about electrification, showcase the latest technological advancements and innovations that are facilitating the electrification of mobility, and inform our stakeholders about the unique contributions Exro brings to the market, we have crafted this comprehensive article. Our Chief Technology Officer, Eric Hustedt, with over 20 years of experience in leading innovations in automotive power electronics, has provided invaluable assistance in creating this three-part article, which covers a range of topics related to traction inverters.

The first part of this article introduces what a traction inverter is, followed by a discussion of early developments in motor drives and differences between AC and DC machines in part two. Finally, part three will explore the latest advancements in traction inverter technology in greater detail. Our goal is to inform readers about the latest developments in electric vehicle technology and highlight the crucial role that power electronics play.

What Is a Traction Inverter?

Electric and hybrid electric vehicles are gaining popularity as a sustainable alternative to gasoline or diesel-powered vehicles. One key component that determines the vehicles' operation, efficiency, and performance is the traction inverter.

In modern life inverters are a common electronic device utilized in a wide range of applications. Although they often go unnoticed, their significance is increasingly being recognized, and the term inverter or variations on it will become even more prevalent in media and marketing as the popularity of EVs continues to grow.

The term "traction" denotes the act of pulling or drawing something over a surface, thus a traction inverter is an inverter utilized to provide motion over a surface in coordination with an electric motor. Consequently, traction inverters can be found in all types of electric land vehicles, including trains, mining equipment, and increasingly, cars and trucks.

There are various names used to refer to these systems, such as variable frequency drives, motor drives, traction drives, variable speed drives, and traction inverters. If these systems work with electric motors that require AC power, they are considered inverters.

Common Terms Related to Traction Inverters

Motor drive or motor controller:

The term "Motor drive" or "Motor controller" refers to any device used to operate an electric motor, whether it's an AC or DC machine. This term is quite generic and can encompass a range of systems, from simple brushed DC machine controllers found in early battery-operated tools, to complex industrial drives. If a device can "drive" or "control" a motor, it can be referred to as a motor drive or motor controller. An inverter that operates a motor is considered a motor drive, but not all motor drives are inverters. However, the term "motor drive" is becoming increasingly synonymous with "inverter" as the use of brushed DC machines decreases.

Variable Frequency Drive (VFD):

A "Variable frequency drive" (VFD) is a motor drive system that controls the motor by altering the frequency. For AC machines, the frequency is directly linked to the rotation speed of the machine. Although this term is still widely used in reference to industrial drives, it is somewhat redundant for modern AC motor drives as all of them must vary the frequency in order to control the machines. As VFDs are specifically designed for AC machines and deliver AC power, they are typically inverters.

High and low voltage:

When discussing high and low voltage in the context of these articles, "low voltage" typically refers to voltage less than 60-100V and "high voltage" refers to voltage over 100V. However, it's important to note that according to ANSI and NEC codes, there are five classifications of voltage, where "high voltage" is above 115,000Vac and "low voltage" falls within the range of 240 to 600Vac (~850Vdc). This means that all electric cars today fall under the classification of "low voltage" as per the NEC code. However, the definitions and classifications of voltage can vary among different organizations.

MOSFET:

A MOSFET, short for Metal-Oxide-Semiconductor Field-Effect Transistor, is a transistor that is utilized for amplifying or switching electronic signals. It is composed of a metal gate, typically made of polysilicon instead of actual metal, an insulating oxide layer, and a semiconductor channel. The channel's conductivity between the source and drain is controlled by the voltage applied to the gate, the result is that a MOSFET behaves like a voltage-controlled resistance device. When no voltage is applied to the gate, the channel's resistance is extremely high, representing an open circuit or "OFF." Conversely, when the correct voltage is applied to the gate, the channel becomes an excellent conductor, with a resistance measured in milli Ohm for power devices, representing “ON”. MOSFETs are widely used in digital and analog circuits, power electronics, and microelectronics applications.

How Does a Traction Inverter Work?

In the context of electric vehicles, an inverter is an essential power electronic device that converts a direct current (DC) supply from the vehicle's batteries into an alternating current (AC) output. This AC output is then used to power the electric motor that drives the vehicle. However, inverters are not limited to just electric vehicles and can be designed to deliver power to the grid or anything else that requires AC power to operate.

In an AC motor, the current in each phase must alternate the direction of flow between positive and negative at the right time according to the motor shaft rotation and desired torque, similar to how a combustion engine requires the right amount of fuel and precise timing of the spark to operate. The faster the motor speed, the more frequent the alternation of current, just like the sparks in a combustion engine. This variable rate of changing current direction or frequency is precisely controlled by a traction inverter. Its primary function is to ensure the correct current is flowing in the motor phases at any instant by continuously monitoring the motor shaft angle and calculating the necessary current to produce the desired torque. The importance of the traction inverter in achieving optimal performance and efficiency in electric and hybrid vehicles cannot be overstated.

In electric and hybrid vehicles, the electric motor can also act as a generator during regenerative braking, converting the vehicle's kinetic energy into AC power. This is then converted back to DC power by the traction inverter, allowing the battery to be charged. It is worth noting that in electronics, a device that changes AC power to DC power is commonly referred to as a rectifier.

Inverters and Rectifiers

In electrical engineering, both inversion and rectification are common terms used to describe specific functions. For example, an audio amplifier serves as an inverter as it converts a DC power source into an AC power source that drives a speaker coil, which essentially functions as a linear AC motor. However, the term "amplifier" is used instead of "inverter" because the primary purpose of an audio amplifier is to increase a small audio signal, such as from an MP3 player, into a larger audio (AC) signal to power loudspeakers. Nevertheless, to achieve amplification, an audio amplifier must also perform inversion.

The somewhat loose application of terms in electrical engineering stems from the fact that rectifiers, inverters, and devices like class D amplifiers have similar circuitry. While these devices are named based on their primary function, there is minimal difference between them from a conceptual standpoint.

The fundamental similarity between these devices is rooted in the structure of the half bridge, a fundamental building block in power electronics. If the half bridge is constructed using diodes as the switches, it can only facilitate rectification. Conversely, if it is assembled with a controllable switch, like a transistor, it has the ability to perform both inversion and rectification. The half bridge is a ubiquitous architecture in power electronics.

Three-phase inverters, which are commonly used in electric vehicles, share a similar basic circuit, consisting of three half bridges, one for each phase. While most modern electric motors are three-phase, it is possible to add more phases by duplicating the half bridge circuit for each additional phase desired.

The half bridge circuit has been around for a long time, but its practical application for operating electric motors became possible with the invention of the semiconductor switch in the 1950s. However, it is worth noting that when it comes to medium or high voltage power inverters, various circuit configurations are utilized to overcome the voltage limitations of semiconductor switches.

The invention of a specific type of semiconductor switch, the IGBT, and advancements in chip manufacturing technology, which allowed for fast switching and high efficiency, made the use of DC-AC conversion to drive high-power electric motors a practical reality. Since then, when multiple half bridges and other components are integrated into a device to drive an electric motor, it is often referred to simply as an inverter, even though it has the capability to rectify. It can also be called a motor drive, VFD, or any number of other names.

In part 2 of this series, we will take a closer look at the evolution of inverter technology, starting with early advancements in electric motor drive technology, and tracing the progress that has led to the advanced inverters found in today's electric and hybrid vehicles. We will also examine the differences between AC and DC drives. Finally, in part 3 we will discuss the promising future developments in the field, giving our readers a comprehensive understanding of the technology and its potential.