Dominic Bowker, general manager UK and Ireland at Inovance, looks at how AC drive technology have evolved from simple speed controllers into intelligent power conversion systems, and why silicon carbide could represent the next important stage in their development.
Dominic Bowker
Over six decades, AC drive technology has transformed the way machines use electric motors. At its most basic level, an AC drive solves a simple problem. When an AC motor is connected directly to the mains supply, its speed is largely determined by the supply frequency and the motor design.
However, most industrial applications don’t want a motor to run at one fixed speed. Production machinery, pumps and fans may need to respond to changing operating conditions and demands. Engineers historically relied on gearboxes, belts, clutches, valves or dampers, but this added mechanical complexity and, in many flow-control applications, wasted energy.
AC drives control the conversion of electrical energy into mechanical motion as efficiently and smoothly as possible. This article explores how this has been achieved over the years and how technology is evolving to push the limits of what is possible.
The early days of AC motion control
Modern AC drives began to emerge during the late 1960s and became widely established through the 1970s. Early drives used relatively crude switching techniques, including quasi-square-wave or stepped output waveforms. These systems could vary speed, but the waveform supplied to the motor was far from ideal. It contained significant harmonic content, which could increase motor heating, torque ripple, acoustic noise and electrical interference. What the industry needed was a way to switch high voltages and currents quickly, efficiently and reliably.
A major step forward came in the 1990s with the commercial adoption of the Insulated Gate Bipolar Transistor, or IGBT. IGBTs gave drive designers a practical way to switch high voltages and currents rapidly and reliably at industrial power levels. Using pulse width modulation (PWM), the drive rapidly switches its DC bus to synthesise a variable output voltage. Although this voltage consists of high-frequency switching pulses, the motor’s inductance smooths the resulting current into a waveform much closer to the sinusoid required for efficient torque production.
IGBTs reduced harmonics and improved speed and torque control. Combined with increasingly capable digital control algorithms, they enabled effective flux vector control, allowing AC motor packages to deliver strong low-speed torque and dynamic performance previously associated mainly with DC motors.
From DC motors to more efficient AC motor control DC motors, although highly controllable, use brushes and commutators that require regular maintenance. As vector-controlled AC drives matured, many applications moved away from DC systems towards lower-maintenance AC motor and drive packages.
IGBTs also began to appear on the input side of drive systems. Conventional drives use diode or thyristor rectifiers, so braking energy is typically dissipated as heat in braking resistors. Active Front End (AFE) technology replaces these with a controlled IGBT switching stage, allowing energy to be returned to the electrical supply, while also achieving low harmonic distortion and near-unity power factor.
As IGBT-based power stages became the established architecture for drives, drive technology also advanced in other areas. Over the past two decades, many of the major developments have been in control electronics, software, communications, diagnostics, connectivity, functional safety and ease of use.
Intelligent, safer, more user-friendly drives
Modern drives can communicate over high-speed, industrial Ethernet networks such as EtherCAT and PROFINET, allowing multiple axes to be synchronised accurately within a machine. This supports faster, more coordinated production, while the drive itself can provide diagnostic information, support integrated safety functions and be commissioned through intuitive software tools.
Functional safety has also become an important part of drives, including Safe Torque Off (STO), which prevents the drive from producing torque when required. Newer drive systems support advanced safety functions that monitor stopping, speed and direction. These functions can help reduce external safety hardware and wiring, simplify panel design, improve diagnostics and support compliance with modern safety requirements.
The MD630 LV AC drive from Inovance
AC drives: a key component in modern automation
The AC drive has evolved from a standalone speed controller into an intelligent component within the wider automation system. As drive technology becomes more cost-effective and easier to apply, its use has expanded beyond industrial machinery into everyday infrastructure applications. Pumps, fans, compressors and HVAC systems are good examples of this move. Rather than controlling flow mechanically through valves, dampers or other restrictions, motor speed can be matched more closely to process demand. This reduces energy consumption, improves controllability and reduces reliance on some mechanical control components.
Today, drives support much of the infrastructure behind modern life. They help move water, control air, manufacture food, handle materials, operate lifts and cranes, support marine systems, run production equipment and test new technologies. Most people rarely see them, but their contribution to industrial productivity and energy efficiency is considerable.
The emergence of silicon carbide
The growth of electric vehicles (EVs) is driving major investment in motors, inverters and test equipment. EV traction systems demand compact, efficient power conversion, while battery simulators, motor test rigs and e-mobility development systems are creating similar requirements within industrial environments.
This is where Silicon Carbide (SiC) semiconductor technology is beginning to play a more important role. Compared with conventional silicon devices, SiC offers lower switching losses, higher switching frequencies and improved thermal performance. For drive designers, this creates opportunities for greater power density, improved efficiency and more compact cooling and filtering. These advantages are particularly valuable in demanding applications such as high-speed motor, battery simulation and e-mobility applications.
IGBT technology will continue to be used alongside SiC
Conventional IGBT-based drive architectures will remain the mainstream choice for many industrial applications because they continue to offer an excellent balance of performance, reliability and cost. SiC technology is more likely to be adopted where efficiency, switching frequency, power density or high motor speeds justify the additional investment.
In practice, the first step is often not a full SiC power stage but a hybrid architecture. For example, Inovance’s MD520HS AC drive combines conventional silicon IGBT switching devices with SiC diodes, improving efficiency and power density while maintaining a practical balance between performance, cost and commercial viability. Inovance is also developing SiC-based technologies for selected e-mobility power conversion applications, including battery simulation and DC/DC conversion, where high-frequency power conversion, efficiency and power density provide significant system benefits.
Final thoughts
The history of variable speed drives can’t be defined by one single development. It combines advances in power electronics, digital processing, communications, functional safety and software. From early quasi-square-wave drives to IGBTs providing better motor control and regeneration, and now SiC technology, engineers continue to push the physical limits of what drive systems can achieve.
Variable speed drives are now fully integrated into the wider automation system and, for me, the interesting point is that, even though drives have become much more intelligent, the power semiconductor remains central to what is physically possible. It affects efficiency, switching performance, thermal management and system size. That’s why the move from early quasi-square-wave technology to IGBT-based PWM drives was so important, and why silicon carbide is now attracting attention.
The future of drive technology will not be defined by a single breakthrough. It will combine more efficient power devices with better control algorithms, better connectivity and easier system integration. For machine designers, drives are likely to become even more efficient, compact, easy to commission and better suited to high-performance applications.
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