Embedded AC drive
Alternating current (AC) motors have over a century of history. During the last decades, they have dominated the market for motor drives used in industrial and consumer applications. But in the case of variable speed drives, there were some areas where DC motors were the only option for quite long, especially when torque control or a high dynamic control was required. This is now changing as the availability of compact and effective hardware solutions lets vector control expand to cost-sensitive areas like home appliances and consumer electronics. It is also a perfect choice for the traction motors of electric vehicles. Continual improvements of motor drive technologies resulted in unique modulation schemes, sensor-less state estimators and hardware solutions which are now immediately available.
The most important milestones in the development of variable-speed AC drives were the work of R. H. Park in 1929 and field-oriented control (FOC), known also as vector control, developed in the late 1960s by K. Hasse, F. Blaschke, and W. Leonhard. At that time, the proposed control strategy was quite difficult to implement. Lack of efficient processors that could perform the required real time calculations was the most important obstacle. Almost five decades later, FOC and its derivatives became one of the most significant control strategies for variable speed AC drives providing a high dynamic of torque control. (Direct torque control is worth mentioning as an alternative for some cases).
What’s inside a low-cost AC drive?
An AC drive consists of power switches that supply the motor phases, a control circuit, sensing units, and a power supply with DC link capacitors. Nowadays, the hardware part of a low-power (below 1kW) inverter, excluding the power supply and passive components, can be built out of just two integrated circuits: a microcontroller and an integrated power module. A general diagram of a typical AC drive solution is presented below.
Novel, single-chip solutions for motor control include efficient cores, high-resolution pulse width modulators, analog-to-digital converters, and other peripherals, allowing efficient and reliable implementation of variable speed control for AC drives. It is significant that a separate kind of chips appeared on the market, called DSC (digital signal controllers) for the above-mentioned and other digital power conversion applications. The C2000 controllers from Texas Instruments and the MC56F8xxx family from Freescale/NXP are good examples of DSCs. Both manufactures offer reference designs and software libraries for quick control system composition. The InstaSPIN solution from TI is also worth mentioning as it is a comprehensive set of tools that facilitate implementation, tuning, and verification of motor drive design.
For the needs of power electronics, manufactures offer integrated power modules which include power switches with drivers, auxiliary power sources for high-side switches, and protection circuits, so no glue logic is required between power modules and microcontrollers. The SLLIMM power modules offered by ST represent this kind of product.
To sum up, complete inverter systems allowing effective variable-speed control of AC machines can be embedded in every device requiring a reliable and efficient drive.
Why consider an AC drive?
The most common, non-inverter-based drive solutions used in consumer electronics are variable speed DC drives and constant speed induction motor drives.
So what is the real benefit of using an inverter instead of a DC variable speed drive? First of all, AC motors have a simpler mechanical construction. They have no commutator (which is noisy) and no brushes which require maintenance. In pumps, AC drives offer “wet rotor” configuration, which removes the need of a seal between the rotor and the pump impeller, reducing maintenance and increasing pump reliability. Another advantage is that an inverter offers much more than simple direction and speed control. Direction and speed control are of course also possible with DC drives, but direction control of the most common DC solution – universal motors – requires additional hardware. Another benefit of AC drives is simple switching between different modes of operation such as regenerative breaking, dynamic breaking, and driving, which is important in the case of traction systems. This feature is available on separately exited DC motors, but it is not as straightforward in the case of series-DC machines.
Compared to simple, single-phase induction drives running at a constant speed, inverter drives offer the possibility of speed and torque control, which is the most obvious benefit. But inverter-based drives can also perform additional measurements and calculations to analyze the load and working conditions without any additional sensors. Good examples are a pump drive which can detect discontinuities in the flow of the medium and a washing machine’s drum drive which can evaluate the mass of the machine load. Inverters can also offer motor overload protection, speed profiles to reduce starting currents, and functional safety features. AC inverters can also drive permanent magnet synchronous motors (PMSM) which have higher power density and higher efficiency (in most applications) and are quite difficult to control in other way.
So what speaks against embedded AC drives? First of all, the development of high-quality AC drives requires highly skilled personnel. It is also important to notice that we are not talking about industrial motor drive units but moderately simple drive control solutions developed for particular applications and specific AC motors. Significant changes in working conditions or motor parameters could require tuning the control software.
AC drives and their control techniques integrate a number of brilliant ideas into technical masterpieces which are available now, immediately.
Marek Kabala is Chief Design Engineer, working as software team leader in product development projects. Has PhD degree in robotics, and experience in multidisciplinary projects involving mathematical modeling and simulations, algorithms development, implementation and verification. You can reach Marek at firstname.lastname@example.org.