Speed control provides an essential efficiency optimisation

There have been three landmark events in the history of electric drive technology:

• The invention of the DC motor in 1833
• The invention of the three-phase motor in 1889
• The first series production of frequency converters in 1968

This article focuses on the evolution of drive technology since the introduction of speed control with frequency converters. The advent and rapid development of microelectronics, followed by signal processing, were essential factors in this evolution. These two technologies were also key factors for the development of localised solutions and networked automation.

Danfoss pioneered mass-produced frequency converters in 1968. This marked the start of continuously adjustable, energy-efficient speed control of three-phase asynchronous motors in drive technology. The first devices used thyristors for the inverter and controlled them using pulse amplitude modulation. The power stage was cooled by an oil bath, which made the devices large, heavy and unwieldy. However, commissioning was very easy: the motor data was set with jumpers, and only five potentiometers were used to adapt the motor to the plant.

In the late 1970s, smaller devices with air cooling were launched on the market. They had more functions and allowed extensions to be implemented with accessories. For example, Danfoss offered a manual control kit, a ramp generator and an isolation transformer for devices without galvanic isolation as accessories for the VLT 1 to VLT 4 series. PLC control could also be implemented with these devices.

The functional scope of most devices available at that time was still very limited. For example, users had access to start/stop and reverse motion functions.

Digital technology opens the door to new possibilities

Increasingly demanding requirements from the industrial sector accelerated the evolution of frequency converters. The devices needed to be flexible, intelligent and cost effective. These requirements were met by the first digital frequency converter series in the 1980s. The key to providing the appropriate functions was the availability of the first microprocessors. In 1989, Danfoss presented the VLT 3000 series of digital devices, which featured an integrated ramp function, eight digital control inputs and, more importantly, a consistent user interface over the full power range, which was 750W to 250kW at that time.

In the early 1990s communication technology expanded the functional scope. From the initial situation with a confusing variety of individual approaches from different manufacturers, the modern field bus approach quickly became established as a future-proof and non-proprietary solution, with the Profibus interface assuming a leading role and developing into its current position as the most important and most widely used field bus interface.

The evolution of frequency converter technology was rapid in the following years, driven by progress in microelectronics. However, the industrial sector was still looking for even faster and more flexible devices, easier commissioning and better dynamic performance in order to optimise plants and production processes. Other significant factors, of course, were increasing globalisation as well as greater competition and price pressure.

Along with the ongoing evolution of digital frequency converters, which in the case of Danfoss saw the introduction of different series for differing applications, there was a demand from the industrial sector for an integrated product, consisting of a modern frequency converter and a standard induction motor, for compact systems with localised architecture. This led to the launch of the frequency converter motor. These drives typically had power ratings in the range of 300W to 7.5kW, and their key advantage is cost savings resulting from the elimination of the motor cable, reduced wiring effort, less space demand in electrical cabinets and easier commissioning.

Modular system design and smarter drives

The first years of the present century are marked by compact, modular systems. These can be put together easily and effectively from modules, enabling users to obtain systems matched to their needs. They allow users to expand the system at a later date if necessary, and they make it easier to adapt the drives to a modified plant design or new processes, simply and without major effort or expense.

In recent years, the trend is towards more and more intelligence in the drive. Many functions that previously were handled by a central controller are now migrating to the frequency converter, which frees up control systems for other tasks and more complex program sequences. Safety functions, such as Safe Stop, as well as monitoring of explosion-proof motors in explosion hazard areas can also be integrated. Following a change in the approval procedure, it is now easier to use frequency converters to drive motors specifically designed and certified for use in explosion hazard areas.

User-specific adaptations are now possible with freely programmable extensions. EMC measures are also becoming increasingly important. The rising use of frequency converters is increasing the level of harmonic distortion in the supply network. For this reason, integrated EMC filters that comply with current limits have been standard in Danfoss frequency converters for a long time. This aspect will become increasingly significant in the future in order to ensure the security of supply of our power grids. Low-harmonic frequency converters, devices with active front ends, and active filters that mitigate harmonic distortion for a whole set of motors driven by frequency converters are crucial elements for this - especially because power levels now extend to several megawatts, even with low-voltage devices.

Drive technology is taking on another important duty for the future: using energy efficiently. Frequency converters are the first choice for energy-efficient drives, and they control the manufacture of virtually every product in the supermarket. They control pumps for reliable water supply and wastewater treatment, and they drive air conditioners. In all of this, they achieve considerable energy savings and optimise processes. With their diminishing size, increasing capability and growing intelligence, they have become indispensable for us, and most of them do their jobs quietly in the background.

To learn more about electric drive technology from Danfoss, please go to www.danfoss.co.uk.