Switching converters in the 4th Industrial Age
In the first blog, we saw how industry and electric motors consume most global energy production. Given that future energy consumption is likely to increase, cost-effective power conversion in industrial applications will prove vital. This blog explores the widespread use of electric motors in industry and the switching technologies that control them. Engineers now have access to new components and architectures, enabling precise, energy-efficient motor control.
Speed Matters
Typically, industrial induction motors can be quite efficient – around 95 percent at full load, but simple on/off control leads to process inefficiencies. For example, the maximum speed is rarely the right speed for a particular operation, which opens up opportunities for efficiency improvements. According to a leading factory automation supplier, a speed reduction of 20 percent to a more appropriate level can result in a 50 percent increase in energy savings. Variable Frequency Drive (VFD) allows the control of a motor’s speed by varying its electrical input frequency and voltage. Such systems can also facilitate a soft start, allowing the motor to ramp-up and ramp-down during the start or stop cycle.
Self-Monitoring and reporting technology (SMART)
Advances in technology and the implementation of the IIoT have made wireless monitoring for variable speed assets possible. SMART (Self-Monitoring and Reporting Technology) sensors with integrated speed detection ‘turn on’ and acquire data only when the motor is in use or up to speed. The use of electronic motor control with a variable frequency drive and smart monitoring reduces mechanical stress, enabling better productivity and less down-time. The right switching component makes all the difference.
The semiconductor switch technology landscape
Figure 1 shows a suggested landscape for the applicability of different switch technologies. Choosing the right switch can make significant efficiency improvements. Industrial motor drives and general-purpose power supplies are invariably switched-mode types – the go-to technologies for the semiconductor switches have been IGBTs for high power with mainly silicon MOSFETs at lower power.
IGBTs have been around since the 1960s and are incredibly robust, with a track record of reliability. However, despite incremental performance advances over the years, they have characteristics that limit their practical switching speed. IGBTs turn off relatively slowly with a residual ‘tail current’ that causes high dissipation as the device voltage rises in the off-state.
This transient dissipation is a small contribution at low frequencies but becomes problematic at frequencies above 10kHz. However, higher frequencies are desirable, as they allow more precise motor control, lower ‘torque ripple,’ and smaller, more effective EMI filters. These keep emissions within statutory levels and reduce mechanical effects such as bearing wear caused by common-mode currents – so-called electric discharge machining (EDM).
MOSFETs do not exhibit the tail current problem of IGBTs and can be switched much faster (up into hundreds of kHz). But they do come with limitations. IGBTs have a relatively constant voltage drop when in their ‘on’ state. MOSFETs, however, exhibit a constant resistance in their ‘on’ state (RDS(on)). At higher powers, I2R losses become exponentially more significant than the VI losses of an IGBT, due to the ‘squared’ term.
Again, technology advances have reduced MOSFET RDS(on) values over the years. However, silicon-based MOSFETs are still only commonly available up to around 1000V ratings, thereby confining them to the lower power applications. Wide band-gap (WBG) semiconductors such as Silicon carbide (SiC) and Gallium nitride (GaN) are now alternatives to IGBTs and silicon-based MOSFETs. SiC-based MOSFETs promise faster-switching speeds than silicon ones, but with lower RDS(on) due to high electron mobility, much better temperature performance (thanks to SiC’s superior thermal conductivity), and a higher critical breakdown voltage.
Gate drivers for SiC-based MOSFETs is a little more critical than for silicon ones. However, versions of SiC devices are available as ‘cascodes,’ which are as easy to drive as an IGBT or silicon MOSFET while requiring a much lower gate charge and, consequently, a reduced drive power.
Industry 4.0 is a seismic change
While conversion to smart motor control has been in progress for a decade or more, other seismic industry changes are also underway. Industry 4.0 or the Industrial Internet of Things (IIoT) combines robotics, advanced automation, artificial intelligence (AI., cloud computing, and network communication. Helping industries become more productive, flexible, and energy-efficient.
Connection to the cloud allows secure consolidation of process variables, outputs, and surrounding environmental conditions, with centralised command and control over multiple sites. There is also a movement toward adding intelligence to the multitude of sensors and actuators – referred to as ‘edge computing.’ This intelligence will allow for autonomous, local, and fast decision making.
In turn, the architecture of equipment power supplies in the industrial environment will be affected. Control cabinets with 24VDC buses derived from single- or three-phase A.C. mains DIN-rail power converters will give way to distributed, lower power converters at the sensors where the most accurate and clean power is needed.
Input power to these converters could be provided by:
- replaceable batteries, suitable for the lowest power applications
- small AC-DC adapters (if mains is available)
- Power-over-Ethernet (PoE) with an on-board DC-DC
- energy harvesting
We will take a more in-depth look at these in the next two blogs in this series.
Conclusion
Using the latest components, SMART control, and high-frequency drives, optimising an electric motors’ performance and efficiency is possible. Industry 4.0 and the IIoT facilitate the constant monitoring of critical parameters, including temperature, motor speed, and vibration. Communication is also possible across multiple sites. With access to such data, engineers can plan scheduled maintenance, avoiding costly downtime and equipment failures. Alternative switch technologies are, therefore, continually evaluated for better speed and efficiency.
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