Solid-state switching: advantages and precautions
July 2001
PLCs, DCSs & Controllers
Solid-state switching devices are fast and wear-and-tear free. They only require a low energy control signal. Hence, they are increasingly used in favour of electromechanical switches (relays).
Solid-state switches now incorporate features that vastly simplify control and supervision. This opens new avenues for PC/PLC control and scada systems. Latest versions feature serial communication interface, internal current transformers, restorable electrical fuses and master-slave control. Consideration to heat dissipation and protection against excessive voltage and current greatly enhances the reliability of solid-state switches.
Traditionally, a distinction is made between solid-state relays (SSRs), which operate in on-off mode and thyristor drives which regulate the supply voltage proportional to an analog input signal. This line becomes blurred due to the introduction of burst-firing solid-state switches, which vary the output duty cycle in proportion to an analog input signal.
* Logic input solid-state relays: Logic control solid-state relays are used where frequent switching occurs. Typical applications are PID control of heating elements and solenoid valves. Zero crossover switching reduces interference on the power supply line. The control signal is usually a logic signal of 4 to 32 V d.c. Less expensive SSRs have an internal triac and are only suitable for resistive loads (AC1-rating), whereas solid-state relays with internal thyristors have an AC3 rating and are suitable for resistive and inductive loads (valves, solenoids...).
* Analog input solid-state relays: proportional (analog) input, burst firing control solid- state relays are used to control fast processes where a typical PID cycle time of 1 s is too slow. Zero crossover switching is maintained and proportional control is achieved by varying the duty cycle (ratio of on pulses to off-pulses). The control signal is usually 4-20 mA or 0-10 V d.c. Typical applications include long-to-medium wave infrared heating arrays and air heating systems that have a fast response time.
* Phase angle solid-state power controllers (thyristor drives): proportional (analog) input, phase angle control devices are used for processes where a smooth modulation of the power supply is required. Through phase angle control part of the sine wave is sliced off, resulting in power reduction. The control signal is typically 4-20 mA or 0-10 V d.c. Applications include short-to-medium wave infrared heating arrays, transformers, vibrators and speed control on fans.
* Three-phase switching: For three-phase applications either three-phase SSRs or three single phase SSRs can be used. Care should be taken that single phase SSRs are rated at 400 V or higher since the loss or absence of a neutral connection on an unbalanced (or faulty) load can place the full phase voltage across the switching device. Some manufacturers offer a master-slave connection that synchronises switching and reduces current pulsation on the neutral conductor.
* Protection: Solid-state devices are susceptible to overheating, overvoltage, high-voltage spikes and fault currents. Adequate heatsinks, proper ventilation, ultra-rapid fuses and transient arrestors (MOV, RC-circuit) are therefore essential considerations for installation of SSRs. Solid-state switching devices are most likely to fail in the on mode. Safety override (eg high temperature cut out) should therefore be considered to switch off the main contactor.
The recent acquisition of Coreci by Gefran has made a comprehensive range of solid-state switches available to the South African industry via local distributor Unitemp. Gefran offers advanced power control units up to 600 A with power soft-start, current limit, current feedback; electronic (resettable) fuse and RS485 interface with Modbus protocol. This allows multidrop remote control of power and offers various diagnostic functions including the ability to read line voltage and load currents.
Unitemp
(021) 762 8995
[email protected]
www.unitemp.com
Further reading:
ABB updates distributed control system
ABB South Africa
PLCs, DCSs & Controllers
Leveraging 30 years of continuous innovation and reliability, ABB’s updated Freelance 2024 distributed control system (DCS) offers greater plant adaptability, faster and more reliable device communication, improved system security, and seamless data exchange.
Read more...
Four ways modern operations control can boost sustainability and efficiency
PLCs, DCSs & Controllers
With the growing importance of digital transformation, HMIs and scada have evolved from control panels to vast operational hubs. Next-gen HMI/scada can bring together data, personalisation, and advanced insights to successfully achieve organisational goals, it is important to think about HMI/scada holistically within the operations ecosystem.
Read more...
The convergence of intelligence: DCS, SCADA and TLC
Schneider Electric South Africa
PLCs, DCSs & Controllers
In the early days of industrialisation, control systems were primarily mechanical, and relied on manual intervention and simple feedback loops to manage processes. Now, in the 21st century, industrial process automation systems are mind-blowingly intelligent, and provide almost unparalleled control and monitoring capabilities, making them integral to modern industrial systems.
Read more...
PC-based control for advanced hydrogen storage technology
Beckhoff Automation
Editor's Choice PLCs, DCSs & Controllers
The proportion of renewable energies from solar, wind and water is rising continuously. However, sufficient storage options are of the essence to use these energies as efficiently as possible. GKN Hydrogen offers a particularly compact and safe option, low-pressure metal hydride hydrogen storage systems with PC-based control from Beckhoff.
Read more...
ABB modernises key board mill
ABB South Africa
PLCs, DCSs & Controllers
ABB has secured a landmark contract to modernise Smurfit Kappa’s Paper Machine 5 at its corrugated cardboard mill near Mexico City. ABB will provide Smurfit Kappa with DCS, accompanied by a comprehensive paper machine drives system, encompassing some of the market’s most advanced drives and motors meticulously designed to optimise PM5’s performance.
Read more...
The synapses of the distributed control system
Schneider Electric South Africa
PLCs, DCSs & Controllers
Industrial operations require a distributed control system (DCS) to coordinate and control their process subsystems in real time. Like the brain, a DCS is a multitasking maestro, controlling and coordinating complex processes in a myriad of industrial setting such as large manufacturing plants, providing valuable top-down control.
Read more...
Modular assembly platform for clean manufacturing
Beckhoff Automation
PLCs, DCSs & Controllers
JR Automation delivers custom automated solutions for numerous industries. It has done this through its scalable, modular automation platform, FlexChassis, which speeds up time to market while cutting costs. The company chose the XTS linear transport system from Beckhoff because of its speed, and modular design that allows for multiple configurations.
Read more...
Selecting the best remote access solution for your application
PLCs, DCSs & Controllers
In today’s Internet of Things (IoT) world, remote mobile access is a necessity for many industrial applications. There are several ways of implementing this connectivity with routers and virtual private networks.
Read more...
PLCs and PACs simplify data acquisition
PLCs, DCSs & Controllers
Data acquisition, data logging and data analysis are required functions for most modern industrial control systems. The simplest and lowest cost way to provide these functions is often by using the same platform providing real-time control, namely the PLC or the PAC.
Read more...
Small-scale custom development in the information age
H3iSquared
PLCs, DCSs & Controllers
In the twenty-first century, the ability to put together custom electronic devices has become much more accessible to smaller companies, and even the private user. With the open nature of these platforms, potential developers can start learning systems easily, and do not need a massive capital output to do so.
Read more...
printer friendly version