Silicon Controlled Rectifier (SCR) power controls are great step of power control that have a minimal maintenance costs that if properly maintained could last for many years to come. The article posted by Industrial Heating (below) explains the advantages of SCR power controls including precise control of the heating process, extended heater life, improved product quality at faster production speeds, and all while reducing the maintenance costs.
“Equipment manufacturers and maintenance engineers need a reliable, flexible approach to controlling electric-heating processes in today’s competitive, cost-conscious industrial environment. SCR (silicon controlled rectifier) power control offers advantages including precise control of the heating process, extended heater life, improved product quality at faster production speeds and reduced maintenance costs. Proper selection of an SCR power control can result in many years of reliable service.
Comparing power controls
A silicon controlled rectifier is a solid state switching device that can provide fast, infinitely variable proportional control of electric power; a SCR can cycle as fast as 0.08 seconds on a 60-Hz power line. With proper selection and use, a SCR power control can cycle on and off 1,000,000,000 times without any problems.
SCR selection and care
Three factors that will destroy all solid-state power controls are over-temperature, short circuits and transient voltage spikes. Protection against over-temperature, voltage spikes and short circuits should be taken into account at the outset of a system design.
One problem with some SCRs and solid-state relay controls is packaging. In an effort to reduce the size of the heat sink, the fin area is not adequate to remove excess heat. DIN rail-mounted heat sinks can save panel space and installation time. However, with many controls mounted close together on a DIN rail, the power density (W/area) inside the enclosure is increased. At the same time, airflow to the heat sinks is decreased or blocked. If such an arrangement is used, it is important that the manufacturer has not asked that the DIN rail heat sink be fan cooled or mounted with the heat sink fins outside the cabinet. It also is important to check the derating curve for the device at the power level at which it will be used.
Wattage and airflow
Semiconductors are destroyed by a short circuit. One of the simplest ways to protect a SCR power control is proper fusing. While a SCR is a robust, reliable device, semiconductor, subcycle, and I2T current-limiting fusing must be used to ensure maximum performance and service life. Nearly all SCR power-control manufacturers incorporate these fuses on their controls. I2T fuses will clear within 2 milliseconds and limit the current as they clear. They are reliable and easy to replace.
Note that 99.9% of fuse failures are due to shorted heaters, loose connections, incorrect (too large) load matching and miswiring of the SCR power control. With high inrush loads (such as with tungsten heaters, transformer primary drivers, molybdenum heaters and shortwave heaters), use of anything other than a soft start, phase-angle fired SCR control will blow fuses. Never switch on a cold inrush heater bank after the soft start has been activated. Changing taps on a transformer while in use also may blow fuses.
The simplest protective device for over-voltage is a metal-oxide varistor (MOV), which is wired in across the SCR. By using an MOV having a voltage rating higher than the line voltage, but lower than the SCR peak voltage rating, the MOV becomes an effective guard against voltage spikes. If a transient voltage spike goes over the rating of the MOV, the MOV will block the spike. If the spike is powerful enough, the MOV will explode, protecting the SCR.
Selecting a SCR power control
Three ways a SCR power control can deliver electrical power to heaters are: phase-angle fired, zero-voltage switched and on/off control.
Phase angle-fired controls proportionally turn on a percentage of each power line half cycle, which gives smooth, infinitely variable application of power to the heaters. It is the most precise method of control. Phase-angle firing can increase heater life up to seven times depending on heater type, and also allows options such as soft start, voltage limit and current limit. These options are not available using any other methods of control.
Zero voltage-switching controls proportionally turn on and off each power line full cycle. The SCR provides power to the heaters by varying the number of ac power line cycles. With a variable time base, the optimum number of cycles turned on/off is achieved. This method produces less RFI line noise than phase angle-fired SCRs.
On/Off controls function in the same way as a mechanical or mercury relay, but with the advantage of much faster cycle times.
SCR power controls are highly reliable, providing trouble-free operation with a minimum maintenance that is economical. It has an infinite resolution controlling power, current, or voltage from 0 – 100%. SCR power controllers can toggle the load power on and off immediately, allowing the response to command, load, and power supply changes.”
If you like to read more about SCR power controls, click here.
If you like to speak with an expert regarding to industrial heating, contact Heatrex! We have experienced engineers and designer staff that will help meet your specific heating needs. Call us at (800)394-6589 or visit us at http://www.heatrex.com/– We can build it!.