The Ins and Outs of SCR Power Controls
An SCR is a solid-state switching device that can provide fast, infinitely variable proportional control of electric power. It can cycle as fast as 0.083 sec on a 60 Hz power line. If selected and used properly, an SCR power control can cycle on and off 1 billion times without any problems.
Unlike a mechanical relay or contactor, an SCR power control has no mechanical parts to wear out. The SCR power control will not arc or be affected by dirty contacts. A mechanical relay will have to be replaced after a certain number of cycles. Due to the slow (30 sec minimum) cycle time inherent in mechanical relays, control is poor, so the heated process may be damaged and heater life could be shortened due to thermal shock.
Mercury displacement relays can cycle faster than mechanical relays. However, if overheated due to excessively fast cycling or overloading, the mercury relay will explode. This results in a hazardous materials problem. Because of more stringent governmental regulations, shipping and disposing of mercury relays also are becoming increasingly difficult.
Solid-state relays are a popular alternative to mechanical power controls. Common to all solid-state devices, solid-state relays dissipate the heat that needs to removed, and they are capable of dissipating more heat than SCRs. But, solid-state relays typically are not supplied with lugs that make a solid electrical connection for higher power levels. Also, they are not always sold with heat sinks, voltage protection or the fuses needed to protect and safely operate the relay.
Further problems could arise from the ratings of the solid-state relay. Almost all solid-state relays are rated for maximum output at 77oF (25oC). In real-world operating conditions where electrical enclosures' internal temperatures reach over 104oF (40oC), the solid-state relay may fail if used at full output. Most manufacturers have a derating chart for their products to compensate for this discrepancy. Unfortunately, many users rely on the only the maximum rating when selecting a solid-state relay. Be sure to read the manufacturer's data before selecting which solid-state relay will work best for your process.
Most SCR power controls are rated for ambient operating temperatures of 122oF (50oC) at full power ratings. Fuses, heat sinks, voltage protection and power lugs usually are included with SCR power controls.
Extending the Life of Your Power ControlThree things will destroy all solid-state power controls:
- Short circuits.
- Transient voltage spikes.
Here's how to keep them from disrupting your production line.
Overtemperature. Almost all semiconductors will be destroyed at an internal junction temperature of 257oF (125oC). All solid-state power devices, thyristors such as SCRs, triacs and solid-state relays will dissipate heat. Voltage drops across a power device generate heat. This drop can be anywhere from 1 and 2 V depending on the device. The more current (amps) put through the device, the higher the wattage the device will dissipate in the form of heat. This heat must be removed or the device will fail.
The simplest and most common way to remove heat is with a heat sink. If a properly sized heat sink is used, the SCR can operate at its full rating with an ambient temperature of 122oF (50oC). The higher the amperage output, the greater the heat that is dissipated. Many manufacturers use fans to remove excess heat from high output SCR power controls. On some super-high-output SCR power controls (more than 1,000 A), water-cooled heat sinks are used.
One problem with some SCR or 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 near one another on a DIN rail, the watt density inside the enclosure is increased. At the same time, airflow to the heat sinks is decreased or outright blocked. If you employ such an arrangement, be sure 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. Also, check the derating curve for the device at the power level it will be used.
Even at low power such as 25 A, each controlled leg of a solid-state relay will dissipate close to 50 W of dissipated heat. If you have 20 DIN-rail-mount solid-state relay power controls in a small enclosure, you will have to get rid of 1,000 W of heat! For spacing power controls, a quick rule of thumb is to use twice the footprint of the device when mounting. For example, if the SCR power control has a 12 x 12" footprint, use a 24 x 24" area for mounting.
Proper ventilation of the electrical enclosure is critical to keep an SCR power control functioning. Even a power control with an adequate heat sink will fail if the heat cannot escape the enclosure. When venting an enclosure, the basic formula is 10 in2 of inlet and outlet area for every 50 A per controlled leg. Because heat rises, vent holes should be at the top and bottom of the enclosure.
To determine the heat generated by an SCR controller use this formula: For each controlled leg (C), multiply the amperage load (I) by 1.5.
C x I x 1.5 = watts dissipated (W)
Fan cooling is an efficient way to keep the enclosure at safe operating temperatures. To determine the size of the fans needed, use this simple formula: Watts dissipated (W) x 3.16, divided by temperature rise (oF) above ambient (TR).
W x 3.16 / TR = cfm Suppose you wanted to calculate the size of fan necessary to limit the heat rise in an enclosure to 10oF above ambient (TR) with 1,000 W of dissipated heat. Using the equations presented, the calculations would be: 1,000 W x 3.16 = 3,160
3,160 / 10 = 3,160 cfm Fan
Plastic enclosures act as thermal insulators. You most likely will destroy an SCR power control if you install it inside a plastic enclosure. Having a through-hole-mount heat sink -- with the heat sink fins on the outside of the enclosure -- is the only reliable way to use a plastic enclosure.
The fuse is the easiest component to replace if there is a heater short. Make sure to clear the shorted heater or wiring before installing a new fuse. Not using a semiconductor fuse is penny-wise and dollar-foolish. Without fuse protection, the SCR can be damaged when it need not be.
Remember that 99.9 percent of fuse failures are due to shorted heaters, loose connections, incorrect (too large) load matching, or miswiring of the SCR power control. On high in-rush loads (tungsten lamps, short-wave heaters), use of anything other than a soft-start, phase-angle fired SCR control will blow fuses. Never switch on a cold in-rush heater bank after the soft start has been activated.
Make sure to size the SCR power control to your heater load. Remember that heaters and power lines have tolerances. To be safe, use an SCR power control with a rating of at least 1 percent to 10 percent of your maximum heater load potential.
The simplest-to-use protective device is a metal-oxide varistor (MOV). The varistor is wired in across the SCR. By using a varistor with a voltage rating higher than the line voltage but lower than the SCR peak voltage rating, the metal-oxide varistor becomes an effective guard against voltage spikes. If a transient voltage spike exceeds the varistor's voltage rating, the varistor will block this spike. If the spike is powerful enough, the metal-oxide varistor will explode, protecting the SCR.
The use of a DV/DT suppression board is the next step in power line noise and voltage spike protection. With a network of power resistors, high voltage capacitors and metal-oxide varistors, the SCR has greater protection from line noise and voltage spikes. This network helps eliminate damage to the SCR as well as SCR misfiring.
Continuous overvoltage will destroy the SCR. Make sure the SCRs used on the power control are rated with a voltage high enough to withstand industrial voltage peaks. The higher the peak voltage rating of the SCR, the safer it will be.
Selecting an SCRThe SCR power control can deliver electrical power to heaters in several ways: 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. This gives smooth, infinitely variable application of power to the heaters. The most precise method of control, phase-angle firing also can increase heater life up to seven times, depending on heater type. In addition, phase-angle firing allows options such as soft start, voltage limit and current limit. These options are not available with any other means of control.
Zero-voltage switching controls proportionally turn on and off each full cycle of the power line. By varying the number of AC power line cycles, the SCR provides power to the heaters. With a variable time base, the optimum number of cycles turned on or off is achieved. This method produces less radio frequency interference (RFI) line noise than phase-angle-fired SCRs.
On/off controls function the same way as a mechanical or mercury relay but with the advantage of much faster cycle times.
The use of an SCR power control offers the most precise means of controlling your electric heaters. Heater life is extended; production speeds are increased; and product quality is improved. Just remember to protect against over temperature, voltage spikes and short circuits at the outset of your system design.
By following the few simple steps provided, an SCR power control can give superior performance with minimal maintenance costs for many years to come.