SCR Power Controls Provide Reliable Industrial Electric ...

1 Reprinted fromIndustrial Heatingoctober 2002process control & instrumentationEquipment manufac-turers and mainte-nance engineersneed a Reliable , flex-ible approach tocontrolling Electric -heatingprocesses in today s competi-tive, cost-conscious industrialenvironment. SCR (silicon con-trolled rectifier) Power controloffers advantages includingprecise control of the heatingprocess, extended heater life,improved product quality atfaster production speeds andreduced maintenance selection of an SCRpower control can result inmany years of Reliable Power controlsA silicon controlled rectifieris a solid state switching devicethat can Provide fast, infinitelyvariable proportional controlof Electric Power ; an SCR cancycle as fast as secondson a 60-Hz Power line.

2 Withproper selection and use, anSCR Power control can cycleon and off 1,000,000,000times without any a mechanical relay orcontactor, an SCR Power con-trol has no mechanical parts towear out. An SCR Power con-trol will not arc and is notaffected by dirty contacts. Bycomparison, a mechanicalrelay must be replaced after acertain number of cycles. Dueto the slow cycle time (30 sec-onds minimum) inherent inmechanical relays, control ispoor, the heating process maybe inaccurate and heater lifecould be shortened due tothermal mercury displacementrelay (MDR) cycles fasterthan a mechanical , overheating therelay due to excessively fastcycling or overloading willcause it to explode, a situa-tion which creates a haz-ardous-materials problem.

3 Inaddition, shipping and dis-posing of MDRs is becomingincreasingly more difficultdue to more stringent federalenvironmental regulations. A solid-state relay (SSR) is apopular alternative to mechan-ical Power control . However,solid-state relays are not sup-plied with lugs that make asolid electrical connection forhigher Power levels, and theyare not always supplied withheat sinks, voltage protectionor the fuses needed to protectand safely operate the common characteristic of allsolid-state devices (includingSSRs) is that they generateheat that needs to relays generatemore heat than consideration ofsolid-state relays is their rat-ing, which could cause a prob-lem. Almost all solid-staterelays are rated for maximumoutput at a temperature of25 C (77 F).

4 However, in realworld operating conditionswhere temperatures insideelectrical enclosures exceed40 C (104 F), a solid-staterelay will fail if used at fulloutput. Most manufacturershave a de-rating chart for theirproduct to compensate for thisdiscrepancy. Unfortunately,many plant personnel use theadvertised maximum ratingwhen selecting a solid-staterelay. It is important to under-stand the manufacturer s databefore determining whichsolid-state relay will work bestfor a specific SCR Power controlsare rated for ambient operat-ing temperatures of around50 C (122 F) at full powerratings. Also, SCR Power con-trols usually are supplied withfuses, heat sinks, voltage pro-tection and Power selection and careThree factors that willdestroy all solid-state powercontrols are over-temperature,short circuits and transientvoltage spikes.

5 Protectionagainst over-temperature, volt-age spikes and short circuitsshould be taken into account atthe outset of a system solid-state Power de-vices (that is, thyristors suchas SCRs TRIACs and SSRs)generate heat. Voltage drops,which can be anywhere fromSCR Power Controls Provide ReliableIndustrial Electric Heating Paul Evalds, Avatar Instruments, Broomall, following the few simple steps provided here, an SCR Power control can givesuperior performance with minimal maintenance costs for many Power Controls have been used since the late 1950s. Today, SCR Power con-trols are smaller, safer, versatile and very Reliable . process control & instrumentation1 to 2 volts depending on thedevice across a Power device,generate heat.

6 The more cur-rent (amps) put through thedevice, the higher the wattagethe device will generate in theform of heat, which must beremoved or the device willfail. Almost all semiconduc-tors are destroyed at an inter-nal junction temperature of125 C (260 F).The higher the amperageoutput, the greater the heatthat is generated. The sim-plest, most common way toremove heat is to use a heatsink. Many manufacturersuse fans to remove excessheat from their high-outputSCR Power Controls . Onsome superhigh-output SCRpower Controls (over 1000A), water-cooled heat sinksare used. If a properly sizedheat sink is used, an SCR canoperate at its full rating at anambient temperature of50 C (122 F).

7 One problem with someSCRs and solid-state relaycontrols is packaging. In aneffort to reduce the size of theheat sink, the fin area is notadequate to remove excessheat. DIN rail-mounted heatsinks can save panel spaceand installation time. How-ever, with many controlsmounted close together on aDIN rail, the Power density(W/area) inside the enclosureis increased. At the sametime, airflow to the heat sinksis decreased or blocked. Ifsuch an arrangement is used,it is important that the manu-facturer has not asked thatthe DIN rail heat sink be fancooled or mounted with theheat sink fins outside the cab-inet. It also is important tocheck the de-rating curve forthe device at the Power levelat which it will be at low Power (such as25 A), each controlled leg ofan SSR will generate close to50 W of heat.

8 Therefore, if 20 DIN rail mount SSR powercontrols are used in a smallenclosure, it is necessary toremove 1000 W of determine the properspacing for Power Controls , arule of thumb is to use twicethe footprint of the devicewhen mounting. For exam-ple, if the SCR Power controlhas a 12 in. 12 in. (305 305 mm) footprint, use a 24in. 24 in. (610 610 mm)area for ventilation of theelectrical enclosure is criticalto keep an SCR Power con-trol functioning. Even apower control having an ade-quate heat sink will fail if theheat cannot escape from theenclosure. When venting anenclosure, the basic formulais to use 10 ( cm2) ofinlet and outlet area for every50 A per controlled leg.

9 Ventholes should be at the top andbottom of the and airflowThe heat (as watts generat-ed) generated by each con-trolled leg (C) of an SCR con-troller is equal to timesthe amperage load (I). Fancooling is an efficient way tokeep the enclosure at safeoperating temperatures. Thesize of the fan required can bedetermined by multiplyingthe watts generated by dividing by the tempera-ture rise (10 F) above ambi-ent. The resulting value issimply taken as the fan vol-ume size in ft3/min (cfm).For example: To limit theheat rise in an enclosure to10 F above ambient tempera-ture, with 1000 W of generat-ted heat, you have:Plastic enclosures functionas thermal insulators. AnSCR Power control mostProperly designed SCR Power control built with fan cooled heat sink, MOV voltagespike protection and I2T p r e v e n t premature Power control failure, the SCR Power control containsthermostats to Provide an alarm if ambient temperature is too high, fans havefailed, or air supply is ( )=3160 = 316-cfm fan1010likely will be destroyed ifinstalled inside a plasticenclosure.

10 The only reliableway to use a plastic enclosureis to have a through-holemount heat sink, with theheat-sink fins on the outsideof the protectionand fusingSemiconductors are destroy-ed by a short circuit. One ofthe simplest ways to protectan SCR Power control isproper fusing. While an SCRis a robust, Reliable device,semiconductor, subcycle, andI2T current-limiting fusingmust be used to ensure maxi-mum performance and serv-ice life. Nearly all SCRpower- control manufacturersincorporate these fuses ontheir Controls . I2T fuses willclear within 2 millisecondsand limit the current as theyclear. They are Reliable andeasy to replace. Not using a semiconductorfuse is penny wise and dollarfoolish.