WHAT YOU SHOULD KNOW ABOUT SCR POWER CONTROLLERS
WHAT YOUSHOULD KNOWABOUT SCRPOWERCONTROLLERS
"What You Should Know About SCR Power Controllers" was written as a practical guide to the selection and successfulapplication of SCR controllers. As such, it is not meant as a comprehensive discussion of power controllers and theirfunctions. If you require more in-depth information on this subject, please contact us at:Control Concepts, Inc.Distributed Worldwide by www.mcgoff-bethune.comNorcross, GA USAPhone: 1-770-840-9811Fax: 1-770-840-7514WATS: 800-303-4705 Copyright, Control Concepts, Inc., 1988, 1992, 1993, 1995, 2000, 2002
1.0 INTRODUCTION TO SCR POWER CONTROLSince the development of SCR power controllers in thelate 1950 s, the power handling capabilities of SCR s(silicon controlled rectifiers) have advanced from a fewhundred watts to many megawatts. So, too, the use ofSCR power controllers in industrial applications hasincreased dramatically and they are now used in almostevery major industry.2.0 MAJOR ADVANTAGESSCR power controllers provide a relatively economicalmeans of power control. SCR power controllers costless and are more efficient than saturable core reactorsand variable transformers. Compared to contactors,SCR power controllers offer a much finer degree ofcontrol and do not suffer from the maintenance problemsof mechanical devices. Features and benefits of SCRpower controllers over other forms of control include:160 AMP SINGLE PHASE SCR CONTROLLERHigh reliabilityBecause the SCR power controller is a solid-statedevice, there are no inherent wear-out modes. Thus,they provide virtually limitless and trouble free operation.Infinite resolutionPower, current or voltage can be controlled from zero to100% with infinite resolution. This capability allowsextremely accurate, stepless control of the process.Extremely fast responseThe SCR controller can switch load power on and offextremely fast providing the means to respond rapidly tocommand changes, load changes and power supplychanges. This feature allows the control of fast respondingloads and eliminates the negative effects of variations inload or supply voltages that can occur with other typesof control.Selectable control parametersThe SCR power controller can control the average loadvoltage, the RMS value of the load voltage, the RMS orthe average load current or load power. It can alsoprovide useful features such as current and voltagelimiting. The ability to control the desired parameter asa function of a command signal and to incorporatelimiting features is not normally available with othertypes of control.Minimum maintenanceBecause they are solid state there are no moving partsto wear out or replace. Therefore, the routine replacementrequired in some forms of control is eliminated.425 AMP THREE-PHASE SCR CONTROLLER3.0 GENERAL DESCRIPTIONBasically, an SCR power controller consists of thefollowing: semiconductor power devices (SCR s and Diodes) a control circuit normally referred to as the firingcircuit a means to dissipate the heat generated from thesemiconductor devices and protective circuits (fuses and transientsuppressors).
THE SCRThe heart of the SCR power controller is the SCR (siliconcontrolled rectifier, also sometimes referred to as athyristor).The SCR has two states, ON and OFF, and allowscurrent to flow in only one direction. SCR s can remainin the off state even though the applied potential may beseveral thousand volts; in the on state, they can passseveral thousand amperes. When a small signal isapplied between the gate and cathode terminals (Figure2.1), the SCR will turn on in 10-100 microseconds. Onceturned on it will remain on until the current through it isreduced below a very low value called the holdingcurrent.CURRENT FLOWANODECATHODEGATEFigure 2.1. SCR symbolBecause the SCR allows current to flow in only onedirection, two SCR s are connected in a back to back configuration to control AC current (Figure 2.2).Figure 2.2. AC "Back to Back" SwitchThree types of construction styles are available: thedisc or hockey puck, the module, and the stud mount.Modern SCR controllers generally use either the hockeypuck or the module construction.Hockey puck SCR'sThe hockey puck style is an assembly that hasessentially the same physical shape as a hockey puck.The construction provides excellent cooling of thesemiconductor material and is generally used in highercurrent applications.SCR ModulesSCR modules are assemblies in which the SCR s arecontained in a plastic enclosure with an electricallyisolated mounting plate. SCR modules are becomingincreasingly popular and modules containing a variety ofSCR configurations are available. They are easy andinexpensive to mount to a heatsink, they typically havelarge surge current ratings and they provide electricalisolation, allowing multiple modules to be mounted on acommon heat sink.ELECTRONIC CIRCUITThe electronic circuit controls the operation of theSCR s such that the desired energy applied to the loadis proportional to the command signal.Important tasks of the circuit include:Timing: It is imperative, particularly in applicationsinvolving inductive loads such as transformers, that noDC be applied to the load. For the reasons describedunder Trans-Guard (p.8), DC components can alsocause supply transformers to over heat. This requiresthat the ON time of the back-to-back SCR s be exactlyequal. Modern circuits use sophisticated digital phaselock loop techniques which are immune to the electricalnoise and varying voltages that are often found inindustrial environments.Electrical Isolation: The command signal must beisolated from the supply and load voltage. Plus, excellentisolation is required between the circuits controlling thesignals to the gates of the SCR s to prevent false turnon of the SCR s.HEATSINK REQUIREMENTSSCR s emit about 1.5 watts of energy in the form of heatper ampere conducted. Failure to dissipate this energyis perhaps one of the main sources of SCR failure. Thereliability of SCR s decreases about 50% for every 10degrees centigrade increase of it's semiconductortemperature. Other critical parameters such as the dv/dt (See glossary) rating and the blocking voltage ratingalso decrease rapidly with temperature.The heat generated by the SCR must be dissipated,thus, all controllers have some means to cool theSCR s. Typically an aluminum heatsink with fins toincrease the surface area is used to dissipate thisenergy to air. Controllers with relatively small currentcapacities rely on natural convection; larger currentcapacity controllers use a fan to force air past the finsto increase the rate heat is dissipated. Occasionally,SCR controllers with very large current ratings use watercooled heatsinks.
4.0 BASIC CONTROL MODESThe power delivered to a load may be regulated orproportioned by SCR power controllers using either thephase-angle or the integral cycle (zero-cross voltageswitching) control mode. Each control mode has its ownspecific advantages and disadvantages and eachapplication should be reviewed to determine the mostcompatible mode of control.Phase-angle: In phase-angle control each SCR of theback-to-back pair is turned on for a variable portion of thehalf-cycle that it conducts (Figure 3.1). Power is regulatedby advancing or delaying the point at which the SCR isturned ON within each half cycle. Light dimmers are anexample of phase-angle control.AC SUPPLYSCR "ON" time, shownby shaded area, isvaried to apply thedesired load voltage.LOAD VOLTAGEPower is regulated byadvancing or delayingthe point at which theSCR's are turned on.Figure 3.1. Phase-Angle WaveformsPhase-angle control provides a very fine resolution ofpower and is used to control fast responding loads suchas tungsten-filament lamps or loads in which theresistance changes as a function of temperature. Phaseangle control is required if the load is transformercoupled or inductive.Phase-angle controllers are typically more expensivethan zero-cross controllers because the phase-anglecircuit requires more sophistication than does a zerocross circuit. Phase-angle control of three-phase powerrequires SCR s in all three legs and is appreciably moreexpensive than zero-cross control which only requiresSCR s in two of the three legs.Zero-cross: The term zero-cross or synchronousoperation of SCR s is derived from the fact that theSCR s are turned on only when the instantaneous valueof the sinusoidal waveform is zero. In zero-crossoperation, power is applied for a number of continuoushalf-cycles and then removed for a number of half-cyclesto achieve the desired load power in the same manneras power would be controlled with a mechanical switchingdevice. The difference is that the SCR controllers alwaysswitch power when the instantaneous value of theapplied voltage is zero. Also, the frequency of the on-offcycles can be extremely fast because there is no limitto the number of switching operations the SCR canperform.Zero-cross controllers can provide two rather distinctivelydifferent types of control. Time proportioning control issometimes used when switching large amounts ofcurrent can cause voltage variations which affect ambientlighting or other equipment. The disadvantage is thatpower is applied in longer bursts which can in turn causecontrol problems and shorten heater life. Distributivecontrol is typically somewhat less expensive, providesa much faster cycle rate giving better controllability andlonger heater life. It can also be used with much fasterresponding loads than can time proportioning.Zero-cross time proportioning: Zero-cross timeproportion control is accomplished with a fixed orconstant time base, therefore the total of power ON time and power OFF time is always equal to a fixedvalue. For example, if the time base is ten seconds andthe desired power is 50%, then power is applied for 5seconds and removed for 5 seconds. If the desiredpower were 25% then power is applied for 2.5 secondsand removed for the remaining period of 7.5 seconds.The disadvantage of time proportioning particularly asthe time base is increased is that the load temperaturevaries considerably between the on-off cycles. This canshorten the life of heater elements and decrease theability to obtain precise process control.Distributive zero-cross: Distributive zero-cross doesnot use a fixed or constant time base as is used in timeproportioning. The technique used by Control Conceptsapplies load power for 3 electrical cycles and removesload power for 3 electrical cycles at 50% power. At lowerpower requirements the controller will apply power for 3electrical cycles and is then off for the appropriatenumber of electrical cycles. For example, at 25% powerthe controller is on for 3 electrical cycles and off for 9electrical cycles, or on for 3 electrical cycles out of 12.At higher power levels the controller is off for threeelectrical cycles and on for the appropriate number ofelectrical cycles. For example at 75% power thecontroller is on for 9 electrical cycles and off for 3electrical cycles.
The Control Concepts distributive control techniquecombines power pulses of short duration to obtain theexact power level proportional to the command or setpoint signal. For example, 60% power is achieved bycombining two power pulses. The first power pulseconsists of 4 on cycles and 3 off cycles; the secondpower pulse consists of 5 on cycles and 3 off cycles,providing a total of 9 on cycles during 15 cycles. Ofcourse, such rapid, short bursts of power would beimpossible with mechanical contactors. Zero-crosscontrol is typically less expensive than phase-anglecontrol and generates fewer harmonics. However, zerocross can only be used to control power to resistiveloads that do not change appreciably with temperatureor time and which are directly-coupled (no transformerbetween the SCR controller and the load). Zero-crosscontrol of large power levels can cause supply voltagefluctuations resulting in ambient lighting fluctuations orother problems (See Sync-guard, page 8), and cancause overheating of transformers supplying power tothe controller and load (See Trans-guard, page 8).5.0 SPECIAL CONSIDERATIONS: POWER FACTORAND HARMONICSThe operation of zero-cross or phase-angle SCRcontrollers can lower the power factor, resulting in ahigher electrical cost. Harmonics can be generatedwhich may cause radio frequency interference (RFI) orpossibly affect the operation of other equipment.Power FactorThe power factor of a single-phase circuit is the ratio oftrue power in watts, as measured with a wattmeter, tothe apparent power in volt-amperes, obtained as theproduct of voltage and current.P.F. KWKVA(The power factor of a balanced polyphase circuit is thesame as that of the single phases. When the phases arenot balanced the true power factor is indeterminate.)COMPARISON OF PHASE-ANGLE AND ZERO-CROSSPARAMETERPHASE-ANGLEZERO-CROSSCost: 1-phaseSlightly moreSlightly lessCost: 3-phaseAppreciably higher2-leg control is appreciably less due to lowercircuit cost and because only 2 of the 3supply lines require SCR s. 3-leg control isslightly less because of circuit cost.Type of loads:Transformer coupled loads, fastresponding loads, loads with largeresistance changes, loads requiringcurrent limiting, or soft start.Resistive loads only. Power can not be appliedto a transformer. Moderately fast loads canbe controlled with distributive control.Power factor:(Ref Section 5)Theoretical value equals (% ofapplied power/100)0.5. Power factorobserved on typical utility meter isvery close to theoretical power.Resistive loads only. Power can not be appliedto a transformer. Moderately fast loads canbe controlled with distributive control.Theoretical value equals (% of applied power/100)0.5 Power factor observed on typical utilitymeter approaches unity.RFI and Harmonics:(Ref Section 5)Higher harmonics are generated andthe potential for RFI is higher.Harmonics and RFI are very low.Reliability:Lower than zero-crossHigher than phase-angle because fewercomponents are required and because theSCR turns on when the voltage and currentare zero.Serviceability:More complexEasier because of fewer components.
In figure 6.1, consider the supply voltage E s to be 480volts RMS and the load voltage EL controlled by the SCRcontroller to be 240 volts RMS. The load current IL is, ofcourse, equal to the supply current I S and is equal to E L/RL or 100 amps. The KW of the system is 24. (240 voltsX 100 amps) and the KVA is of the system is 48 (480volts X 100 amps).P.F. 24 E L X I L E L 0.548 ES X IL ES (% applied power/100)0.5The power factor with resistive loads controlled by eitherphase-angle or zero-cross control is the ratio of the loadvoltage to the supply voltage and is therefore proportionalto (% applied power/100) 0.5.Tests have shown that most power factor meters suppliedby utility companies respond correctly to phase-anglecontrol, but provide near unity power factor for zerocross control. Newer meters using solid state technologyand which sample for a longer time period provide powerfactor measurements very close to the theoretical valuesfor both phase-angle and zero-cross control.Harmonics and RFIHarmonic wave forms are generated when electricalpower is switched and are therefore generated whenpower is controlled by mechanical contacts, saturablecore reactors, SCR controllers and all other powerswitching devices.Harmonics are sinusoidal waveforms with frequenciesthat are integral multiples of the fundamental frequency.For example, a waveform that has twice the frequencyof the fundamental frequency is called the secondharmonic. All repetitive waveforms are formed by theaddition of harmonics. In general, abrupt changes in thewaveform or complex waveforms cause the magnitudesof the harmonics to increase, increasing the possibilityof interference problems. In SCR control the magnitudeof the harmonics are the greatest when the load poweris controlled at 50%. Tables 5.1 & 5.2 show the relativemagnitude and frequency for zero-cross and phaseangle control.Although harmonics and RFI are a common concern,very few if any problems have occurred in actualapplication. This is because harmonics are attenuatedby inductance, and because the effects of harmonicsare attenuated by proper shielding and grounding ofelectrical equipment.Table 5.1 ZERO-CROSSTable 5.2 PHASE-ANGLEHarmonicFrequencyMagnitude 0.002.030.001.500.001.180.000.090.00Note: The 1st or fundamental harmonic of zero-crossis 10 Hertz because power is applied for 3 cycles andremoved for 3 cycles at 50% requencyMagnitude 500.002.900.00
6.0 LOAD CONFIGURATIONS AND CONSIDERATIONSLoad configurations and considerations for single-phasecontrol should be obvious from the previous discussion.However, there are several unique load configurationsand some important considerations which need to beexamined in three-phase applications.THREE-PHASE LOAD CONNECTIONSPhase-anglePhase-angle control of three-phase requires a total of 6power switching devices. These devices can beconfigured as hybrid (3 SCR s & 3 diodes), 6 SCR in-lineor 6 SCR inside delta.SINGLE-PHASE LOAD CONNECTIONSThe connections for single-phase are the same forphase-angle (Figure 6.1) and zero-cross (Figure 6.2),the difference between the two being the manner inwhich the SCR s are controlled.Hybrid ControlThree-phase hybrid controllers use three SCR s andthree diodes (Figure 6.3) to control load power. Thesecontrollers are intended primarily for three-wire wye or delta type resistive loads connected directly to thecontroller (i.e., a transformer is not connected betweenthe load and the controller). The three-phase hybridcontroller should not be used in a four wire circuit. Thefourth wire common return would allow uncontrolledconduction through the power diodes resulting inapproximately 50% power output even though the SCR swere off.The advantage of hybrid controllers is that the cost issomewhat less than six SCR control because the circuitis less complex and diodes are less costly. However,hybrid controllers should not be used if the load isunbalanced. An unbalanced load controlled with a hybridcontroller will result in a DC current flow which, for thesame reasons discussed in the section entitled TransGuard, can result in supply transformer saturation.ISILESELFigure 6.1. Phase angle control at 50% powerSCRDiodeILSCRDiodeESELSCRDiodeFigure 6.2. Zero-cross at 50% powerFigure 6.3 Hybrid control
Six SCR in-line controllersThe Six SCR controller, as the name implies, uses sixSCR s (Figure 7.1) to control the load power. Thisconfiguration is an ideal configuration for inductive loadapplications, unbalanced resistive loads and transformercoupled loads. Caution: If the controller is operating atransformer, either the primary or the secondary windingmust be a delta configuration.Figure 7.1. Six SCR in-line controlSix SCR inside delta controlThree phase inside delta control is essentially threesingle-phase controllers operating from the samecommand or set point. Typically a six SCR controllercan be configured to operate inside delta. It is alsopossible, of course, to configure three single phasecontrollers controlled by the same command signal.Because the current in each phase is 57.74% of the linecurrent, smaller and less expensive controllers can beused for inside delta control.Caution: If inside delta control is used in the primary ofa three-phase transformer, a six SCR controller must beused and the secondary must be a delta. (Three singlephase controllers can not be used inside the delta tooperate a three-phase transformer)ZERO-CROSS CONTROLTwo leg control:This approach uses a pair of bac
SCR power controllers provide a relatively economical means of power control. SCR power controllers cost less and are more efficient than saturable core reactors and variable transformers. Compared to contactors, SCR power controllers offer a much finer degree of control and do not suffer from the maintenance problems of mechanical devices .
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