WHAT YOU SHOULD KNOW ABOUT SCR POWER CONTROLLERS

7/28/2019 WHAT YOU SHOULD KNOW ABOUT SCR POWER CONTROLLERS

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WHATYOU

SHOULD KNOW

ABOUT SCR

POWER

CONTROLLERS


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"What You Should Know About SCR Power Controllers" was written as a practical guide to the selection and successfuapplication of SCR controllers. As such, it is not meant as a comprehensive discussion of power controllers and thei

functions. 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 USA

Phone: +1-770-840-9811Fax: +1-770-840-7514WATS: 800-303-4705

Copyright, Control Concepts, Inc., 1988, 1992, 1993, 1995, 2000, 2002


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160 AMP SINGLE PHASE SCR CONTROLLER

425 AMP THREE-PHASE SCR CONTROLLER

1.0 INTRODUCTION TO SCR POWER CONTROL

Since the development of SCR power controllers in thelate 1950s, the power handling capabilities of SCRs(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 ADVANTAGES

SCR 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:

High reliabilityBecause the SCR power controller is a solid-state

device, 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 responding

loads 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.

3.0 GENERAL DESCRIPTIONBasically, an SCR power controller consists of thefollowing:

semiconductor power devices (SCRs 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).


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Figure 2.1. SCR symbol

Figure 2.2. AC "Back to Back" Switch

SCR ModulesSCR modules are assemblies in which the SCRs 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 electrical

isolation, allowing multiple modules to be mounted on acommon heat sink.

ELECTRONIC CIRCUITThe electronic circuit controls the operation of theSCRs 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 SCRs be exact lyequal. 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 SCRs to prevent false turn-on of the SCRs.

HEATSINK REQUIREMENTSSCRs emit about 1.5 watts of energy in the form of heatper ampere conducted. Failure to dissipate this energy

is perhaps one of the main sources of SCR failure. Thereliability of SCRs 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 theSCRs. 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 fins

to increase the rate heat is dissipated. Occasionally,SCR controllers with very large current ratings use watercooled heatsinks.

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. SCRs can remainin the off state even though the applied potential may beseveral thousand volts; in the on state, they can pass

several 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.

Because the SCR allows current to flow in only onedirection, two SCRs are connected in a back to backconfiguration to control AC current (Figure 2.2).

Three 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.

CATHODE

CURRENT FLOW

GATE

ANODE


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Figure 3.1. Phase-Angle Waveforms

SCR "ON" time, shown

by shaded area, is

varied to apply the

desired load voltage.

LOAD VOLTAGE

Zero-cross: The term zero-cross or synchronousoperation of SCRs is derived from the fact that theSCRs 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 switching

device. 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 proportioningcontrol 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. Distributivecontrolis 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 ONtime 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 appropriate

number 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.

Power is regulated by

advancing or delaying

the point at which the

SCR's are turned on.

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.

Phase-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. Phase-angle control is required if the load is transformer-coupled or inductive.Phase-angle controllers are typically more expensivethan zero-cross controllers because the phase-anglecircuit requires more sophistication than does a zero-cross circuit. Phase-angle control of three-phase power

requires SCRs in all three legs and is appreciably moreexpensive than zero-cross control which only requiresSCRs in two of the three legs.

AC SUPPLY


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COMPARISON OF PHASE-ANGLE AND ZERO-CROSS

PARAMETER PHASE-ANGLE

Slightly more

Appreciably higher

Transformer coupled loads, fast

responding loads, loads with largeresistance changes, loads requiringcurrent limiting, or soft start.

Theoretical value equals (% ofapplied power/100)0.5. Power factorobserved on typical utility meter isvery close to theoretical power.

Higher harmonics are generated andthe potential for RFI is higher.

Lower than zero-cross

More complex

Slightly less

2-leg control is appreciably less due to lowercircuit cost and because only 2 of the 3supply lines require SCRs. 3-leg control isslightly less because of circuit cost.

Resistive loads only. Power can not be applied

to a transformer. Moderately fast loads canbe controlled with distributive control.

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.

Harmonics and RFI are very low.

Higher than phase-angle because fewercomponents are required and because theSCR turns on when the voltage and currentare zero.

Easier because of fewer components.

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. Of

course, 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, zero-cross 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 generated

which 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.)

ZERO-CROSS

Cost: 1-phase

Cost: 3-phase

Type of loads:

Power factor:(Ref Section 5)

RFI and Harmonics:(Ref Section 5)

Reliability:

Serviceability:


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In figure 6.1, consider the supply voltage Esto be 480

volts RMS and the load voltage ELcontrolled by the SCR

controller to be 240 volts RMS. The load current ILis, of

course, equal to the supply current ISand is equal to E

L/

RLor 100 amps. The KW of the system is 24. (240 volts

X 100 amps) and the KVA is of the system is 48 (480volts X 100 amps).

P.F. = 24 = EL X IL = EL = 0.548 E

SX I

LE

S

= (% applied power/100)0.5

The 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 zero-cross 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 the

waveform 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 phase-angle 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 5.2 PHASE-ANGLE

Harmonic Frequency Magnitude %

12

34

5

67

89

1011

1213

141516

1718

1920

2122

60120

180240

300

360420

480540

600660

720780

840900960

10201080

11401200

12601320

59.700.00

31.800.00

10.60

0.0010.60

0.006.40

0.006.40

0.004.60

0.004.600.00

3.500.00

3.500.00

2.900.00

Table 5.1 ZERO-CROSS

Magnitude %FrequencyHarmonic

1

23

45

678

910

111213

1415

1617

18

1920

2122

10

2030

4050

607080

90100

110120130

140150

160170

180

190200

210220

Note: The 1st or fundamental harmonic of zero-crossis 10 Hertz because power is applied for 3 cycles and

removed for 3 cycles at 50% power.

10.90

0.0014.10

0.0034.71

50.0029.800.00

8.490.00

4.500.002.88

0.002.03

0.001.50

0.00

1.180.00

0.090.00


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IL

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 be

examined in three-phase applications.

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 in

which the SCRs are controlled.

Figure 6.1. Phase angle control at 50% power

Figure 6.2. Zero-cross at 50% power

Diode

Figure 6.3 Hybrid control

IL

EL

ES

ES

EL

IS

THREE-PHASE LOAD CONNECTIONSPhase-anglePhase-angle control of three-phase requires a total of 6power switching devices. These devices can beconfigured as hybrid (3 SCRs & 3 diodes), 6 SCR in-line

or 6 SCR inside delta.

Hybrid ControlThree-phase hybrid controllers use three SCRs andthree diodes (Figure 6.3) to control load power. Thesecontrollers are intended primarily for three-wire wye ordelta 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 SCRs

were 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 Trans-Guard, can result in supply transformer saturation.

Diode

Diode

SCR

SCR

SCR


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Six SCR in-line controllersThe Six SCR controller, as the name implies, uses sixSCRs (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 7.2. Six SCR inside-delta control

Figure 7.1. Six SCR in-line control

Figure 7.3. 2-leg zero-cross

Figure 7.4. 3-leg zero-cross

Zero-cross inside deltaZero-cross using the inside delta technique as describedpreviously can be used to reduce the current rating ofthe SCRs and in some instances system cost can bereduced.

3-leg controlSome installations require that all three legs of the loadbe controlled and therefore back-to-back SCRs areplaced in all three supply leads. The cost advantage ofzero-cross control is reduced with 3-leg control becauseof the added SCRs and heat sink costs, however, thecircuit cost is less than an equivalent phase-anglecontroller and the advantage of low RFI is retained.

Two leg control:This approach uses a pair of back-to-back SCRs in twoof the supply leads to the load (Figure 7.3). If all of theSCRs are off no power can be applied to the load. Thetwo leg control can be used on either "wye" or "delta"loads, however, it can not be used to control four wire

loads. As previously mentioned, the primary advantagesof the zero-cross control technique are lower cost andextremely low RFI.

ZERO-CROSS CONTROL

Six 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)


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7.0 FEATURES TO IMPROVE PERFORMANCESoft Start & Missing Cycle DetectionThese features are required in phase-angle control toassure that the load power is gradually increased fromzero to the value set by the command signal if loadpower is interrupted. This gradual increase of powerprevents surge currents and avoids the possibility ofsaturating inductive loads like transformers.

Current LimitingThis feature, available only on phase-angle controllers,provides a means to prevent the load current fromexceeding a preset value. Current limiting is used toprotect, the load, the SCR controller, fuses, and thesystem supply from large surge currents that couldoccur at start up due to loads that have a low resistance

when cold.

Feedback/FeedforwardEither feedback or feedforward provides the means toachieve a linear relation between the desired output andthe control or command signal. Feedback implies thatthe desired parameter is measured and fed back to theinput of the control such that the output can be increasedor decreased if corrective action is required. Feedforwardinvolves a less expensive technique in which the outputis simulated in the circuitry and corrective action istaken based upon the simulated circuit response. Bothtechniques provide a linear output with respect to thecontrol or command signals and eliminate the effects ofload and supply variations.

Over Current TripThe over current trip feature prevents the SCRs frombeing turned on if the SCR current has exceeded a

preset value during the last half cycle. Essentially, thisfeature is an electronic fuse which eliminates the needfor expensive and troublesome semiconductor fuses.The controller can be reset by either removing andreapplying power or by momentarily closing a remoteswitch. This feature has proven to be a more reliableprotection technique than fast acting fuses.

Shorted SCR DetectionIf an SCR does fail, it typically becomes shorted allowingload current to flow continuously. In the event an SCRshorts, this feature energizes a relay which can be usedto activate an alarm or remove system power.

ES

EL

DiodeRS = Source Resistance

Load voltage EL

Transformer voltage ES

low

high

Figure 8.1. Transformer saturation due to secondary DC current

The saturation problem is not likely to occur when theload is a small percentage of the transformer capacity,or when the source resistance and inductance aresmall. However, the potential problem always existsunless the controller is designed with the featuredescribed above.

Sync-GuardThis feature, available on some zero-cross controllersproduced by Control Concepts, reduces the possibilityof synchronous operation of two or more controllers. Itreduces the variations in power demand resulting in amore stable supply voltage and improved power factor.The Sync-Guard feature does not alter the power appliedto the load, but adjusts the time when power is applied

in such a manner as to reduce the possibility of thecontrollers being ON and OFF at the same time.

Trans-GuardThe use of zero-cross controllers on the secondary of atransformer can cause saturation of the transformerresulting in excess transformer temperatures and earlyfailure.Transformers can be caused to saturate if a DC voltageis applied to the primary. DC voltage can be induced onthe primary by DC components in the secondary. Thesimple half wave rectifier circuit shown in figure 8.1 willinduce a DC voltage on the primary due to the fact that

the voltage drop across the source resistance during thecycle the diode conducts will lower the primary voltagewhen the diode conducts. The effect is the same as if azero-cross controller were not providing an equal numberof positive and negative half cycles to the load. Althoughnot shown in figure 8.1, the source inductance can alsocause a DC voltage to occur on the transformer primary.The Trans-Guard feature eliminates the problem byalways supplying an odd number of ON half-cycles andan even number of OFF half-cycles. This techniqueguarantees that no DC will occur independent of thesource impedance or the load configuration.


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IP

VPI

L

IL

VL

VL

VL VL

IP

IP

IL

IL

IP

VP

VP

VP

VP

VL

VL

VL

= line-to-line voltage

VP

= phase voltage

IL

= line current

IP

= phase current

Iline

= Iphase

Vline

= Vphase

For a balanced delta-connected

resistive loads only:

For a balanced wye-connected

resistive loads only

Iline

= 1.732 X Iphase

Vline

= Vphase

P = 1.732 X Vline

X Iline

P = 1.732 X Vline

X Iline

P = 3 X Vline

X Iphase

P = 3 X Vphase

X Iline

IL

IL

IP

for VL

= 480 IL

1.2 X P (inKW)

for VL

= 240 IL

2.4 X P (in KW)

for VL

= 208 IL

2.8 X P (in KW)

Figure 9.1. Calculations to determine power and current.

2.0 Determine the control modeDetermine the type of control mode (zero-cross orphase- angle) and features to enhance performance:Transformer coupled loads, fast responding loads,loads that change resistance with age or temperaturetypically require phase-angle control. Figure 10.1 isa guide to aid in determining the type of control modeto be used.Zero-cross control is typically less expensive thanphase-angle control.

3.0 Choose a reliable and serviceable controller.Expect more than just the obvious. The following,often overlooked features and requirements are criticalto the reliability and serviceability of SCR powercontrollers.

8.0 SELECTING THE CORRECT CONTROLLER1.0 Determine the current and load configurationDetermine the current and voltage requirements and determine whether the application is to be three-phaseor single-phase. (Single-phase controllers will typically be less expensive than three-phase controllers.)Figure 9.1 shows the calculations to determine power and current.

DELTA-CONNECTED LOAD WYE CONNECTED LOAD

To determine the line current

(in Amps) for balanced 3-

phase resistive loads (wye

or delta-connected) where P

= total 3-phase power in KW.

THREE-PHASE LOADCALCULATIONS

SPRING LOADED CONNECTIONSThe use of spring washers on all electrical junctionsalways insures a good connection. Heat is generatedin electrical connections because of the resistanceof one conductor or bus bar to another. This heatcauses expansion and deformation of the connectionand in turn can cause the resistance to increase,generating even more heat. The spring applies aconstant force to the junction, allowing for theexpansion. In addition, all connections should have

an electrical compound applied to improve boththermal and electrical conductivity.


12/16

TRANSFORMER COUPLED LOADS

PHASE-ANGLE

PHASE-ANGLE

PHASE-ANGLE

PHASE-ANGLE

ZERO-CROSS

DESIRED FEATURESLOAD TYPE CONTROL

TYPE

Figure 10.1 SCR Controller Selection Guide

MULTIPLE CONTROLLERSSYNC-GUARD

LARGE CURRENTSTRANS-GUARD

CURRENT LIMITINGPOWER FEEDBACK

SOFT-STARTCURRENT LIMITING

HIGH SURGE CURRENT

OVERCURRENT TRIPSOFT START AND MISSING CYCLE

HIGH SURGE CURRENT

SOFT-START AND MISSING CYCLEHIGH SURGE CURRENT

DIGITAL PHASE-LOCK LOOP TIMING

NICKEL CHROMIUMIRON CHROMIUM

LOADS IN WHICH THE RESISTANCECHANGES LESS THAN 10%

SILICON CARBIDE

LOADS IN WHICH THE RESISTANCECHANGES WITH TIME

MOLYBDENUMGRAPHITEQUARTZ DISCILLCIDESTAINLESS 304

LOADS IN WHICH THE RESISTANCECHANGES WITH TEMPERATURE

INFRARED LAMPSLOW MASS HEATERSFAST RESPONDING PROCESS

FAST RESPONDING LOADS


13/16

LONG LIFE FANSFans are often required to cool the semiconductors inSCR power controllers. Fans with ball bearings orextended life sleeve bearings provide superiorperformance and long life over less expensive fans withconventional sleeve bearings.

POWER CONNECTORS

Connectors used to connect the supply leads and theload leads should be designed to remain secure whenfield wiring is installed. (The typical installer of thecontroller will over- torque the connectors; if theconnectors become loose under these conditionsproblems with the controller will develop.) The connectorsshould also be located such that it is convenient to usea wrench and apply the right torque during installation.

Additionally, a design which allows the air from thesemiconductor cooling fans to pass over the largecurrent connectors cools the connection and improvesreliability.

FORCE INDICATING CLAMPSSCRS and diodes of the hockey puck design must besqueezed between the heat sinks by spring loadedclamps to assure electrical and thermal conductivity.To facilitate field replacement (serviceability) of thesemiconductor, the spring clamps should have a forceindicator to determine the proper force.

STATUS INDICATORSStatus indicators consist of small LEDs that provideinformation regarding the magnitude of the commandsignal, the magnitude of the load current, the status offeatures such as over-current trip and shorted SCRdetection. Controllers so equipped can be easily and

safely serviced by inexperienced personnel.

TRANSIENT PROTECTIONRapid voltage changes (dv/dt) and high voltage transientsare common in industrial applications. Under suchconditions, the SCRs can turn on, causing undesiredsurge currents or outputs if proper protection is notprovided. Unprotected SCRs can actually be destroyedby repeated transients. If an SCR is turned on due to atransient, it is possible for very high current densities todestroy a small portion of the SCR. If this is repeatedeven higher current densities occur until failure. Toprevent damage or failure due to high voltage transients,

SCRs should have a high blocking voltage (1400 voltsfor 480 Vac) and must be protected with fast actingMOVs (metal oxide varistors). Further, a series resistorand capacitor circuit should be connected across theSCRs to provide a shunt for high frequency transients(dv/dt).

SINGLE PLUG-IN CIRCUIT BOARDThe use of a single circuit plug-in circuit board greatlyimproves serviceability. For example, circuit cards canbe switched from one controller to another whendiagnosing problems. Also, use of a plug-in boardeliminates the possibility of mis-wiring during service.In addition, if the circuit can be used on all controllersindependent of current rating then only one card is

required for spare parts.

CIRCUIT SYNCHRONIZATION AND TIMINGTo synchronize with line frequency, many controllersuse the zero-cross (the start of the AC voltage cycle) asa reference to determine the conduction or ON time ofthe SCR. However, this may be difficult because ofelectrical noise in the industrial environment. Use of afilter in the zero-cross detection circuit helps to rejectelectrical noise and is acceptable for some applications.Controllers intended for transformer coupled loads requirea very precise and noise immune technique. Here,request a phase-locked digital circuit to determine thezero-cross by a voltage integration technique.Transformer load applications require that all SCRs beturned ON for precisely the same amount of time toavoid applying DC. The phase-locked self adjustingdigital clock technique provides the means to achieveextremely accurate timing and can be used as is on 50or 60 Hertz applications.

THE GATE DRIVESCRs are turned ON by the injection of current into thegate. The input impedance of the gate changes drasticallyduring turn on. Therefore, a gate drive circuit, particularlyon large SCRs, is required to inject the desired currentindependent of the gate impedance.

A technique using optical couplers to init iate a fastrising constant current source has proven to providereliable operation and excellent electrical isolation. AnSCR is turned on approximately 2 million times in eighthours and can be slowly destroyed if the gate drive isinadequate.

COOLINGA 10oC increase in semiconductor temperature willdecrease reliability by 50%. It is difficult for the user todetermine if adequate cooling is provided. Howevercharacteristics of adequate cooling include: a) Theability to operate continuously in a 55oC ambient

environment with no de-rating. b) Extremely smoothsurfaces on which the SCRs are mounted. c) The useof a thermal compound on all electrical connections toaid thermal conduction and to prevent oxide formation.d) Proper containment of the cooling air such that all ofthe air provided by the fan passes over the heat sink.


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SURGE CURRENTCertain loads, such as transformers or loads with a lowcold resistance like infrared lamps, can have a largesurge current. Therefore, the surge current rating of thecontroller must be adequate to protect the SCRs fromdamage in the event the SCRs are turned on bytransients. Any surge current that may occur isdependent upon the load and the source resistance andis difficult to determine. However, a surge current ratingof 12 times rated current has proven to provide verysatisfactory reliability.

PANEL SPACE REQUIREMENTSElectrical enclosures are expensive. Therefore, aphysically smaller controller saves enclosure costs. Inaddition a smaller and relatively light weight controllercan be shipped faster and less expensively.

EXPERIENCE AND KNOWLEDGE OFTHE VENDORThere are a great many factors which affect theapplication of SCR power controllers. An experiencedand knowledgeable vendor is best able to provide theassistance and recommend the best product for theapplication.

9.0 ENCLOSURE SELECTIONAs is true of any solid-state device, the reliabil ity of anSCR power controller is greatly increased if operated

within its temperature range and if kept free ofcontaminates. Therefore, the selection of the enclosureand the means to cool it are very important.

HEAT DISSIPATION CONSIDERATIONSThe enclosure should have proper ventilation or coolingto dissipate the heat generated by the equipment withinit. An SCR power controller dissipates about 1.5 wattsper ampere of current it controls. Therefore, a 100 ampsingle-phase controller dissipates about 150 watts, a 2-leg 100 amp zero-cross controller dissipates about 300

watts and a 100 amp 3-phase phase-angle controllerdissipates about 450 watts. The heat generated within

the enclosure can be dissipated by one of the following:

CONVECTIONThe internal temperature of a NEMA 1 enclosureincreases above the external ambient temperature about4oF per watt dissipated per square foot of exposedsurface area. The temperature rise in degrees Fahrenheitcan be approximated by dividing the total watts dissipatedinside the enclosure by the exposed surface area insquare feet and multiplying this number by 4. Forexample if a NEMA 1 enclosure with 100 square feet ofexposed surface area contained equipment dissipating900 watts, then the watts dissipated per square foot is9 and the internal temperature can be estimated at 36oFabove the outside ambient temperature.

FORCED AIRIf the enclosure can not be cooled by convection then itmay be possible to ventilate the enclosure by forcing air

through it with a fan. The size of the fan can bedetermined by the following equation relating air flow incubic feet per minute (CFM) to the power in watts (W)dissipated within the enclosure and the allowabletemperature rise (DF) above ambient that can be tolerated

within the enclosure.

CFM = 3.16 X WDF

For example, if the power to be dissipated by forced aircooling is 900 watts and the maximum allowabletemperature rise within the enclosure is 15oF then the air

flow required would be:

CFM = 3.16 X 900 = 189.615

COOLING WHEN A SEALED ENCLOSURE ISREQUIREDIf the enclosure must be totally sealed because of watertight requirements, or extremely dusty conditions, etc.,then other means of cooling such as air to air exchangers,

water to air exchangers or air condit ioning unitsspecifically designed for cooling electrical enclosurescan be used. (A kilowatt-hour equals 3413 BTUs)


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10.0 FUSING AND DISCONNECT MEANSThe proper selection of fuses and other protectiontechniques can greatly affect the reliability of SCRcontrollers. Some SCR controllers are provided withfuses to protect the SCRs, some controllers haveelectronic fusing, and low cost controllers are provided

with no SCR fusing. I t is important to note that fusesprovided with the controller are intended to protect the

SCRs and may not be the correct fuse for load protection.The type of fuse depends, of course, on the load and thelocal and national codes. In general a fast acting fusesuch as the JJS or JJN fuses provided by Bussmannhave proven to be a good compromise for reliability, costand protection. The vendor of the controller should beable to recommend the type of fuses required for anyspecific application.SCRs have a small leakage current in the off state.Therefore, it is imperative that a mechanical disconnectbe used to completely remove power prior to servicing.Controllers with over-current trip can be configured tooperate shunt trip or under voltage trip circuit breakersto remove power in the event a load fault occurs.

11.0 MAINTENANCEThe SCR controller should be inspected periodically todetermine that the electrical connections are tight andthat the heat sinks are clean. The abil ity of the heat sinkto dissipate heat is greatly reduced if dirt and grime haveaccumulated on it and it must be cleaned if dirty.Normally the cause of dirty heat sinks is failure toprovide filtered air to air cooled enclosures or failure toclean the filters.The electrical connections on new installations shouldbe periodically inspected and tightened per themanufacturers instructions. Proper maintenance of the

equipment will greatly increase the reliability of thecontroller.

12.0 GLOSSARY OF TERMSANODE - The positive power terminal of an SCR orDiode.

AVE - The average value of a particular parameter suchas voltage or current.

CATHODE - The negative power terminal of a SCR orDiode.

CURRENT LIMITING - A means to limit the maximumamount of current applied to the load.

COMMAND SIGNAL - An input variable applied to anSCR power controller to adjust its output.

CONDUCTANCE - The ability of a material to conductelectricity. Conductance is the inverse of resistance.

CURRENT FEEDBACK - A means to maintain theapplied current to the load. The applied current ismaintained as a linear function of the command signalregardless of line voltage and load changes.

CT - Current Transformer; a device used for sensingcurrent. Its output is an electrically isolated signalproportional to the measured current.

di/dt - The rate of rise of applied current to an SCR asthe unit turns on.

DIODE - A semiconductor which allows current to passin one direction only.

DISTRIBUTIVE CONTROL - A means of controlling theon/off time of the SCRs to obtain a desired power outputto the load.

dv/dt - Refers to the maximum rate of rise of appliedvoltage across an off SCR that will NOT cause a falseturn-on. Usually stated as volts/second.

FORWARD DROP - The voltage drop across asemiconductor when that device is conducting currentin its normal forward direction.

GATE - The signal terminal of an SCR. The terminalused to turn on an SCR.

HEATSINK - Device used to transfer heat away fromeither an SCR or Diode.

HYBRID - Refers to a particular arrangement of SCRsand Diodes in an SCR power controller. Three SCRsand three Diodes.

I2T - Amperes squared times seconds. Refers to thesubcycle current characteristics of either an SCR or afast clearing fuse.

INLINE - Refers to the manner in which the SCRs areconnected in three-phase applications.

INSIDE-DELTA - Refers to the manner in which the

SCRs are connected in three-phase applications.

INRUSH CURRENT - That current which surges into alow impedance load or that which is drawn by atransformer during saturation.

LED - Light Emitting Diode. Often used as statusindicators.


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MOV - Metal Oxide Varistor. A device used to protectSCRs and Diodes from voltage transients.

OVER CURRENT TRIP - A detection and shutdown

circuit which interrupts the operation of the SCRs if theSCR current has exceed a preset value.

PEAK - Refers to maximum instantaneous value of avoltage or current.

PHASE-ANGLE - An SCR firing mode in which theSCRs are turned ON for a portion of each half of an ACcycle.

PHASE LOCK LOOP - An electronic circuit whichautomatically adjusts itself to maintain synchronization

with the line frequency for precise SCR timing.

PIV - Peak Inverse Voltage; refers to the voltage ratingof an SCR or Diode.

PT - Potential Transformer; used to isolate and/orchange a voltage from one level to another.

RC - Resistance/Capacitance network. Connectedacross the SCRs the RC is used to slow down the rateof applied voltage (dv/dt) to the SCRs to prevent theSCRs from being falsely turned ON by a voltagetransient. This is often referred to as a dv/dt circuit.

RESISTANCE - The characteristics of a material whichinhibit the flow of current, measured in ohms. Resistanceis the inverse of conductance.

RESPONSE - The ability to respond to a commandchange measured in time or cycles.

RFI - Radio Frequency Interference; High frequencyinterference generated by phase-angle fired SCRs andother sources.

RMS - Root Mean Squared; refers to the heating valueof voltage and current parameters.

SCR - Silicon Controlled Rectifier; The device used toswitch the power to the load in SCR power controllers.

A three terminal member of the thyristor semiconductorfamily.

SEMICONDUCTOR - Solid-state device for controllingelectrical signals or electrical power.

SOFT START - A gradual application of voltage or powerto the load to prevent current inrush.

SPAN ADJUSTMENT - Typically, a potentiometeradjustment to match the controllers output to thecommand signal. Also sometimes referred to as gain.

Adjusted with the command signal at 100% so thepower controllers output is full ON.

SUPPRESSOR- A device for clamping excessive voltagetransients. See MOV.

THYRISTOR - The name of a semiconductor family,including SCRs, Triacs, and others.

TIME PROPORTIONING - A control mode in which theON and the OFF time of an SCR are adjusted to providethe desired load power.

VOLTAGE CONTROL - A circuit which provides a linearvoltage to the load proportional to the command signaland compensates for line voltage changes.

ZERO ADJUSTMENT - Typically, a potentiometeradjustment to match the command signal to thecontrollers output. Adjusted with the command signal at0% so the power controllers output is OFF.

ZERO-CROSS FIRING - A method of controlling loadpower in which SCRs are turned ON only at thebeginning of an AC cycle. Therefore, switching alwaysoccurs when the applied voltage is zero.

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WHAT YOU SHOULD KNOW ABOUT SCR POWER CONTROLLERS

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