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SCR Power Control The Watlow Educational Series Book Six
SCR Power Control
The Watlow
Educational Series
Book Six
2
SCR Power Control
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3SCR Power Control in the Thermal System . . . . . . . . . . . . . . . . . . . . . . . . . . . .3Silicone Controlled Rectifier (SCR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6SCR - A Modified Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6Triac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
SCR Firing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8Electrical Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8Zero-Cross Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8Contactor Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9Burst Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9Phase Angle Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
SCR Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Power Control and Heater Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Electromechanical Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Mercury Displacement Relay (MDR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14Solid State Relay (SSR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15Burst Firing SCR Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15Phase Angle Firing SCR Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Power Control Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17Stable Resistance Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17Variable Resistance Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Phase Angle Soft Start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18Phase Angle Current Limiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Transformer-Coupled Heating Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20SCR Selection Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
3 Phase SCR Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22Booklet Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23Answers to Exercises and Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . .25
© 1995 Watlow Electric Manufacturing Company. This document is protectedunder U.S. copyright law. Any duplication, other than by Watlow employees,without the express written consent of the publisher is forbidden.
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Watlow Educational Series
Introduction
SCR Power Control in
the Thermal System
The objectives of this booklet are to enable you to:
• Name at least 2 reasons for using SCR power controllers.
• Define what a SCR is and how it is used to allow AC to flow in both directions.
• Define zero-cross switching and explain its benefit
• Name the 2 types of burst firing and explain the effect time base has on them.
• Explain phase angle firing and describe when it should be used.
• Name and explain the 2 features provided with many phase angle fired powercontrollers.
• Explain the effects of using a SCR power controller on heater life.
• Describe which SCR firing methods should be used for which type of heater.
• Identify the proper SCR configuration for 3 phase heaters.
A SCR power controller is an output device used for fast heater switching, toswitch higher amperage electric heaters, to control variable resistance heaters andtransformers coupled to resistance heaters. The many places where SCRs can beapplied gives you many possibilities to successfully provide unique customer solutions!
This book provides a thorough introduction to SCR power control principles. Wefirst discover how a power controller fits into the thermal system. We thenexplore SCRs themselves, examine the firing methods used in SCRs and then lookat how a power control affects heater life. From there, we journey through theapplication of SCRs for switching various heating loads. Finally, we give you therecommended SCR 3 phase configurations for various applications.
The thermal system has a work load, heater, temperature sensor and temperaturecontroller (Figure 1). As you learned in Book 5 (Temperature Control), the temperature controller output signals an output device to switch electric currentON and OFF to the heater. The output device is usually built into the controllercase if possible (and that is what Figure 1 illustrates).
The ON and OFF switching causes the resistance element inside of a heater to gothrough a continuous heating and cooling cycle (Figure 2). We know from Book2 (Thermal Systems and Electric Heaters), that large temperature swings acceler-ate the oxidation of the resistance element in a heater. This is bad, since it reducesheater life.
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SCR Power Control
SCR Power Control in Figure 1A Typical Thermal System
Figure 2Resistance Element Temperature Swings Due to Slow ON/OFF Cycling
How can we reduce or eliminate the temperature swings of the heater's resistanceelement? We use a SCR power controller in the thermal system! A SCR powercontroller is made up of 3 distinct parts: the SCR (silicon controlled rectifier),sophisticated electronics which switch the SCR ON and OFF and a heat sink todissipate the heat a SCR produces. A SCR is illustrated in Figure 3a. Examples ofSCR power controllers are shown in Figures 3b and 3c.
Figure 3
Firing Gate (Ring)
SensorWorkLoad
Heat Transfer Medium
TemperatureControllingDevice
SensorInput
Output
Heat Source
Time
Temperature
a. A 75 amp SCR “Chip” b. Watlow QPAC Power Controller c. Watlow DIN-a-mite Power Control
Temperature cycling acceleratesoxidation by repeatedly crackingthe oxide coating on a resis-tance element. Cracking andbreaking off the protective oxidelayer exposes fresh metal tooxygen attack.
1⁄2"
SCR Power Control in theThermal System (con’t)
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Watlow Educational Series
A SCR power controller is connected into the thermal system in Figure 4 below.
Figure 4The Thermal System with a SCR Power Controller
SensorWorkLoad
Heat Transfer Medium
TemperatureControllingDevice
PowerSupply Process
Output
SCROutput
SCRPower
Controller
Sensor
Input Heat Source
What has changed? The temperature controller output is now connected to theSCR power controller, not the heater. Notice that the current flowing to the heaterpasses through the power controller. The SCR power controller can now controlthe amount of electric current supplied from the power supply to the heater(called load current).
How does the SCR control load current to the heater? First, the SCR power controller receives a process output signal from a temperature controller. Theelectronics use the signal to calculate how often the SCR must switch the heaterON and OFF. The electronics then adjust load current by very rapidly switchingthe SCR ON and OFF. This switching is often so fast, that the resistance elementexperiences very little temperature fluctuation (Figure 5).
Figure 5Reduced Element Temperature Swings Due to Very Fast ON/OFF Switching
Temperature
Time
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SCR Power Control in theThermal System (con’t) Because the temperature swings are greatly reduced or even eliminated, heater
life increases dramatically! That is one of the major benefits of using SCR powercontrollers. Especially for large, expensive heaters, the SCR allows the customerto extend heater life as long as possible.
Of course, the next question (if you haven't asked it already) is: What is a SCRand how does is control electric current to the heater? Excellent question! Weaddress this next
Silicon Controlled
Rectifier (SCR)The silicon controlled rectifier (SCR) is a semiconductor. Another name for a SCRis a thyristor. Semiconductors, you may remember, are materials which are usually electric insulators. However, under certain conditions, they become electrical conductors. The most basic type of semiconductor is the diode. Diodesare occasionally used in SCR power controllers as well as SCRs.
Diode
SCR - A Modified Diode
The diode is a semiconductor device that allows current to flow in only one direction. Electric current flows through a diode ONLY when the anode ispositive and the cathode is negative (Figure 6a). If the anode becomes negative
(and cathode positive), the diode shuts off.
Figure 6
- Cathode+ Anode
Current Flow 1stcycle
2ndcycle
3rdcycle
+0_
V
a. Diagram of a diode b. Alternating Current (AC) through a Diode
Thus, when AC flows through the diode, only half of the wave can get through(Figure 6b). Why? AC alternates its polarity every half cycle. Whenever AC is ina negative alternation, the diode shuts off and doesn’t let any current through!This is a problem, since the heater will only receive 1/2 of the available current.Also, we can’t switch the diode ON and OFF to control current flow to the heater.Is there something better? Yes! We can use a SCR.
The SCR is a modified diode (Figure 7a). The diode now has a “gate.” The gateopens and closes the SCR (switches it ON and OFF). When the gate is “fired”(opened), the SCR conducts current just like a diode. The gate stays open untilthe polarity reverses (anode becomes negative and cathode positive). When thegate closes, current flow stops (just like a diode). Until the gate if once again fired,the SCR cannot conduct current in either direction!
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Watlow Educational Series
SCR - A Modified Diode(con’t) Thus, when a SCR is conducting AC, the gate must be fired at the beginning of each
cycle for current to flow (Figure 7b). If the gate is not fired, no current is conducted by the SCR, even if the anode and cathode are the correct polarity. Soa SCR’s gate allows us to control current flow through the SCR. We did not havethis ability to control current flow with a diode.
Figure 7
a. Diagram of a SCR b. Alternating Current through a SCR
- Cathode+ Anode
Current Flow
Gate
1stcycle
2ndcycle
3rdcycle
+0_
V
Unfortunately, the SCR (like the diode) only allows current to flow in one direction. Since AC switches polarity every half cycle, only half of the availablepower in AC is conducted through the SCR. This is not good. What can we doto get current flow in both directions?
Simple. Two SCR’s are simply placed back-to-back in parallel (Figure 8a). Whenthe current is positive on the right side of the SCRs, SCR #1’s gate is fired and conducts current. When AC changes polarity, SCR #2’s gate fires and conductscurrent in the other direction. The result is that the full AC sine wave is sent tothe heater, allowing us to use the full power available (Figure 8b). This cycle continues then until the gates are no longer fired (to stop current to the heater).
Figure 8
+
+
--
SCR #2
SCR #1 +0_
V
a. Diagram of a Pair of Back-to-Back SCRs
b. Alternating Current throughBack-to-Back SCRs
TriacYou may have heard of a triac before. A triac is simply a modified set of back-to-back SCRs. Instead of having two gates, there is only one (common) gate (Figure9). This saves on space. Triacs are not used for voltages over 240 volts or largecurrents. Why? A triac generates 1/3 more heat than a SCR pair due to the com-mon gate, thus making it less efficient.
SCR Power Control
8
Triac (con’t)Figure 9
Diagram of a Triac
Common Gate
We wanted to mention the triac, as you may hear of it from time to time. In fact,some Watlow temperature controllers still use a triac as an output device toswitch small load currents.
We have learned about SCR’s, but still don’t know much about how SCR’s areswitched ON and OFF. We explore this next.
SCR Firing MethodsBefore we explore SCR firing methods, let’s take a small step back and reviewwhat electrical noise and zero-cross switching are. This will help us better under-stand advantages and disadvantages of the various SCR firing methods.
Electrical noise (also known as radio frequency interference or RFI) is an electri-cal impulse which travels along any electrically conductive medium or is radiat-ed through the air. Sensitive electronic equipment using microprocessors andintegrated circuits may not function properly when they are exposed to electricalnoise. Why? It interferes with their ability to operate properly.
What are sources of electrical noise? Any electrical device which produces rapidor large changes in voltage potential will generate electrical noise. Examples areswitches, relays, inductive loads (like motors, coils, solenoids), phase angle firedSCRs, electrical welding machinery, fluorescent and neon lights, etc.
You can see and hear electrical noise when listening to an AM radio station. Whena light switch is switched or an electric motor is operated near the radio, you hear“static” from the speakers. That static is really electrical noise picked up by theradio and converted into sound. The electrical noise interferes with the radio’sability to process and output a clear AM radio signal. You can just imagine whatthis “noise” does to sensitive electronic equipment!!
Electrical Noise
The higher the voltage switched,the more electrical noise is generated. This explains whythe static you hear on an AMradio is vastly greater from alightning strike than a lightswitch turning on and off!
How do we eliminate electrical noise? Good question. At what point is the SCR’sgate normally fired (opened)? Look at the AC waveforms through the SCR inFigures 6 to 8. What do you see? The gate is always fired (switched ON) at zerovoltage. Why? To eliminate (or at least greatly reduce) electrical noise, we wantto switch the SCR ON and OFF at the lowest possible voltage - zero! See Figure10. Switching the SCR ON when the AC sine wave crosses zero voltage is calledzero-cross switching.
Zero Cross Switching
Zero Cross Switching (con’t)
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Watlow Educational Series
Figure 10
+0_
V
SCR Switch-on-Points
Because it takes a bit of time for the gate to fully open after it has been “fired,” aSCR typically switches at 1 to 1.5 volts. This “forward voltage drop” produces avery small amount of electrical noise and generates some heat. How do we getthe heat away from the SCR? As with SSRs, we use a heat sink to dissipate theheat from the SCR. All SCR power controls usually use heat sinks.
A SCR power control can be used as a contactor. Contactor is just another wordfor a relay. Typically, it is only used as a contactor if it is switching very highamperage loads (usually over 75 to 100 amps). The control output signal requiredby the SCR is some type of ON/OFF signal, like a switched DC signal. Thus, theSCR can function just like a large relay or SSR. Of course the SCR is zero-crossswitched to minimize electrical noise.
Using SCRs as contactors to control heaters is not recommended. You don’t gainmuch. You gain much more by using burst firing SCR power controllers.
Contactor Firing
Burst FiringBurst firing SCR power controllers provide short “bursts” of load current to theheater. These short bursts of current provide just the right amount of powerrequested by the temperature control output. How many and how often these“bursts” of current occur depends on the time base of the power controller.
The time base is basically the same thing as the cycle time in a temperature con-troller. With SCR power controllers, however, the time base is set by the powercontrol itself. It is not set in the temperature controller. The burst firing SCRpower controller can use two types of time bases: fixed and variable.
Fixed Time Base
A fixed time base means that the “cycle time” on the SCR power controller is permanently set or fixed (not adjustable). During this time period (1 second, forexample), the SCR power controller will turn the SCR ON and OFF to provide thecorrect percentage of power to the heater. If this sounds similar to how a SSR isswitched ON and OFF, you are absolutely right. Let’s work through some exam-ples to clarify this.
Time base is basically the“cycle time” programmed intothe SCR power controllerinstead of being set in the tem-perature controller. However,the time base is often not userselectable.
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Burst Firing (con’t)Example: A temperature controller’s process output commands the SCR powercontroller to provide 40% power. The SCR has a fixed time base of 1 second. Thepower supply is 60 hertz (60 cycles per second) AC. How is the SCR burst firedto provide 40% power to the heater during this 1 second time period?
The SCR power controller uses the percent power requirement (40%) to calculatehow many AC cycles should be allowed through the SCR during 1 second. Theresult (shown in Figure 11a) is that the first 24 cycles (40% of 60 cycles) will beallowed through the SCR. The SCR is then turned off for the other 36 cycles (60%of 60 cycles).
Example: If the process output demands a 90% power requirement, how manycycles will the SCR allow through (time base = 1 second)?
The SCR is fired enough to allow 54 cycles (90% of 60 cycles) through (Figure11b). After 54 cycles, the SCR is not fired for the remaining 6 cycles (10% of 60cycles).
Figure 11
24 Cycles ON 36 Cycles OFF
a. Burst Firing with Fixed Time Base - 40% Power Requirement
54 Cycles ON 6 Cycles OF
b. Burst Firing with Fixed Time Base - 90% Power Requirement
See how this works? Notice that we are still switching the heater ON and OFF.We are just switching it ON and OFF (or cycling) within the 1 second time base.As the power requirement changes, the number of AC cycles allowed through theSCR per second also changes. The load current flowing to the heater is not trulycontinuous, but provides a much smoother flow of power to the heater.
Exercise One
A burst firing SCR is used with a 1 second time base. If 20% power is required,how many ON cycles and OFF cycles are there each second. A 60 Hz powersupply is used. Draw a diagram to illustrate this. Answer on page 25.
Variable Time Base
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Watlow Educational Series
A SCR power controller with a variable time base changes the time base according to the power requirement. Burst firing with a variable time base usesthe smallest possible number of AC cycles to deliver the required percentagepower to the heater. Again let’s do 2 examples and compare these to the fixedtime base examples on the previous page.
Example: A temperature controllerís process output commands the SCR powercontroller to provide 40% power. The SCR has a variable time base. The powersupply is 60 hertz (60 cycles per second) AC. How is the SCR burst fired to provide 40% power to the heater during a 1 second time period?
First, the variable time base SCR calculates the smallest possible number of cyclesto produce 40% power. For 40% power, 2 AC cycles out of every 5 cycles (2⁄5 =40%) are allowed through the SCR (Figure 12a). Compare this to a fixed time baseat 40% power - 24 cycles are ON and 36 cycles are OFF.
Example: If the process output demands a 90% power requirement, how manycycles will the SCR allow through using a variable time base?
Now the SCR will be ON 9 out of every 10 cycles (9/10 = 90%). Notice how thetime base changed from 5 cycles in Figure 12a to 10 cycles in Figure 12b! Nowyou know how variable time base got its name? The time base changes to deliver the shortest possible bursts of power to the heater.
Figure 12
2 Cycles ON 3 Cycles OFF
+0_
V
a. Burst Firing with Variable Time Base - 40% Power Requirement
9 Cycles ON
1 Cycle OFF
+0_
V
b. Burst Firing with Variable Time Base - 90% Power Requirement
What is the main advantage ofvariable time base over fixedtime base? Because variabletime base typically switches theheater ON and OFF more quick-ly, heater life is greatlyincreased.
Why is variable time base preferred over a fixed time base? As you can see fromour analysis above, the ON/OFF switching of the heater happens much morequickly with variable time base. We know that the more quickly the heater isswitched, the less temperature variations the resistance element has. The nearlyconstant load current to the heater keeps the heater’s resistance element temper-ature nearly constant. This provides a longer heater life.
SCR Power Control
12
Exercise Two
A burst firing SCR uses a variable time base. If 20% power is required, howmany ON cycles and OFF cycles will provide 20% power to the heater? Drawa diagram to illustrate this. Compare this to Exercise One. Answer on page25.
Phase-Angle Firing
Variable Time Base (con’t)
In this method of SCR firing, the proportioning action takes place every consecu-tive half cycle in the AC sine wave. The time base then, is equal to one half ACcycle! This is fast! The firing point is variable within this half cycle time periodto achieve a very accurate proportional control of electric current through theSCR.
If the SCR gate is fired early in the half cycle, the power output of the heater ishigh. Why? If most of the current gets through the SCR, the heater will producelots of power. If the gate is fired late in the half cycle, only a small increment ofpower passes through the SCR. Then the amount of power (heat energy) produced by the heater will be very little. If we again use the 40% and 90% powerrequirement examples, you can see the affect of phase angle firing on the currentflow through the SCR (Figure 13).
Figure 13
+0-
V
a. Phase Angle Firing - 40% Power Requirement
+0-
V
b. Phase Angle Firing - 90% Power Requirement
The point at which the SCR’s gate is fired in the AC sine wave is continuouslyvariable across the half cycle. Where it is fired depends on the power required bythe heater. Current flow to the heater is practically continuous! Thus, there is notemperature variation experienced by the resistance element in the heater. As aresult, phase angle firing provides us the best resistance element control and thelongest life possible.
Phase-Angle Firing (con’t)
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Watlow Educational Series
As you may have noticed, when the SCR is phase angle fired, it is NOT zero-crossswitched! What problem will we have if it is not zero-cross switched? Right!Phase angle fired SCRs generate a lot of electrical noise! The electrical noiseappears as voltage “spikes” on the AC sine wave (Figure 14). These voltagespikes were left out of Figure 13 for clarity.
Figure 14
+0-
V
The “choppy” sine wave pattern in Figure 14 also produces harmonic frequencieswhich can add to the electrical noise produced. In general, the electrical noiseproduced is not a problem if electronic equipment is properly protected from thiselectrical noise interference.
This is a good place to stop and review what we have discovered so far. Pleasetake a few minutes to answer the following questions. Answers are on page 25.
1. A SCR power controller is like a relay, except that it can switch the heater ONand OFF much more rapidly. This increases heater life. True or false.
2. Name the 3 main parts of a SCR power controller.
3. Diodes and SCRs conduct current in only both directions. True or false.
4. Why are back-to-back SCRs used?
5. Why do we prefer to use zero-cross switched SCRs?
6. Explain the difference between fixed and variable time base burst firing.
7. a.) Describe what phase angle firing is. b) In the left margin use diagrams to compare variable time base burst firing and phase angle firing. Comparethem using a 60% power output on an AC 60 Hz power supply.
SCR Review
Questions
SCR Power Control
14
As we learned in Book 2 (Thermal Systems and Electric Heaters), all resistanceelements eventually burn out. We also know that we can greatly extend heaterlife by reducing the temperature cycling which the resistance element experiences. That is where the true advantage of the SCR power control lies - itgreatly reduces the temperature cycling on the resistance wire.
Well, how much does it reduce this temperature cycling? At this point, let’s sum-marize and compare what we know about the various types of output devices.The point we want to compare is cycle time (or time base) and what effect it hason resistance element temperature..
Power Control and
Heater Life
ElectromechanicalRelay
Mercury DisplacementRelay (MDR)
Normally, this type of relay operates on a cycle time of 10 seconds or longer.When an ON/OFF cycle takes place over 10 to 30 seconds, the resistance elementtemperature experiences very large temperature swings (Figure 15). Large temperature swings accelerate resistance wire oxidation and burn out the heatermuch more quickly.
Because the MDR uses liquid mercury to make contact and conduct electricity,the life of the MDR is much longer. Consequently, we can use a shorter cycletime. The typical cycle time for MDRs is 4 to 5 seconds. Notice in Figure 16 thedramatic reduction in resistance element temperature swings. This is a result ofquicker switching (cycling every 5 versus every 30 seconds).
The advantage of the MDR is not only in the longer life of the heater, but in thelonger life of the mercury relay itself. Even when switching so quickly, the MDRwill outlast an electromechanical relay.
Figure 15Time-Temperature Profile for an
Electromechanical Relay
Figure 16Time-Temperature Profile for a MDR
Electromechanical
Process Set Point:Overshoot:Droop:Internal Temperature
30 Second Cycle Time
1600°F300°F (1900°F)254°F (1346°F)
2100°F
2000°F
1700°F
1600°F
1500°F
1300°FMercury Displacement Relay
Process Set Point:Overshoot:Droop:Internal Temperature
5 SecondCycle Time
1600°F50°F30°F
1830°F
Solid State Relay (SSR)
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Watlow Educational Series
Since a SSR is solid state, it does not fail due to the mechanical wear or arcing ofthe contacts cycling ON and OFF. It can operate on a cycle time of about 1 second. Again, because of the shorter cycle time, we see a further reduction in thetemperature swings of the resistance element (Figure 17). These are the sametemperature swings we expect to see if using a burst firing - fixed time base SCR(1 second time base).
Burst Firing SCR PowerControl A variable time base - burst fired SCR further reduces the time base to a few AC
cycles. The switching time is now so short that the resistance element sees verylittle if any temperature swings (Figure 18). This type of switching makes resistance element heaters have a very long life. There is still temperature cyclingof the resistance element with very high watt density heaters.
Solid State RelayProcess Set Point:Overshoot:Droop:Internal Temperature
1 Second Time Base1600°F
4°F5°F
1730°F
2000°F
1700°F
1600°F
1500°F
1300°FSCR (with burst firing)Process Set Point:Overshoot:Droop:Internal Temperature
Minimum16.6 millisecond time base1600°F
00
1720°F
The time base for phase angle firing is one half cycle of the AC sine wave. Theheater switching is now so fast that there is NO temperature cycling of the resis-tance element (Figure 19)! Even high watt density heaters will not experiencetemperature cycling when phase angle firing is used. The resistance element (andthus the heater) have the longest life with phase angle firing!
Phase Angle FiringSCR Power Control
Figure 19Time-Temperature Profile for Phase Angle Fired SCR
SCR (with phase angle)Process Set Point:Overshoot:Droop:Internal Temperature
8.3 to 10 millisecond time base1600°F
00
1680°F
Figure 17Time-Temperature Profile for a SSR or SCR
with 1 Second Time Base
Figure 18Time-Temperature Profile for a Burst Firing SCR
(Variable Time Base)
16
Burst Firing SCR PowerControl (con’t) An accelerated life test was done to determine which output device provides the
longest life. The results in Figure 20 once again confirm what we knew...quickerON/OFF switching leads to lower resistance element temperature swings. This,in turn, greatly extends heater life!
Figure 20Heater Life Vs. Time Base Relationship
SCRPhaseAngleFiring
Electro-Mechanical
Relay
SCRBurstFiring
SSR
MDR
8.3 ms 33 ms 1 5 3000
100
300
400
500
3000
4000
5000
6000
7000
8000
200
Life (Hours)
Cycle Time (Seconds)
Exercise Three
A customer wants to order a Watlow microprocessor-based PID temperaturecontroller. This controller is required because of the fast response and accu-racy it offers. She is thinking of using electromechanical relays to switch theheater loads. Is this a good choice? Based on what we just discussed andwhat you know about output devices, what control output-output device combination would you recommend? Explain your answer. Answer on page25.
Power Control
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Watlow Educational Series
If SCR power controllers and PID temperature controllers are so great, WHY docustomers still use thermostats and relays in many types of equipment? Thinkabout it a minute.... Good question, hey? Let’s find a good answer!
Most of the time, a customer uses the least expensive control system which willdo the job required. So, if a thermostat controls temperature within +- 25˚F andthe thermal system performs well, then a thermostat will be used. As more accu-rate temperature control is required, the customer must use either ON/OFF orPID controllers. These controllers are more expensive, but must be used to provide accurate temperature control.
The mistake many customers make is to invest in an expensive PID temperaturecontroller and then to use electromechanical relays to do the power switching!That defeats much of the purpose in using a PID temperature controller in thefirst place! A PID control can do things very fast...an electromechanical relay canonly switch very slowly (due to long cycle time). It’s a mismatch.
To make the best use of a PID control’s capabilities, you must use a SSR or SCRpower controller to switch the heater (or cooling system) load.
That is basically how the selection of the temperature and power controls aremade. Let’s journey more specifically into application of SCR power controllers.There are 3 types of heating loads which SCRs switch: Heaters with stable resistance elements, heaters with variable resistance elements and transformer-coupled heaters. We explore these types of heater control next.
These heaters have resistance elements which maintain a fairly constant resis-tance value. For example, nichrome wire elements typically experience about a5%+ increase in resistance at operating temperature. The resistance value mayincrease 10%+ above the cold resistance if the temperature moves up towardnichrome’s operating limit (close to 2000˚F or 1200˚C). However, a 5 to 10%increase is not much. Other “stable” resistance elements increase somewhat morethan 10% as temperature increases. The majority of Watlow heaters use nichromeor other stable resistance elements.
When switching nichrome element heaters, a SSR with a 1/2 to 1 second cycletime setting is recommended. If higher amp switching or faster switching isrequired to get better heater life (or quicker control response), use a variable timebase burst firing SCR power controller. Variable time base burst firing providesexcellent control, very little electrical noise and provides very long heater life.The SCR Selection Chart (Figure 25 on page 20) shows this selection process moreclearly.
Notice that the chart does not recommend phase angle fired SCRs to control stable resistance heating loads! Variable time base burst firing works just fine.
Only a SSR or a SCR powercontroller allow a customer totake full advantage of a propor-tioning (PID) temperature con-troller’s capabilities.
Stable ResistanceHeaters
All Watlow heaters use nichromeor other type of stable resis-tance element. Specialdesigned heaters, however, mayemploy temperature dependentresistance elements.
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There are a few resistance element materials which show large resistance valuechanges with temperature. Examples are tungsten, molybdenum, silicon carbideand graphite element heaters. Tungsten elements are often used in high temper-ature tungsten-quartz tubular heaters. Molybdenum, silicon carbide and graphiteheaters are used for high temperature ovens and applications as well. The graphin Figure 21 shows the temperature dependent resistance values of tungsten andmolybdenum.
Variable ResistanceHeaters
Figure 21Temperature Dependent Resistance Values of Tungsten and Molybdenum
Temperature ˚C
Specific Resistance microohm-cm
0
10
20
30
40
50
60
70
80
90
100
400 800 1200 1600 2000 2400 2800 3200
Tungsten
Molybdenum
Why is this large resistance value change a problem? Let’s use Ohm’s Law tohelp us out. Ohm’s Law states that V = IR. The supply voltage to a heater is con-stant. Therefore, the only terms that can change are current (I) and resistance (R).If, for example, V = 230V and R = 23Ω at operating temperature, what is the cur-rent through the tungsten element? It is 10 amps (230V/23Ω). What happenswhen the heater is shut off? Per Figure 21, the resistance value of the tungstendrops to about 1/15th its value at operating temperature! So the cold resistanceis about 1.53Ω.
What happens when the heater is switched on again? Since the resistance ele-ment is cold, the current will be 15 times as large as at operating temperature (150amps)! Why? Since the resistance dropped to 1/15th its “hot” value, the currentmust increase by 15 times to make the equation 230V = 150 amps x 1.53Ω valid.This large in-rush current will blow out the SCR and may damage the heater aswell.
How do we eliminate this problem? For these kinds of heaters, a phase anglefiring SCR power controller is required. To prevent the SCR from burning up onheater start up, we use the soft start feature on a phase angle firing SCR. Figure25 on page 20 shows you the selection process just described above.
The high in-rush current on startup when using tungsten ormolybdenum-based resistanceelements will destroy a SCRalmost instantly (unless using I2tfusing and soft start).
Variable Resistance Heaters(con’t)
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Watlow Educational Series
Phase Angle Soft Start
Soft start only operates during heater start up. When the signal is given to startthe heater, the temperature controller tells the SCR to switch full ON. However,we know that if this happens, the SCR will be destroyed. So in this start up con-dition, the soft start feature allows only a small amount of current through to theheater. As time goes on, the current is gradually increased until full power isfinally applied to the heater (Figure 22). Soft start usually lasts for about 5 to 10seconds, depending on the SCR power controller used.
Figure 22Soft Start Illustrated
+0_
VTime
Soft start essentially allows the resistance element to preheat or warm up. Thisincreases element resistance before the full load voltage is applied through theSCR to the heater. This ensures that the SCR is not harmed and that the heaterwill have a very long life.
Phase Angle Current Limiting
Soft start gradually increases the current flow and thus the power supplied to theheater during initial heater start up. It is still possible, however, that after the softstart (or at some other time during operation) too much current may pass throughthe SCR. This, as we know, will destroy the SCR.
To prevent this from happening, a current sensing transformer is built into theSCR power controller. The transformer senses the current flow through the SCR.If the current is higher than the maximum, the power controller electronics reducethe current to the maximum limit value (Figure 23).
When the load current through the SCR drops back below the maximum currentlimit, then the full power of the AC sine wave is allowed through.
Figure 23Current Limiting in Action
+0_
Soft Start Current Limiting
V
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Variable Resistance Heaters(con’t) Current limiting cannot help, though, when a short circuit occurs. When a short
circuit occurs there is almost no resistance in the circuit. The rise in current is soswift that it destroys the SCR. The only way to prevent a short circuit fromdestroying the SCR is by using an I2t fuse (pronounced eye-squared-tee). Thisfuse will blow within 1⁄2 cycle of the short circuit condition. ALWAYS use I2t fuseswhenever SCR power controls are used! It is a small price to pay to protect thecustomers investment!
Transformer-CoupledHeating Loads
SCR Selection Chart
Heaters used in very high temperature applications tend to have low voltage ratings (say, 12 to 60V). Low voltage ratings are required, because the heaters typically have very low resistance values. Using a lower voltage provides theproper amount of current required to power the heaters.
A transformer is used to step down the high voltage power supply to the lowvoltage required by the heater. A phase angle fired SCR is again required to con-trol current flow through the transformer (and thus the heater). The transformerand phase angle fired SCR are usually connected as shown in Figure 24. Noticehow the SCR power controller is connected on the HIGH VOLTAGE side of thecircuit. It is much less expensive to control high voltage and low current than theother way around!
Figure 24Typical Connection Scheme for Transformer-Coupled Heaters
L1
L2
LowVoltageSide
Heater
SCR Power Controller
Transformer
HighVoltageSupply
A phase angle SCR is required, because it has a built-in circuit which ensures thatthe transformer always receives alternating plus and minus AC voltage pulses. If,for some reason, the transformer received two pulses of the same polarity, it couldoverheat the transformer and blow the SCR. The SCR Selection Chart in Figure25 again shows this for transformer-coupled heating loads.
To simplify our selection of the proper SCR power controller, we can use the flowchart shown in Figure 25. First, we determine the type and resistance character-istic of the load. Then, we find the recommended type of SCR firing method.Finally, we find the type of temperature controller output required to signal theSCR power controller.
Example: You have quoted a Watlow radiant panel heater to a customer. Whattype of SCR power control can you recommend?
Low voltage Watlow ceramicfiber heaters are often trans-former-coupled.
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Watlow Educational Series
SCR Selection Chart(con’t) From Figure 25, we see that Watlow heaters are stable resistance loads. Thus, we
can choose a solid state contactor (SSR) or a burst firing SCR power controller.The temperature controller output required for a SSR is a time proportioning out-put (like switched DC). The temperature controller output required for a burstfiring SCR power controller is a process output (like 4-20mA).
Figure 25SCR Selection Chart
Characteristicsof the Load
TypicalLoad
SCRFiringMethodRequired
TemperatureControlOutputRequired
Stable *1Resistance
Resistance *2Change
Inductance *3
NichromeCartridgeCirculationStripTubularMica StripQuartzRadiant
TungstenQuartzSilicon CarbideGlo BarsMolybdenumGraphite
Transformer
Solid StateContactor
BurstFiring
PhaseAngle
PhaseAngle
TimeProportioning
Process(Analog)
Process(Analog)
Process(Analog)
Exercise Four
A customer wants to use a Watlow tubular immersion heater. The heater israted for 480VAC, 3 phase, 50kW. He wants to use a thermostat connected upto an electromechanical relay to control thermal system temperature.
The customer also requires accurate temperature control and fastresponse to temperature changes. Based on the heater and customerrequirements, recommend the best temperature controller (thermostat,ON/OFF or PID), control output and switching device to meet customerrequirements. Use Figure 25 if necessary. Answers on page 25.
Notes:*1. Nichrome heater elements
change resistance less thantwo times in their operat-ing temperature range.
*2. Heaters that change resis-tance include:
• Tungsten changes over 16times from cold to hot
• Silicon carbide changeswith temperature and age
• Molybdenum and graphitechange resistance with thetemperature and are oftenused on the secondary of a transformer
*3. Transformers can becomeDC saturated if two pulsesof the same polarity areapplied in sequence whichcan cause overheating andhigh currents that willdamage the SCR. Burst fir-ing should not be applied.
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It is actually quite simple to apply SCR power controllers to single and threephase powered heaters. Single phase heaters are connected as shown in Figure26. Notice that only one set of back-to-back SCRs is used to control current to theheater. Of course, proper fusing must be used!
Figure 26Single Phase SCR Power Control
L1
L2
There are many types of 3 phase connections for SCR power controllers. Each isdependent on the type of SCR firing required. The first is used for burst fired, 3 phase control. Figure 27 shows that only 2 back-to-back SCRs are required tocontrol a burst fired, 3ph heater. This is the least expensive way to control burstfired, 3 phase heaters. Remember that burst firing means that the SCRs are zero-cross switched.
Figure 27Three Phase-Two Leg SCR Power Control (Burst Firing Only)
L1
L2
L3
When phase angle fired power control is required, the least expensive way to gois to use a hybrid 3 pair SCR/Diode combination (Figure 28). This is less expensive than 3 pairs of back-to-back SCRs and generates less heat. This hybriddesign can be used for burst fired control of 3ph heaters, but is normally not recommended.
Figure 28Three Phase-Three Leg Hybrid SCR Power Control (Phase Angle Firing Only)
L1
L2
L3
Contact a Watlow sales agent orWatlow Controls factory for moredetailed information on switch-ing heaters with SCR powercontrollers.
3 Phase SCR Power Control
3 Phase SCR Power Control(con’t)
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Watlow Educational Series
Another alternative for phase angle fired 3ph heaters, is to use a set of 3 back-to-back SCRs (Figure 29). This control scheme must be used when there is an unbalanced load (resistances of various legs are different). It also must be usedfor delta-to-delta 3ph transformer control.
Figure 29Three Phase-Three Leg SCR Power Control (Phase Angle Firing Only)
L1
L2
L3
Finally, when a 3ph, 4 wire (grounded wye) heater must be controlled, 3 back-to-back SCRs are again used with an uncontrolled neutral (Figure 30). This is forburst fired control applications.
Figure 30Three Phase-Four Wire SCR Power Control (Burst Firing Only)
L1
L2
L3
N
We’ve come a long way on our journey through SCR power control! Now, let’sreview by putting into practice what you have so diligently studied. Answer thefollowing questions. If you can’t answer a question, go back into the book andreview that section. Answers to all questions are on page 26.
1. Why do we use a SCR power controller in a thermal system?
a) To increase work load life.b) To increase electromechanical relay life.c) To increase heater life.d) To increase the ability of a PID temperature controller to respond to
temperature changes in the work load and increase heater life.e) Both a and c are correct.
2. Briefly explain why SCR's are placed back-to-back in parallel.
Booklet Review
Questions
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Booklet Review Questions(con’t) 3. What does "time base" mean?
a. It is similar to cycle time on a PID temperature controller.b. The number of cycles per second (hertz).c. The time required for the work load to reach set point.d. The time during which a SCR power controller cycles the heater ON and
OFF.e. Only a and d are correct. f. Only a and c are correct.
4. A PID temperature controller has a 0-5VDC process output. The process signal is fed to a burst firing SCR power controller (with a variable time base).The power supply has 230VAC, 60Hz. The process output signal is 3.5 volts.What is the current flow through the SCR to the heater?
a. 42 cycles ON, 18 cycles OFF.b. 7 cycles ON, 10 cycles OFF.c. 7 cycles ON, 3 cycles OFF.d. 3 cycles ON, 1 cycle OFF.
5. Which of the answers in question #4 above is correct if the SCR power controller has a fixed time base of 1 second?
6. A quartz tubular heater uses tungsten resistance elements. What method ofSCR firing and options should be chosen for this heater?
a. Burst firing, fixed time base, current limiting.b. Burst firing, variable time base, current limiting.c. Phase angle firing, soft start, current limiting.d. Both a and b are correct.
7. A set of Watlow Multicell heaters is used to heat gases to very high tempera-tures. The heaters are rated 480VAC, 3ph, 20,000W. Which temperature control method, control output and output device would you choose to control these heaters. Please explain your answer.
8. Molybdenum disilicide heating elements are used in a high temperature furnace application. Which temperature control method, control output andoutput device would you choose to control these heaters. Please explain youranswer.
9. A low voltage ceramic fiber heater is operated on 60VAC. A transformer isused to step down the voltage. What method of SCR power control do yourecommend for this application? Please explain your answer.
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Watlow Educational Series
Answers to Exercises
and Review QuestionsAnswers to Exercises
Exercise 1: First we multiply 20% (0.2) by 60Hz to calculate the number of ONcycles per second. 60 x 0.2 = 12 cycles. Therefore, in one second, at 20% power,there are 12 ON cycles followed by 48 OFF cycles. See Figure 31a below.
Exercise 2: With variable time base, we have to find the smallest combination ofON/OFF cycles which will provide 20% power. From Exercise One, we knowthat 20% power is 12 cycles ON, 48 cycles OFF. We then reduce this proportion toits smallest possible value. We can divide both numbers by 12. The result is 1 cycle ON/4 cycles OFF. This will provide 20% power to the heater. See Figure31b below.
Figure 31
48 Cycles OFF12 Cycles ON
+0_V
a. Fixed Time Base of 1 Second - Burst Firing Under 20% Power
4 CyclesOFF
4 CyclesOFF
1 CycleON
1 CycleON
+0_V
b. Variable Time Base - Burst Firing Under 20% Power
Exercise 3: In any case, electromechanical relays are not a good choice. This isespecially true since fast response and accuracy are required. It is much better touse at least a SSR, or perhaps a SCR power controller.
Assuming an external SSR is used, either a switched DC control output or inter-nal SSR must be used to switch the external SSR. Use a fast cycle time (about 1second).If a SCR power controller is used, use a process control output and a variable timebase burst firing SCR.
Exercise 4: DO NOT USE THE THERMOSTAT! Use a PID temperature controllerwith a process output (like 4-20 mA) and a variable time base burst firing SCRpower controller
Answers to SCR Review Questions - Page 13
1. True. A SCR power controller switches power ON and OFF to the heater inmuch the same way as a relay. The faster switching increases heater life.
2. The SCR, control electronics and heat sink.
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Answers to Exercises andReview Questions (con’t) 3. False. They conduct current in only one direction.
4. This allows current to flow in both directions. One SCR conducts current inone direction, the other SCR conducts current in the other direction.
5. This greatly reduces electrical noise, because SCR switching takes place at ornear zero volts.
6. In fixed time base, ON and OFF switching of the SCR must always take placeduring a fixed time period (like 1 second). Variable time base continuallyvaries the time period (or number of cycles) to provide the quickest switch-ing possible.
7. a) Phase angle firing fires the SCR each consecutive half cycle. The powerflow through the SCR is varied by firing the SCR either earlier or later in thathalf cycle.
b) The smallest possible number of cycles required to get a 60% power flowis 3 cycles ON out of 5 cycles (3/5 = 60%). Thus the total time base is 5 cycles.Figure 32 below illustrates this.
Figure 32
3CyclesON
5 CyclesTime Base
2CyclesOFF
+0_V
Answers to Booklet Review Questions - Page 23
1. d) To increase the ability of a PID temperature controller to respond to temperature changes in the work load and increase heater life.
2. They are placed back-to-back in parallel so that AC current can flow bothways through the SCR pair. One SCR is used for current in one direction, oneSCR is used for current in the other direction.
3. e) Only a and d are correct.
4. c) 7 cycles ON, 3 cycles OFF
5. a) 42 cycles ON, 18 cycles OFF.
6. c) Phase angle firing, soft start, current limiting
7. Use a PID temperature controller, a process output (like 4-20mA or 0-5VDC), and a burst firing, variable time base SCR power controller. Use the 3 phase, 2 back-to-back SCR controller shown in Figure 27. Thistemperature and power control system will provide the best life and temperature control.
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Watlow Educational Series
Answers to Exercises andReview Questions (con’t) 8. Molybdenum is (per Figures 21 and 25), one of the resistance elements that
has a high change in resistance due to temperature. Therefore, a PID tem-perature control, process control output and phase angle fired SCR powercontroller are required. The phase angle control should have soft start andcurrent limiting.
9. Transformers (per Figure 25) require phase angle fired SCR power controllers. Soft start and current limiting are required features.
COR-WE6-16
Designer and Manufacturer of IndustrialHeaters, Sensors and Controls
Watlow St. Louis • 12001 Lackland Road • St. Louis, MO 63146 USA • Phone: 314-878-4600 • FAX: 314-434-1020
For information on other training books and materials available fromWatlow, please call 314-878-4600 or fax 314-434-1020
