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SCR Power Control
The Watlow
Educational Series
Book Six
SCR Power Control
Contents
Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
SCR Power Control in the Thermal System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Silicone Controlled Rectifier (SCR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
SCR - A Modified Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Triac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
SCR Firing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Electrical Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Zero-Cross Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Contactor Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Burst Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Phase Angle Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
SCR Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Power Control and Heater Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Electromechanical Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Mercury Displacement Relay (MDR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Solid State Relay (SSR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Burst Firing SCR Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Phase Angle Firing SCR Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Power Control Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Stable Resistance Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Variable Resistance Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Phase Angle Soft Start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Phase Angle Current Limiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Transformer-Coupled Heating Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
SCR Selection Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
3 Phase SCR Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Booklet Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
Answers to Exercises and Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . .25
© 1995 Watlow Electric Manufacturing Company. This document is protected
under 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
Objectives
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 power
controllers.
• 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.
Introduction
A SCR power controller is an output device used for fast heater switching, to
switch higher amperage electric heaters, to control variable resistance heaters and
transformers coupled to resistance heaters. The many places where SCRs can be
applied gives you many possibilities to successfully provide unique
customer solutions!
This book provides a thorough introduction to SCR power control principles. We
first discover how a power controller fits into the thermal system. We then
explore SCRs themselves, examine the firing methods used in SCRs and then look
at how a power control affects heater life. From there, we journey through the
application of SCRs for switching various heating loads. Finally, we give you the
recommended SCR 3 phase configurations for various applications.
SCR Power Control in
the Thermal System
The thermal system has a work load, heater, temperature sensor and temperature
controller (Figure 1). As you learned in Book 5 (Temperature Control), the
temperature controller output signals an output device to switch electric current
ON and OFF to the heater. The output device is usually built into the controller
case if possible (and that is what Figure 1 illustrates).
The ON and OFF switching causes the resistance element inside of a heater to go
through a continuous heating and cooling cycle (Figure 2). We know from Book
2 (Thermal Systems and Electric Heaters), that large temperature swings accelerate the oxidation of the resistance element in a heater. This is bad, since it reduces
heater life.
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SCR Power Control
SCR Power Control in
the Thermal System
Figure 1
A Typical Thermal System
Temperature cycling accelerates
oxidation by repeatedly cracking
the oxide coating on a resistance element. Cracking and
breaking off the protective oxide
layer exposes fresh metal to
oxygen attack.
Sensor
Temperature
Controlling
Device
Work
Load
Heat Transfer Medium
Sensor
Input
Heat
Source
Output
Temperature
Figure 2
Resistance Element Temperature Swings Due to Slow ON/OFF Cycling
Time
Firing Gate
How can we reduce or eliminate the temperature swings of the heater's resistance
element? We use a SCR power controller in the thermal system! A SCR power
controller 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 to
dissipate the heat a SCR produces. A SCR is illustrated in Figure 3a. Examples of
SCR power controllers are shown in Figures 3b and 3c.
Figure 3
Firing Gate (Ring)
Long Cycle Time
Short Cycle Time
Process Input
Phase Angle
or Burst Fired
1
⁄2"
a. A 75 amp SCR “Chip”
4
b. Watlow Power Series Controller
Long Cycle Time
c. Watlow DIN-a-mite Power Control
Short Cycle Time
Process Input
Phase Angle
or Burst Fired
Watlow Educational Series
A SCR power controller is connected into the thermal system in Figure 4 below.
Figure 4
The Thermal System with a SCR Power Controller
Sensor
Power
Supply
Process
Output
Temperature
Controlling
Device
SCR
Power
Controller
Sensor
Input
Work
Load
Heat Transfer Medium
Heat
Source
SCR
Output
What has changed? The temperature controller output is now connected to the
SCR power controller, not the heater. Notice that the current flowing to the heater
passes through the power controller. The SCR power controller can now control
the 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. The
electronics use the signal to calculate how often the SCR must switch the heater
ON and OFF. The electronics then adjust load current by very rapidly switching
the SCR ON and OFF. This switching is often so fast, that the resistance element
experiences very little temperature fluctuation (Figure 5).
Figure 5
Reduced Element Temperature Swings Due to Very Fast ON/OFF Switching
Temperature
SCR Power Control in the
Thermal System (con’t)
Time
Process Input
Phase Angle
or Burst Fired
5
SCR Power Control
SCR Power Control in the
Thermal 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 power
controllers. Especially for large, expensive heaters, the SCR allows the customer
to extend heater life as long as possible.
Of course, the next question (if you haven't asked it already) is: What is a SCR
and how does is control electric current to the heater? Excellent question! We
address this next
Silicon Controlled
Rectifier (SCR)
The silicon controlled rectifier (SCR) is a semiconductor. Another name for a SCR
is 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. Diodes
are occasionally used in SCR power controllers as well as SCRs.
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 is
positive and the cathode is negative (Figure 6a). If the anode becomes negative
(and cathode positive), the diode shuts off.
Figure 6
+ Anode
- Cathode
Current Flow
a. Diagram of a diode
V
+
0
_
1st
cycle
2nd
cycle
3rd
cycle
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 in
a 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.
SCR - A Modified Diode
The SCR is a modified diode (Figure 7a). The diode now has a “gate.” To turn
v
the SCR ON, the anode must be positive with respect to the cathode, load current
+
must be available (complete circuit) and the gate must be made positive with
0
respect to the anode. The SCR will then stay energized (in conduction) until the
anode voltage is removed or the anode/cathode polarity
1st reverses.
2nd
3rd
cycle
cycle
cycle
Removing the gate signal while the SCR is conducting, will not de-energize the
SCR.
6
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. So
a SCR’s gate allows us to control current flow through the SCR. We did not have
this ability to control current flow with a diode.
Figure 7
Gate
+ Anode
- Cathode
V
Current Flow
a. Diagram of a SCR
+
0
_
1st
cycle
2nd
cycle
3rd
cycle
b. Alternating Current through a SCR
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 available
current in AC is conducted through the SCR. What can we do to get current flow
in both directions?
Simple. Two SCR’s are simply placed back-to-back in parallel (Figure 8a). When
the 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 SCR #2
conducts current in the other direction. The result is that the full AC sine wave is
sent to the heater, allowing us to use the full power available (Figure 8b). This
v
cycle continues then until the gates are no longer fired.
Figure 8
SCR #1
+
-
-
+
V
SCR #2
a. Diagram of a Pair of
Back-to-Back SCRs
+
0
-
1st
cycle
2nd
cycle
3rd
cycle
+
0
_
b. Alternating Current through
Back-to-Back SCRs
Triac
You may have heard of a triac before. A triac is simply a modified set of back-toback SCRs. Instead of having two gates, there is only one (common) gate (Figure
9). This saves on space. Triacs are typically not used for voltages over 240 volts
v
or large currents. Why? A triac generates more heat than a SCR pair due to the
+
higher watt density of the triac, thus making it less efficient. The triac conducts
0
SCRs alternate the load on two separate
current full time where the back-to-back
silicon chips. Triacs also typically have lower blocking voltage ratings than SCRs.
7
SCR Power Control
Triac (con’t)
Figure 9
Diagram of a Triac
Common Gate
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 to
switch small load currents.
We have learned about SCR’s, but still don’t know much about how SCR’s are
switched ON and OFF. We explore this next.
SCR Firing Methods
Before we explore SCR firing methods, let’s take a small step back and review
what electrical noise and zero-cross switching are. This will help us better understand advantages and disadvantages of the various SCR firing methods.
Electrical Noise
Electrical noise (also known as radio frequency interference or RFI) is an electrical impulse which travels along any electrically conductive medium or is radiated through the air. Sensitive electronic equipment using microprocessors and
integrated circuits may not function properly when they are exposed to electrical
noise. Why? It interferes with their ability to operate properly.
The higher the voltage switched,
the more electrical noise is
generated. This explains why
the static you hear on an AM
radio is vastly greater from a
lightning strike than a light
switch turning on and off!
Zero Cross Switching
8
What are sources of electrical noise? Any electrical device which produces rapid
or large changes in voltage potential will generate electrical noise. Examples are
switches, relays, inductive loads (like motors, coils, solenoids), phase angle fired
SCRs, electrical welding machinery, fluorescent and neon lights, etc.
You can see and hear electrical noise when listening to an AM radio station. When
a 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 the
radio and converted into sound. The electrical noise interferes with the radio’s
ability to process and output a clear AM radio signal. You can just imagine what
this “noise” does to sensitive electronic equipment.
How do we eliminate electrical noise? Good question. At what point is the SCR’s
gate normally fired? Look at the AC waveforms through the SCR in Figures 6 to
8. What do you see? The gate is always fired (switched ON) at zero voltage.
Why? To eliminate (or at least greatly reduce) electrical noise, we want to switch
the SCR ON and OFF at the lowest possible voltage. See Figure 10. Switching
the SCR ON when the AC sine wave crosses zero voltage is called zero-cross
switching.
Watlow Educational Series
Zero Cross Switching (con’t)
Figure 10
V
+
0
_
SCR SWITCH-ON POINTS
SCR Switch-on-Points
All SCRs exhibit a forward voltage drop while in conduction of about 1 to 1.25
volts. This “forward voltage drop” produces a very small amount of electrical
noise and generates some heat. How do we get the heat away from the SCR? We
use a heat sink to dissipate the heat from the SCR. All SCR power controls use
heat sinks if they are switching a significant load current.
Contactor Firing
A SCR power control can be used as a contactor. Contactor is just another word
for a relay. Typically, it is only used as a contactor if it is switching very high
amperage loads (usually over 75 to 100 amps). The control output signal required
by the SCR is some type of ON/OFF signal, like a switched DC signal. Thus, the
SCR can function just like a large relay or SSR. Of course the SCR is zero-cross
switched to minimize electrical noise.
Using SCRs as contactors to control heaters is not recommended at longer time
bases. However, when using a temperature controller with a switched VDC output on a fast time base (4 seconds or less) can make for a simple and effective temperature and power controller combination.
Burst Firing
Burst firing SCR power controllers provide short “bursts” of load current to the
heater. These short bursts of current provide just the right amount of power
requested by the temperature control output. How many and how often these
“bursts” of current occur depends on the time base of the power controller.
Time base is basically the
“cycle time” programmed into
the SCR power controller
instead of being set in the temperature controller. However,
the time base is often not user
selectable.
The time base is basically the same thing as the cycle time within a PID temperature controller. With SCR power controllers, however, the time base can be set
by the power control itself. The burst firing SCR power 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, for
example), the SCR power controller will turn the SCR ON and OFF to provide the
correct percentage of power to the heater. Let’s work through some examples to
clarify this.
9
SCR Power Control
Burst Firing (con’t)
Example: A temperature controller’s process output (4-20mA signal) commands
the SCR power controller to provide 40% power. The SCR has a fixed time base
of 1 second. The power supply is 60 hertz (60 cycles per second) AC. How is the
SCR burst fired to provide 40% power to the heater during this 1 second time
period?
The SCR power controller uses the percent power requirement (40%) to calculate
how many AC cycles should be allowed through the SCR during 1 second. The
result (shown in Figure 11a) is that the first 24 cycles (40% of 60 cycles) will be
allowed 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 many
cycles will the SCR allow through (time base = 1 second)?
The SCR is fired enough to allow 54 cycles (90% of 60 cycles) through (Figure
11b). After 54 cycles, the SCR is not fired for the remaining 6 cycles (10% of 60
cycles).
Figure 11
24 Cycles ON
36 Cycles OFF
a. Burst Firing with Fixed Time Base - 40% Power Requirement
54 Cycles ON
6 Cycles OFF
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 the
SCR per second also changes. The load current flowing through the heater is only
continuous for a full ON condition. Therefore, it is difficult to measure load current with a meter when controlling at a short time base.
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 power
supply is used. Draw a diagram to illustrate this. Answer on page 25.
10
Watlow Educational Series
Variable Time Base
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 uses
the smallest possible number of AC cycles to deliver the required percentage
power to the heater. Again let’s do 2 examples and compare these to the fixed
time base examples on the previous page.
Example: A temperature controller’s process output commands the SCR power
controller to provide 40% power. The SCR has a built in variable time base. The
power supply is 60 hertz (60 cycles per second) AC. How is the SCR burst fired
variable time base 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 cycles
to 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 base
of one second at 40% power - 24 cycles are ON and 36 cycles are OFF.
Example: If the process output demands a 90% power requirement, how many
cycles 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 the
time base changed from 5 cycles in Figure 12a to 10 cycles in Figure 12b! Now
you know how variable time base got its name? The time base changes automatically to deliver the shortest possible bursts of power to the heater.
Figure 12
V
+
0
_
2 Cycles ON
3 Cycles OFF
a. Burst Firing with Variable Time Base - 40% Power Requirement
V
+
0
_
9 Cycles ON
1 Cycle OFF
b. Burst Firing with Variable Time Base - 90% Power Requirement
What is the main advantage of
variable time base over fixed
time base? Because variable
time base typically switches the
heater ON and OFF more often,
heater life is greatly increased
as well as the SCR.
Why is variable time base preferred over a fixed time base? As you can see from
our analysis above, the ON/OFF switching of the heater happens much more
often with variable time base. We know that the more often the heater is
switched, the less temperature variations the resistance element has. The nearly
constant load current to the heater keeps the heater’s resistance element temperature nearly constant and therefore less thermal fatigue. This provides a longer
heater life.
11
SCR Power Control
Variable Time Base (con’t)
Contrary to the definition a solid state device (SCR in this case) is not totally solid
state. There are mechanical connections that can fatigue just like a heater element. Therefore variable time base also extends the life of an SCR.
Exercise Two
A burst firing SCR uses a variable time base. If 20% power is required, how
many ON cycles and OFF cycles will provide 20% power to the heater? Draw
a diagram to illustrate this. Compare this to Exercise One. Answer on
page 25.
Phase-Angle Firing
In this method of SCR firing, the proportioning action takes place every consecutive half cycle in the AC sine wave. The time base then, is equal to one half AC
cycle! This is fast! The firing point is variable within this half cycle time period
to achieve a very accurate proportional control of electric current through the
SCR.
If the SCR gate is fired early in the half cycle, the power output of the heater is
high. Why? If most of the current gets through the SCR, the heater will produce
lots of power. If the gate is fired late in the half cycle, only a small increment of
power 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% power
requirement examples, you can see the affect of phase angle firing on the current
flow through the SCR (Figure 13).
Figure 13
+
V0
-
40% Power Requirement
a. Phase Angle Firing - 40% Power Requirement
+
V0
+
- V0
-
40% Power Requirement
b. Phase Angle Firing - 90% Power Requirement
The point at which+ the SCR’s gate is fired in the AC sine wave is continuously
variable across the
V 0half cycle. Where it is fired depends on the power required by
the heater. Current- flow to the heater is practically continuous! Thus, there is no
temperature variation experienced by the resistance element in the heater. As a
12
V
+
0
-
Watlow Educational Series
Phase-Angle Firing (con’t)
result, you may think that phase angle firing provides us the best resistance element control and the longest life possible. However, Watlow recommends zero
cross variable time base for nichrome resistive elements. The reasons why are
discussed later in this SCR study guide
As you may have noticed, when the SCR is phase angle fired, it is NOT zero-cross
switched! What problem will we have if it is not zero-cross switched? Phase
angle fired SCRs generate a lot of electrical noise! The electrical noise appears as
voltage “spikes” on the AC sine wave (Figure 14). These voltage spikes were left
out of Figure 13 for clarity.
Figure 14
+
V0
-
The “choppy” sine wave pattern in Figure 14 produces harmonic frequencies
which add to the electrical noise in a circuit. Harmonic currents also add to the
total load current in the circuit. The electrical system components must be sized
to account for these extra harmonic currents. Sometimes this extra current is
referred to as voltamps reactive (VARs)
SCR Review
Questions
This is a good place to stop and review what we have discovered so far. Please
take 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 ON
and 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 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. Compare
them using a 60% power output on an AC 60 Hz power supply.
13
SCR Power Control
Power Control and
Heater Life
As we learned in Book 2 (Thermal Systems and Electric Heaters), all resistance
elements eventually burn out. We also know that we can greatly extend heater
life by reducing the temperature cycling which the resistance element
experiences. That is where the true advantage of the SCR power control lies - it
greatly reduces the temperature cycling on the resistance wire.
Well, how much does it reduce this temperature cycling? At this point, let’s summarize 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 has
on resistance element temperature and hours to failure.
Electromechanical
Relay
Mercury Displacement
Relay (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 element
temperature experiences very wide temperature swings (Figure 15). Wide
temperature swings accelerate resistance wire oxidation and burn out the heater
much more quickly. The wire expansion and contraction work hardens (fatigues)
the element and then it breaks.
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 cycle
time. The typical cycle time for MDRs is 4 to 5 seconds. Notice in Figure 16 the
dramatic reduction in resistance element temperature swings. This is a result of
quicker 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 the
longer life of the mercury relay itself. Even when switching so quickly, the MDR
will outlast an electromechanical relay.
Figure 15
Time-Temperature Profile for an
Electromechanical Relay
Figure 16
Time-Temperature Profile for a MDR
2000°F
1700°F
1600°F
1500°F
1300°F
Electromechanical
Process Set Point:
Overshoot:
Droop:
Internal Temperature
2000°F
14
1700°F
30 Second
Cycle Time
1600°F
300°F (1900°F)
254°F (1346°F)
2100°F
Mercury Displacement Relay
Process Set Point:
Overshoot:
Droop:
Internal Temperature
5 Second
Cycle Time
1600°F
50°F
30°F
1830°F
Watlow Educational Series
Solid State Relay (SSR)
Since a SSR is solid state, it does not fail due to the mechanical wear or arcing of
the 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 the
temperature swings of the resistance element (Figure 17). These are the same
temperature swings we expect to see if using a burst firing - fixed time base SCR
(1 second time base).
Burst Firing SCR Power
Control
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 very
little if any temperature swings (Figure 18). Variable time base switching will
extend the life of resistance element heaters. There is still temperature cycling of
the resistance element with very high watt density heaters.
Figure 18
Time-Temperature Profile for a Burst Firing SCR
(Variable Time Base)
Figure 17
Time-Temperature Profile for a SSR or SCR
with 1 Second Time Base
2000°F
2000°F
1700°F
1700°F
1600°F
1600°F
1500°F
1500°F
1300°F
1300°F
Solid State Relay
Process Set Point:
Overshoot:
Droop:
Internal Temperature
Phase Angle Firing
SCR Power Control
1 Second Time Base
1600°F
4°F
5°F
1730°F
SCR (with burst firing)
Process Set Point:
Overshoot:
Droop:
Internal Temperature
Minimum16.6 millisecond time base
1600°F
0
0
1720°F
The time base for phase angle firing is one half cycle of the AC sine wave. The
heater switching is now so fast that there is NO temperature cycling of the resistance element (Figure 19)! Even high watt density heaters will not experience
temperature cycling when
phase angle firing is used. However, as good as phase
2000°F
angle is, it still is not recommended over zero cross variable time base firing for
nichrome element heaters.
1700°F Random turn on into the sine wave at 50% power will
create significant harmonic heater current.
1600°F
Solid State Relay
Process Set Point:
Overshoot:
Droop:
Internal Temperature
Figure 19
Time-Temperature
Profile
for Phase Angle Fired SCR
1 Second Time Base
1500°F
1600°F
4°F
5°F
1300°F
1730°F
SCR (with burst firing)
Process Set Point:
Overshoot:
Droop:
Internal Temperature
SCR (with phase angle)
Process Set Point:
Overshoot:
Droop:
Internal Temperature
16.6 millisecond time base
1600°F
0
0
1720°F
8.3 to 10 millisecond time base
1600°F
0
0
1680°F
15
SCR Power Control
Burst Firing SCR Power
Control (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...quicker
ON/OFF switching leads to lower resistance element temperature swings. This,
FIG.in20turn, greatly extends heater life!
Figure 20
Heater Life Vs. Time Base Relationship
Heater Life Vs. Time Base Relationship
SCR
Burst
Firing
8000
7000
6000
Life (Hours)
5000
SSR
4000
3000
MDR
500
ElectroMechanical
Relay
400
300
200
100
0
0
33 ms
1
5
30
Cycle Time (Seconds)
Exercise Three
A customer wants to order a Watlow microprocessor-based PID temperature
controller. This controller is required because of the fast response and accuracy it offers. She is thinking of using electromechanical relays to switch the heater
loads. Is this a good choice? Based on what we just discussed and what you
know about output devices, what control output-output device
combination would you recommend? Explain your answer. Answer on page 25.
16
Watlow Educational Series
Power Control
Selection
Only a SSR or a SCR power
controller allow a customer to
take full advantage of a proportioning (PID) temperature controller’s capabilities.
If SCR power controllers and PID temperature controllers are so great, WHY do
customers still use thermostats and relays in many types of equipment? Think
about 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 will
do the job required. So, if a thermostat controls temperature within +- 25˚F and
the thermal system performs well, then a thermostat will be used. As more accurate temperature control is required, the customer must use either ON/OFF or
PID 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 temperature
controller and then to use electromechanical relays to do the power switching!
That defeats much of the purpose in using a PID temperature controller in the
first place! A PID control can do things very fast...an electromechanical relay can
only 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 SCR
power controller to switch the heater (or cooling system) load.
That is basically how the selection of the temperature and power controls are
made. 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 transformercoupled heaters. We explore these types of heater control next.
Stable Resistance
Heaters
All Watlow heaters use nichrome
or other type of stable resistance element. Special
designed heaters, however, may
employ temperature dependent
resistance elements.
These heaters have resistance elements which maintain a fairly constant resistance value. For example, nichrome wire elements typically experience about a
5%+ increase in resistance at operating temperature. The resistance value may
increase 10%+ above the cold resistance if the temperature moves up toward
nichrome’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 more
than 10% as temperature increases. The majority of Watlow heaters use nichrome
or other stable resistance elements.
When switching nichrome element heaters, a SSR with a 1/2 to 1 second cycle
time setting is recommended. If higher amp switching or faster switching is
required to get better heater life (or quicker control response), use a variable time
base burst firing SCR power controller. Variable time base burst firing provides
excellent control, very little electrical noise and provides very long heater life.
The SCR Selection Chart (Figure 25 on page 21) shows this selection process more
clearly.
Notice that the chart does not recommend phase angle fired SCRs to control
stable resistance heating loads! Variable time base burst firing works best in most
nichrome element heater applications.
When phase angle firing directly into a resistive heater there is only the heater
element resistance to limit the current in the circuit. When conducting inside the
AC sine wave, say 50% power demand, the SCR will energize from zero to peak
voltage within a few micro seconds. The dv/dt and therefore the di/dt (load current rise with time) will cause the heater element to ring physically and thus
reduce heater life. With zero cross the voltage and current rise and fall with the
sine wave from zero to maximum. This control method is much easier on the
heaters and power controllers.
17
SCR Power Control
Variable Resistance
Heaters
There are a few resistance element materials which show large resistance value
changes with temperature. Examples are tungsten, molybdenum, silicon carbide
and graphite element heaters. Tungsten elements are used in high temperature
tungsten-quartz tubular heaters. Molybdenum, silicon carbide and graphite
heaters are used fig.
for21
high temperature ovens and furnace applications. The graph
in Figure 21 shows the temperature dependent resistance values of tungsten and
molybdenum.
Figure 21
Temperature Dependent Resistance Values of Tungsten and Molybdenum
100
Specific Resistance microohm-cm
90
80
70
Tungsten
60
50
40
Molybdenum
30
20
10
0
400 800
1200 1600 2000 2400 2800
Temperature ˚C
3200
Why is this large resistance value change a problem? Let’s use Ohm’s Law to
help us out. Ohm’s Law states that V = IR. The supply voltage to a heater is constant. 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 current through the tungsten element? It is 10 amps (230V/23Ω). What happens
when the heater is shut off? Per Figure 21, the resistance value of the tungsten
drops to about 1/15th its value at operating temperature! So the cold resistance
is about 1.53Ω.
The high in-rush current on start
up when using tungsten or
molybdenum-based resistance
elements will destroy a SCR
almost instantly (unless using I2t
fusing and soft start).
What happens when the heater is switched on again? Since the resistance element is cold, the current will be 15 greater at operating temperature (150 amps)!
Why? Since the resistance dropped to 1/15th its “hot” value, the current must
increase by 15 times to make the equation 230V = 150 amps x 1.53Ω valid. This
large in-rush current can blow semiconductor fuses, damage SCRs and cause
heaters to fail prematurely.
How do we eliminate this problem? For these kinds of heaters, a phase angle
firing SCR power controller is required. To prevent the SCR from burning up on
heater start up, we use the soft start feature on a phase angle firing SCR. Figure
25 on page 21 shows you the selection process just described above.
18
Watlow Educational Series
Variable Resistance Heaters
(con’t)
Phase Angle Soft Start
Soft start only operates during heater start up. When the signal is given to start
the heater, the temperature controller tells the SCR to switch full ON. However,
we know that if this happens, the SCR could be damaged. So in this start up condition, the soft start feature allows only a small amount of current through to the
heater. As time goes on, the current is gradually increased until full power is
finally applied to the heater (Figure 22). Soft start usually lasts for about 5 to 10
seconds, depending on the SCR power controller used. Some power controllers
allow this soft start time to be adjustable by the operator.
Figure 22
Soft Start Illustrated
V
+
0
_
Time
Soft start essentially allows the resistance element to preheat or warm up. This
increases element resistance before the full load voltage is applied through the
SCR to the heater. This ensures that the SCR is not harmed and that the heater
will have a very long life. Soft start is usually adequate for the tungsten quartz
lamp heaters because the warm up period is so short, usually only a few AC
cycles.
Phase Angle With Current Limiting
Soft start gradually increases the current flow and thus the power supplied to the
heater during initial heater start up. It is still possible, however, that after the soft
start (or at some other time during operation) too much current may pass through
the SCR. This, as we know, can destroy the SCR and the heater.
To prevent this from happening, a current sensing transformer is built into the
SCR power controller. The transformer senses the current flow through the SCR.
If the load current is higher than a preset current limit, the power controller electronics reduce the current to the maximum limit value (Figure 23).
When the load current through the SCR drops back below the maximum current
limit, then the full power of the AC sine wave is allowed through.
Figure 23
Current Limiting in Action
+
V0
_
Soft Start
Current Limiting
19
SCR Power Control
Variable Resistance Heaters
(con’t)
Transformer-Coupled
Heating Loads
Low voltage Watlow ceramic
fiber heaters are often transformer-coupled.
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 so
swift that it destroys the SCR. The only way to prevent a short circuit from
destroying the SCR is by using a semiconductor fuse. This fuse will blow within
1
⁄2 cycle of the short circuit condition. ALWAYS use semiconductor fuses whenever SCR power controls are used! It is a small price to pay to protect the customers investment! Semiconductor fuses have a guaranteed specified I2t value
where the fuse will blow if the short circuit current exceeds that spec. The I2t of
the fuse should always be sized below the I2t specification of the SCR. Normally
the fuse is selected by the power controller manufacturer.
Heaters used in very high temperature applications tend to have low voltage
ratings (say, under 100V). Low voltage ratings are required, because the heaters
typically have very low resistance values. Using a lower voltage provides the
proper amount of current required to power the heaters.
A transformer is used to step down the high voltage power supply to the low
voltage required by the heater. A phase angle fired SCR is again required to control current flow through the transformer (and thus the heater). The transformer
and phase angle fired SCR are usually connected as shown in Figure 24. Notice
how the SCR power controller is connected on the HIGH VOLTAGE side of the
circuit. It is much less expensive to control high voltage and low current than the
other way around! It is important that the transformer be sized, voltage and
KVA, properly to ensure a strong power supply. The voltage should be selected
so that the power controller is using as much of the full sine wave as possible
(phase forward) to reduce the harmonic currents and improve power factor.
Figure 24
Typical Connection Scheme for Transformer-Coupled Heaters
SCR Power
Controller
L1
High
Voltage
Supply
Low
Voltage
Side
L2
Transformer
Heater
A phase angle SCR is required, because it has a built-in circuit which ensures that
the transformer always receives alternating plus and minus AC voltage. If, for
some reason, the transformer received two pulses of the same polarity, it could
overheat the transformer and blow the semiconductor fuse. The SCR Selection
Chart in Figure 25 again shows this for transformer-coupled heating loads.
SCR Selection Chart
20
To simplify our selection of the proper SCR power controller, we can use the flow
chart shown in Figure 25. First, we determine the type and resistance characteristic of the load. Then, we find the recommended type of SCR firing method.
Finally, we find the type of temperature controller output required to command
the SCR power controller.
Example: You have quoted a Watlow radiant panel heater to a customer. What
type of SCR power control can you recommend?
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 output (like switched DC). The temperature controller output required for a burst
firing SCR power controller is a process output (like 4-20mA).
Figure 25
SCR Selection Chart
Characteristics
of the Load
Notes:
*1. Nichrome heater elements
change resistance less than
two times in their operating temperature range.
*2. Heaters that change resistance include:
• Tungsten increases over
16 times from cold to hot
• Silicon carbide changes
with temperature and age
• Molybdenum and graphite
increase resistance with
the temperature and are
often used on the secondary
of
a
transformer
*3. Transformers can become
DC saturated if two pulses
of the same polarity are
applied in sequence which
can cause overheating and
high currents that will
damage the SCR. Burst firing should not be applied.
Typical
Load
SCR
Firing
Method
Required
Stable *1
Resistance
Resistance *2
Change
Inductance *3
Nichrome
Cartridge
Circulation
Strip
Tubular
Mica Strip
Quartz
Radiant
Tungsten
Quartz
Silicon Carbide
Glo Bars
Molybdenum
Graphite
Transformer
Burst
Firing
Phase
Angle
Phase
Angle
Process
(Analog)
Process
(Analog)
Process
(Analog)
Solid State
Contactor
Temperature
Control
Time
Output
Required Proportioning
Exercise Four
A customer wants to use a Watlow tubular immersion heater. The heater is
rated for 480VAC, 3 phase, 50kW. He wants to use a thermostat connected up
to an electromechanical relay to control thermal system temperature.
The customer also requires accurate temperature control and fast
response to temperature changes. Based on the heater and customer
requirements, recommend the best temperature controller (thermostat,
ON/OFF or PID), control output and switching device to meet customer
requirements. Use Figure 25 if necessary. Answers on page 25.
21
SCR Power Control
3 Phase SCR Power Control
It is actually quite simple to apply SCR power controllers to single and three
phase powered heaters. Single phase heaters are connected as shown in Figure
26. Notice that only one set of back-to-back SCRs is used to control current to the
heater. Of course, proper fusing must be used!
Figure 26
Single Phase SCR Power Control
L1
L2
Contact a Watlow sales agent or
Watlow Controls factory for more
detailed information on switching heaters with SCR power
controllers.
There are many types of 3 phase connections for SCR power controllers. Each is
dependent on the type of SCR firing required. Figure 27 illustrates that only 2 sets
of back-to-back SCRs are required to control a burst fired, 3-phase heater. This is
the least expensive way to control burst fired, 3-phase heaters. Remember that
burst firing means that the SCRs are zero-cross switched. The key advantages of
a 3-phase, two leg control over a 3-phase, 3-leg are: best zero cross, less switching noise, lower cost, less power dissipation, simpler design, and a physically
smaller package. In operation the L2 phase is the return path for the switched L1
and L3 phases.
Figure 27
Three Phase-Two Leg SCR Power Control (Burst Firing Only)
L1
L2
L3
When phase angle fired power control is required, one common configuration is
a hybrid 3 pair SCR/Diode combination (Figure 28). This may be less
expensive than 3 pairs of back-to-back SCRs and generates less heat. This hybrid
design can be used for burst fired control of 3ph heaters, but is normally not
recommended. The diodes are the return path for the switched SCRs, this power
control configuration is slowly going away since SCR costs are more comparable
and advanced technology is simplifying SCR gating.
Figure 28
Three Phase-Three Leg Hybrid SCR Power Control (Phase Angle Firing Only)
L1
L2
L3
22
Watlow Educational Series
3 Phase SCR Power Control
(con’t)
The most common configuration for phase angle fired 3-phase heaters, is to use
three pairs of 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 used for delta-to-delta 3ph transformer control.
Figure 29
Three Phase-Three Leg SCR Power Control (Phase Angle Firing Only)
L1
L2
L3
Finally, when a 3-phase, 4-wire (grounded wye) heater must be controlled, 3 pairs
of back-to-back SCRs are again used with an uncontrolled neutral (Figure 30).
This is for burst fired control applications. In operation the SCR switches from
phase to neutral.
Figure 30
Three Phase-Four Wire SCR Power Control (Burst Firing Only)
L1
L2
L3
Neutral
Booklet Review
Questions
We’ve come a long way on our journey through SCR power control! Now, let’s
review by putting into practice what you have so diligently studied. Answer the
following questions. If you can’t answer a question, go back into the book and
review 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.
23
SCR Power Control
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 of
SCR firing and options should be chosen for this heater?
a. Burst firing, fixed time base, current limiting.
b. Burst firing, variable time base.
c. Phase angle firing, soft start.
d. Both a and b are correct.
7. A set of Watlow Multicell heaters is used to heat gases to very high temperatures. 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 and
output device would you choose to control these heaters. Please explain your
answer.
9. A low voltage ceramic fiber heater is operated on 60VAC. A transformer is
used to step down the voltage. What method of SCR power control do you
recommend for this application? Please explain your answer.
24
Watlow Educational Series
Answers to Exercises
and Review Questions
Answers to Exercises
Exercise 1: First we multiply 20% (0.2) by 60Hz to calculate the number of ON
cycles 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 of
ON/OFF cycles which will provide 20% power. From Exercise One, we know
that 20% power is 12 cycles ON, 48 cycles OFF. We then reduce this proportion to
its 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 Figure
31b below.
Figure 31
V
+
0
_
12 Cycles ON
48 Cycles OFF
a. Fixed Time Base of 1 Second - Burst Firing Under 20% Power
V
+
0
_
1 Cycle
ON
4 Cycles
OFF
1 Cycle
ON
4 Cycles
OFF
b. Variable Time Base - Burst Firing Under 20% Power
Exercise 3: In any case, electromechanical relays are not a good choice. This is
especially true since fast response and accuracy are required. It is much better to
use at least a SSR, or perhaps a SCR power controller.
Assuming an external SSR is used, either a switched DC control output or internal SSR must be used to switch the external SSR. Use a fast cycle time (about 1
second).
If a SCR power controller is used, use a process control output and a variable time
base burst firing SCR.
Exercise 4: DO NOT USE THE THERMOSTAT! Use a PID temperature controller
with a process output (like 4-20 mA) and a variable time base burst firing SCR
power controller
Answers to SCR Review Questions - Page 13
1. True. A SCR power controller switches power ON and OFF to the heater in
much the same way as a relay. The faster switching increases heater life.
2. The SCR, control electronics and heat sink.
25
SCR Power Control
Answers to Exercises and
Review 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 in
one direction, the other SCR conducts current in the other direction.
5. This greatly reduces electrical noise, because SCR switching takes place at or
near zero volts.
6. In fixed time base, ON and OFF switching of the SCR must always take place
during a fixed time period (like 1 second). Variable time base continually
varies the time period (or number of cycles) to provide the quickest switching possible and the least amount of change on the heater.
7. a) Phase angle firing fires the SCR each consecutive half cycle. The current
flow through the SCR is varied by firing the SCR either earlier or later in that
half cycle.
b) The smallest possible number of cycles required to get a 60% power flow
is 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
V
+
0
_
3
Cycles
ON
2
Cycles
OFF
5 Cycles
Time Base
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 both
ways through the SCR pair. One SCR is used for current in one direction, one
SCR 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.
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. This
temperature and power control system will provide the best life and
temperature control.
26
Watlow Educational Series
Answers to Exercises and
Review 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 temperature control, process control output and phase angle fired SCR power
controller are required. The phase angle control should have soft start and
current limiting.
9. Transformers (per Figure 25) require phase angle fired SCR power
controllers.
27
Designer and Manufacturer of Industrial
Heaters, Sensors and Controls
Watlow St. Louis • 12001 Lackland Road • St. Louis, Missouri 63146 USA • Phone: 314-878-4600 • FAX: 314-434-1020
For information on other training books and materials available from
Watlow, please call 314-878-4600 or fax 314-434-1020
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