SHZF2/SHZF3 SCR Power Controller
Theory
Rev 2
Ametek HDR Power Systems
21
CHAPTER 6 - Theory
6.1 Functional
Description
The main function of the SHZF2/SHZF3 is to control the amount of power delivered to a load.
The SCR is either on or off and conducts in one direction when turned on. Because the SCR only
conducts in one direction, two SCR’s are connected in “inverse parallel” configuration, Figure 6.1. A
small gate signal is applied to turn on the SCR. Once the SCR is turned on, it will remain On until the
current through the SCR goes below the holding current level.
Figure 6.1 AC Switch
6.2
Digital Firing Control Circuit
Firing (gating) of the SCRs is controlled by a digital firing control circuit synchronized to the line
frequency (50 or 60 Hz). The firing control circuit may be manually controlled by the built-in ZERO
control or a remote potentiometer. Automatic control may be from a temperature or process controller
that provides a demand-proportional low-level voltage or milliampere output signal. The firing control
circuit may also be turned on and off by an external switch or relay contacts. (Chapter 4 discusses
some external control methods.)
6.3 Variable-Time
Base
The firing control circuit of the SHZF2/SHZF3 operates on a variable rather than fixed time base.
Operating on a fixed-time base of 1/3 second (20 cycles), for example, at 50% power the SCRs would
be on for 10 cycles, off for 10 cycles, on for 10 cycles, and so on. Operating on a variable-time-base,
at 50% power, the SCRs are on for one cycle and off for the next. At 75% power, the controller, with a
fixed-time-base of one-third second (20 cycles), would be on for 15 cycles and off for 5. The variable-
time-base, however, would be on for three cycles, off for one, and so on. The time required to
describe the operation of the SHZF2/SHZF3 would be the period of four cycles in this case – three
cycles on and one cycle off.
Table 6.1 Variable-Time-Base Relationship (Example)
Control signal, %
Power Demand
SCR Pair, Complete
On Cycles
SCR Pair, Complete
Off Cycles
1% 1 99
25% 1
3
50% 1
1
60% 3
2
70% 7
3
75% 3
1
80% 4
1
90% 9
1
95% 19
1
99% 99
1
100% ALL
0
LOAD
LINE
SCR-A
SCR-B
Summary of Contents for SHZF2 SCR Series
Page 6: ...Table of Contents SHZF2 SHZF3 SCR Power Controller iv Ametek HDR Power Systems Rev 2...
Page 8: ...Safety Instructions SHZF2 SHZF3 SCR Power Controller 2 Ametek HDR Power Systems Rev 2...
Page 12: ...Options SHZF2 SHZF3 SCR Power Controller 6 Ametek HDR Power Systems Rev 2...
Page 24: ...Operation SHZF2 SHZF3 SCR Power Controller 18 Ametek HDR Power Systems Rev 2...
Page 32: ...Troubleshooting SHZF2 SHZF3 SCR Power Controller 26 Ametek HDR Power Systems Rev 2...
Page 46: ...Appendix SHZF2 SHZF3 SCR Power Controller 40 Ametek HDR Power Systems Rev 2...
Page 47: ......
Page 48: ...503 Lakeview Plaza Blvd Ste C Worthington Ohio 43085 USA...
