Theory Of Operation; Introduction; Silicon Control Rectifiers (Scrs) - Siemens L-828 Manual

L-828 CCRs LC-Type (4, 7.5, and 10 kW/6.6 A/60 Hz)

4. Theory of Operation

Introduction

Silicon Control Rectifiers
(SCRs)
 2002 Siemens Airfield Solutions, Incorporated
All rights reserved
This subsection describes the L-828 CCR theory of operation.
Current regulation is obtained by using SCRs to switch the supply
power (by varying the On duration of the AC cycle) to the power
transformer and by using feedback circuitry to monitor the
transformer's output. The brightness level is selected from one of
three or five preset values. Load variation is detected as a change in
the control voltage by the controller. The controller changes the
conduction angle of the AC voltage to the SCRs to adjust the power
flow into the power transformer. This sequence of events
compensates for the load variation and maintains the output current
at the preset level.
See Figure 8-1 in the Wiring Schematics section. When power is
applied to the CCR at TB3 terminals 1 and 2 and the contactor K1 and
circuit breaker CB1 are closed, no current will flow in the primary of
the power transformer T1 until the silicon control rectifiers (SCRs)
receive a trigger pulse. When this pulse occurs, the SCRs turn on.
This allows the current to flow into the primary of the power
transformer T1 until the SCR is switched off by the zero crossing of
the input voltage.
The path of the current flow is from TB3-2 through the circuit breaker
CB1, contractor K1, SCR1 terminals 1 and 3 to L2, L1, and primary of
T1, and back to K1, CB1, and TB3-1. Since alternating current is used
for the regulator operation, after zero-crossing the current flows in
the opposite direction using the other SCR (see SCR1 terminals 1 and
2).
The power level in the primary of the power transformer is controlled
by varying the phase of the conduction angle. If very low output
current is desired, the conduction angle would be near zero degrees,
that is, the SCR is not triggered until the input voltage is near zero-
crossing. Little power is then available to power transformer T1.
For maximum power to occur in the primary of the power
transformer T1, the conduction angle will be near 180 degrees.
Immediately after zero-crossing of the input power sine wave, the
SCR is triggered and operates continuously throughout the 180-
degree duration of the input sine wave.
For 67% power from the power transformer, the conduction angle
will be nearly 90 degrees. This occurs approximately at the voltage
peak of the sine wave when the SCR switches into conduction.
Switching the transformer on-line at this time creates a rather abrupt
rise time, since the sine wave is at its peak voltage. Choke L1 slows
this rapid rise time from the conduction of the SCRs.
96A0104B
Issued 6/02
Description
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