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General Description 22
Silicon Controlled Rectifiers
Section Contents
General
Description .............. 2
General
Components ............ 3
Troubleshooting ........ 4
Silicon Controlled Rectifiers – SCRs
The primary SCR bridge is
an SCR/Heat Sink assem-
bly with the power SCRs
connected in inverse paral-
lel. The function of the SCR
bridge is to control the
amount of voltage applied
to the motor. The SCR, sili-
con controlled rectifier, is
similar to standard rectifiers
in that it will allow current
to flow in one direction
while blocking current in
the reverse direction. How-
ever, SCRs will conduct
current only when turned
ON. When two SCRs are
connected as shown a
controllable AC switch or
contact is formed that will
carry current in either direc-
tion but only when GATED
ON. SCRs can react very
quickly to gating signals,
quickly enough to be able
to control portions of half
cycles of standard AC
power. When the gating
or firing signal is presented
to the SCR very late in the
half cycle, the SCR will
block all of the cycle up to
the time of firing. At the
time the gating signal is
applied, the SCR will turn
ON and conduct the re-
maining portions of the half
cycle to the load.
As the firing signal is pre-
sented earlier in the half
cycle, the SCR will conduct
more and more of the
cycle on to the load. The
extremes will be maximum
firing when the SCR bridge
passes all of the AC power
on to the motor, and lock
out, zero firing, when the
SCR bridge blocks all of
the AC power.
In static reversing control-
lers, the SCR bridges also
perform a directional con-
trol function as well. As
can be seen in Figures 4 &
5 (on pages 13 & 14),
there are five SCR bridges,
two Hoist bridges, two
Lower bridges, and one
Common bridge. When a
Hoist operation is required,
adjustable voltage three
phase power is supplied to
the motor via the two Hoist
bridges and the Common
bridge. The three phase re-
lationship of the applied
motor power determines
the direction of developed
motor torque and rotation.
A Lower operation will be
performed in one of two
ways depending upon me-
chanical losses and the
weight of the suspended
load. First, adjustable volt-
age power is supplied to
the motor via the two
Lower bridges and the
Common bridge to pro-
duce a driving lower op-
eration. If the loading is
such as to continue to re-
quire a driving lower
torque, system will supply
lowering adjustable volt-
age power to meet the
commanded speed condi-
tion. If an overhauling con-
dition exists, a counter-
torque lowering operation
will occur where the Hoist
SCR bridges and the Com-
mon bridge provide motor
power. True motor and
power system regeneration
can be used, if desired, to
lower overhauling loads at
full speed. For this condi-
tion, the system turns the
Lower SCR bridges and
the Common SCR bridge
fully on to supply full lower-
ing voltage to the motor.
The transition from driving
lower to counter-torque
lower is automatic and is
determined by the me-
chanical loading pre-
sented to the motor. Full
voltage regenerative lower-
ing will produce lowering
speeds 10% to 15%
greater than full load hoist-
ing speeds.
SCR1
Line Load
SCR
Bridge
AC
Power
Source
AC Motor
Inverse Parallel
SCR Bridge Circuit
Primary SCR
Bridge Schematic
SCR2
K1G1
K2 G2
AC
Power
SCR
Bridge
AC
Load
SCR Firing Cycle Waveforms
Minimum
Firing
Medium
Firing
Maximum
Firing
Lockout
No Firing
Minimum
Voltage
Half
Voltage
Maximum
Voltage
Zero
Voltage
4922c
Instruction Manual
Publication 189 SCR
March 1995