JOHNSON CONTROLS
202
FORM 201.23-NM2
ISSUE DATE: 3/9/2015
SECTION 6 - TECHNICAL DATA
The open collector outputs of the Chiller Control Board
energize the DC relays on the Relay Output Board by
pulling one side of the relay coil to ground. When
not energized, both sides of the relay coils will be at
+12VDC potential.
VSD (Variable Speed Drive)
The VSD is a liquid cooled, transistorized, PWM
inverter packaged within the Control/Power cabinet.
The inverter is composed of four major sections:
• ACtoDCrectiersectionwithprechargecircuit.
• DClinkltersection.
• Three phase DC to AC inverter section.
• Output RC suppression network.
AC to DC Rectifier
The AC to DC Rectifier circuit utilizes a semi-convert-
er made of three SCR/diode modules in a three phase
bridge configuration. Each SCR/Diode module con-
tains 1 SCR and 1 diode. The modules are mounted on a
liquid cooled heatsink. This circuit rectifies the
incoming AC voltage to unfiltered DC, which is fil-
tered by the DC Link Filter.
LD10608
A semi-converter (combination SCR/Diode) configu-
ration allows utilizing a separate pre-charge circuit to
limit the current in the DC link filter capacitors when
the VSD is first switched on. This is accomplished by
slowly turning on the SCR’s to initially charge the DC
Bus. Once charged, the SCR’s remain fully gated on
during normal operation. This configuration also pro-
vides a fast disconnect from main power when the
drive is switched off.
When the drive is called to run (leaving chilled liquid
temperature is more than the Setpoint plus CR), the
SCR/Diode modules are turned on by the SCR trigger
Board, allowing the DC link filter capacitors to slowly
precharge for a period of 20 seconds.
The AC incoming line voltage is rectified by the full
three phase semi-converter bridge, made up of three
SCR/Diode modules, which provides pulsating DC to
the DC link Filter in the VSD.
SCR Trigger Board
The SCR Trigger Board controls the firing (gating) se-
quence of the Bridge SCR’s.
LD10609
Command for the SCR Trigger Board to begin firing
the SCR’s is initiated by the VSD Logic Board.
The SCR Trigger Board also monitors the three phase
input voltage to detect the loss of an incoming phase.
DC Link Filter
The DC Link Filter consists of a bank of electrolytic
filter capacitors. The capacitors smooth (filter) ripple
voltage resulting from the AC to DC rectification and
provides an energy reservoir for the DC to AC inverter.
The capacitor filter bank is made up of 2 banks of par-
allel-connected capacitors wired in series. Series banks
of capacitors allow using smaller sized capacitors with
lower voltage ratings.
FILTER
CAPACITORS
LD10610
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