JOHNSON CONTROLS
109
SECTION 4 - VSD OPERATION
FORM 161.01-OM1
ISSUE DATE: 6/8/2018
4
ter power unit is disconnected from the AC line. When
the chiller begins to run, the pre-charge resistors are
switched into the circuit via the precharge contactor for
a fixed time period of 5 seconds. This permits the fil-
ter capacitors in the filter power unit to slowly charge.
After the 5 second time period, the supply contactor is
energized and the pre-charge contactor is de-energized,
permitting the filter power unit to completely charge.
Three power semiconductor fuses connect the power
components to the AC line and serve to help prevent
rupture if a catastrophic failure occurs on the DC bus
portion of the filter power unit.
Three Phase Inductor
The three phase inductor provides some impedance for
the filter to boost bus voltage. It effectively limits the
rate of change in current at the input to the filter to a
reasonable level.
Filter Power Unit
The filter power unit generates the harmonic currents
required by the VSD's AC- to -DC converter so that
these harmonic currents are not drawn from the AC
line. The filter power unit is similar to the PYT model
VSD power unit in the 330 amp VSD, except with 2
fewer capacitors in the filter capacitor “bank”, and a
smaller transistor module and modified gate driver
board. The harmonic filter gate driver board provides
turn on and turn off commands as determined by the
harmonic filter logic board. “Bleeder” resistors are
mounted on the side of the filter power unit to provide
a discharge path for the DC bus capacitors.
Transistor Module
The transistor module contains a temperature sensor
that provides temperature information back to the fil-
ter logic board. This sensor protects the filter transistor
module from over temperature conditions. A bus isola-
tor board is used to ensure that the DC bus capacitors
are properly charged and that the voltage is balanced.
Two output current sensors are used to protect the filter
against an over current or an overload condition. Input
current transformers sense the input current drawn by
the VSD’s AC to DC converter. The line voltage isola-
tion board provides AC line voltage information to the
harmonic filter logic board, which is used to determine
the proper bus voltage setpoint.
High Frequency Trap
The high frequency trap is standard on all VSD's that
contain an optional harmonic filter. The high frequency
trap is composed of a series of capacitors, inductors,
and resistors used to reduce the effects of the PWM
switching frequency of the filter inverter.
CRITICAL LOAD POWER
The VSD cabinet includes an inverter and a battery.
When the source power feeding the chiller is removed,
the inverter-battery combination provides an energy
source to sustain control power until the motor coasts
to a stop and is de-levitated. During normal operation,
the inverter passes 120 VAC through a switch to power
the chiller controls. This inverter is monitoring the in-
coming voltage and when the source is removed, the
inverter begins to provide power to the critical load cir-
cuit, drawing energy from the 12V battery.
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