JOHNSON CONTROLS
108
FORM 161.01-OM1
ISSUE DATE: 6/8/2018
SECTION 4 - VSD OPERATION
DC to AC Converter
The DC to AC inverter section of the VSD serves to
convert the DC voltage to AC voltage at the proper
magnitude and frequency, as commanded by the VSD
logic board. The inverter section is composed of a sin-
gle power unit which contains three transistor modules
mounted on the same liquid cooled heat sink as the con-
verter modules, the DC bus capacitors, and a VSD gate
driver board. The gate driver board provides the turn on
and turn off commands to the transistor modules. The
VSD logic board determines when the turn on and turn
off commands should occur. The gate driver board is
soldered directly to the top of the transistor module and
is held in place with mounting screws.
Output Suppression Network
The DV/DT output suppression network is composed
of a series of capacitors and resistors. The job of the
suppressor network is to increase the time it takes for
the output voltage to switch, as seen by the motor, and
reduce the peak voltage applied to the motor windings.
This network protects the compressor motor from prob-
lems commonly associated with PWM motor VSDs.
Drive Logic Board
The drive logic board performs numerous functions in-
cluding, control of the VSD cooling fans and pumps,
determining when to pre-charge the bus capacitors and
performs PWM calculations. The drive logic board
also determines shutdown conditions by monitoring
the three phases of the motor current, the heat sink
temperature, the baseplate temperatures, the internal
ambient temperature, and the DC bus voltage.
Each transistor module and heatsink have thermistors
which provide temperature information to the VSD
logic board. These sensors protect the VSD from over
temperature conditions. A bus voltage isolator board is
used to ensure that the DC bus capacitors are properly
charged. Three output current transformers protect the
VSD and motor from over current conditions.
Heat generated from the electronic controls and power
components is removed through liquid cooled heat-
sinks and cooling coils, with fans. The inhibited liquid
that flows through the heatsinks and cooling coils is
pumped through the system by an externally mounted
pump. A shell and tube heat exchanger, with removable
heads, is mounted on the back of the VSD cabinet.
PYT MODEL VSD (330, 420, 780, 1020 & 1280
AMP)
All PYT model VSDs function in the same manner and
have the same basic components. The power require-
ments of the higher amperage VSDs require more ca-
pacitors in the DC Bus. The 330 and 420 amp design
use one transistor module per phase on in the inverter
section, the 780 and 1020 use 2 per phase, and the
1280 uses 3 in parallel per phase. The modules and the
boards are not interchangeable between different PYT
models.
HARMONIC FILTER OPTION
The PYT model VSD may also include an optional har-
monic filter and high frequency trap designed to meet
the IEEE Std 519, “IEEE Recommended Practices and
Requirements for Harmonic Control in Electrical Pow-
er Systems”. The harmonic filter is offered as a means
to improve the input current waveform drawn by the
VSD from the AC line. The filter reduces the possibil-
ity of causing electrical interference with other sensi-
tive electronic equipment connected to the same power
source. An additional benefit of the optional harmonic
filter is that it will correct the system power factor to
near unity.
Within the enclosure of the PYT model VSD, the drive
logic board and optional harmonic filter logic board
are interconnected via a 16-position ribbon cable. This
cable provides power for the filter logic board and a
method of communication between the two boards.
The power section of the harmonic filter is composed
of three major blocks:
• The pre-charge section
• The three phase inductor
• The lter power unit
Pre-charge
The pre-charge section contains pre-charge resistors, a
pre-charge contactor and a supply contactor. The pre-
charge network serves two purposes, to slowly charge
the DC bus capacitors associated with the filter power
unit and to provide a means of disconnecting the fil-
ter power unit from the AC line. When the chiller is
turned off, both contactors are de-energized and the fil-
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