York VSD 270 Operation Manual page 16

mounted on the side of the Power Unit to provide a discharge path for the stored energy in the
capacitors.
The DC to AC inverter section of the OSCD serves to convert the DC voltage to AC voltage at the
proper magnitude and frequency as commanded by the OSCD Logic board. The inverter section is
actually composed of one power unit. This power unit contains one very fast switching transistor
module mounted on the same liquid cooled heatsink as the converter modules, the DC Bus
capacitors, and an OSCD Gate Driver board. The gate driver board provides the turn on and turn
off commands to the output transistors. The OSCD Logic board determines when the turn on and
turn off commands should occur. The gate driver board is mounted directly on top of the transistor
module, and it is held in place with mounting screws and soldered to the transistor module.
The OSCD 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 problems commonly associated with PWM motor drives.
Other sensors and boards are used to provide safe operation of the OSCD. The transistor module
and heatsink have thermistors mounted on them to provide temperature information to the OSCD
logic board. These sensors protect the OSCD 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 OSCD and motor from over current conditions.
OSCD 385, 419, 503, 608, 658, 704, 790, 868, 882, 914, 917, 948, and 1055 hp
(high horsepower model)
The high horsepower models' OSCDs function in the same manner as the low horsepower models,
and have the same basic components. The power requirements of these high horsepower drives
require more capacitors in the DC Bus and 3-output transistor sections are needed. One section is
used for each output phase. Each transistor module within the output transistor section contains a
thermistor, which is connected to the OSCD logic board. The transistor gate driver board is mount-
ed on top of the transistor section in the same manner as the low horsepower model, but it only
contains two transistor drivers. The modules and gate driver boards are not interchangeable
between the various models.
Harmonic filter option
The VSD model of OSCD system may also include an optional harmonic filter and high frequency
trap designed to meet the IEEE Std 519, IEEE Recommended Practices and Requirements for Harmonic
Control in Electrical Power Systems. The harmonic filter is offered as a method to improve the input
current wave- form drawn by the OSCD from the AC line. In this way, it reduces the possibility of
causing electrical interference with other sensitive 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 nearly unity.
The power section of the Harmonic Filter is composed of three major blocks: a precharge section, a
three-phase inductor, and a Filter Power Unit.
The precharge section contains precharge resistors, a precharge 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 filter power unit from the
AC line. When the chiller is turned off, both contactors are de-energized and the filter power unit is
disconnected from the AC line. When the chiller starts to run, the precharge resistors are switched
into the circuit by the precharge contactor for a fixed time period of 5 seconds. This permits the
filter capacitors in the filter power unit to slowly charge. After the 5-second time period, the supply
contactor is energized and the precharge contactor is de-energized, permitting the filter power
unit to completely charge. Three power fuses connect the filter power components to the AC line.
Very fast semiconductor power fuses are used to quickly disconnect the transistor module from the
power source if a catastrophic failure were to occur on the DC bus of the filter power unit.
16 Liquid-Cooled OptiSpeed Compressor Speed Drive