Section 4 - Vsd Operation; Pyt Model Vsd Overview; Pyt Model Vsd Components - York YZ MODEL A Operation And Maintenance Manual

FORM 161.01-OM1
ISSUE DATE: 6/8/2018

PYT MODEL VSD OVERVIEW

The YORK PYT model VSD is a liquid cooled, transis-
torized, PWM inverter in a highly integrated package
mounted directly to the chiller. This VSD is specifi-
cally designed to drive the high speed induction motor
with magnetic bearings used on the YORK model YZ
Centrifugal Liquid Chiller. This allows the chiller to be
designed as a complete system, and take full advantage
of the strengths of each major component within the
system.
The PYT drive enclosure contains lethal
AC and DC voltages. Only suitably trained
and qualified personnel are permitted to
open the VSD enclosure for service.
• NEVER place loose tools, debris or any
objects inside the PYT drive enclosure.
• NEVER allow the PYT drive enclosure
doors to remain open. Keep the doors
closed and ensure all latches are en-
gaged on each door unless the unit is
being serviced.
• ALWAYS lockout the disconnect sup-
plying AC to the chiller when servicing
the VSD.
• DO NOT switch on the circuit breaker
if a VSD door remains open.
The power section of the VSD is composed of four ma-
jor blocks:
• The AC to DC rectifier section
• The DC bus filter section
• The three phase DC to AC inverter section
• The output suppression network

PYT MODEL VSD COMPONENTS

At the input of the VSD, an electronic circuit breaker
with ground fault sensing connects the AC line to an
AC line inductor and then to the DC converter. The
line inductor will limit the amount of fault current so
that the electronic circuit breaker is sufficient for pro-
tecting the VSD. The following description of opera-
tion is specific for the 330 amp VSD unless otherwise
noted. See Figure 38 on page 110 through Figure 47
on page 117 for component locations in each PYT
model cabinet.
JOHNSON CONTROLS

SECTION 4 - VSD OPERATION

AC to DC Rectifier
The AC to DC converter uses 3 Silicon Controlled
Rectifiers (SCR's) and 3 diodes. One SCR and one di-
ode are contained in each module. Three modules are
required to converter the 3 phase input AC voltage into
DC voltage. The SCRs are mounted on the bottom of
the liquid cooled heat sink. The use of the SCR's in
the converter permits pre-charge of the DC bus capaci-
tors when the chiller enters the prelube cycle; it also
provides a fast disconnect from the AC line when the
chiller enters the coastdown cycle. During a coastdown
cycle, the SCR's in the converter are no longer turned
on and remain in a turned off condition until the next
pre-charge cycle and the DC bus capacitors will start to
discharge through the bleeder resistors.
When the chiller enters the prelube cycle, the VSD is
commanded to pre-charge and the SCR's are gradually
turned on to slowly charge the DC bus capacitors. This
is called the pre-charge period, which last for 20 sec-
onds. At this time the SCR's are fully turned on. The
VSD logic board provides the command to the SCR
trigger board to pre-charge and the SCR trigger board
provides the turn on commands for the SCR's.
DC Bus Filter
The DC link section of the VSD consists of a series of
electrolytic capacitors. The capacitors provide a large
energy reservoir for use by the DC to AC inverter sec-
tion of the VSD and serve to filter voltage ripple on
the DC bus. The capacitors are contained in the VSD
power unit. "Bleeder" resistors are mounted on the side
of the power unit to provide a discharge path for the
stored energy in the capacitors in the off cycle, and to
allow for balanced charging of the capacitor banks dur-
ing operation.
Although many of these parts are similar
to the parts used in previous VSD designs,
these parts are only compatible with VSDs
having the base part numbers. Failure
to use the correct parts may cause major
damage to these and other components
in the VSD.
4
107