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• Delta P/P (Head pressure)
• Pre-rotation Vanes position
• Surge count
The compressor motor speed can be controlled either
man u al ly in MANUAL mode or automatically in AUTO
mode.
In MANUAL speed control mode, the speed can be
controlled from the VSD Tuning Screen using Key pad
keys. The speed can be set to a pre-selected fre quen cy
over the range of 10 to 60 (50) Hz. Or, it can be in creased
or decreased over the range of 0.0 to 60 (50) Hz in in cre -
ments of 0.1 to 10 Hz. Instructions for man u al con trol are
included in the "System Calibration, Service Setpoints
and Reset Procedures" section of this book. Via Serial
communications, the speed command is sent from the
Microboard to the ACC Board, where it is passed on to
the VSD Logic Board. The VSD Log ic Board con trols
the VSD to operate the motor at this frequency. While
Manual control is selected, VALID POINT LED CR8
is illuminated, indicating that surg es will not be mapped
in the Surge Map. Anytime this LED is il lu mi nat ed,
surges are not mapped, as ex plained be low. In making
the transition from Manual to Auto mode, if the speed
was manually set to <30 Hz, it will au to mat i cal ly be set
to 30 Hz and automatically ad just ed from this val ue. If
not in Current Limit and the actual speed is <60 (50) Hz,
and 60 (50) Hz is selected, the speed will be increased
as follows. If the Leaving Chilled Liq uid tem per a ture
is within 0.2ºF, the PRV will si mul ta neous ly be driven
closed per the following:
Speed Increase
• 0.2 Hz every second if Leaving Chilled Liquid
Tem per a ture >0.2ºF below the Setpoint and motor
cur rent is <80%FLA.
• 0.2 Hz every 1 + (%FLA-80) second if Leaving
Chilled Liquid Temperature is >0.2ºF below the
Setpoint and the motor current is >80% but <
98%FLA.
• 0.2 Hz every 19 seconds if none of the above con-
di tions are present.
PRV Close
A close signal of the following durations is ap plied
every 4.5 seconds:
• 3.9 seconds if PRV position >50%.
• 3.0 seconds if PRV position >25% but <50%.
• 1.5 seconds if PRV position <25%.
YORK INTERNATIONAL
In AUTO speed control mode, the ACC Board con trols
the speed. It determines the optimum compressor mo-
tor speed (frequency) over the range of 30 to 60(50)
Hz and sends this value to the VSD Logic Board via
the Serial com mu ni ca tions link. The VSD logic Board
con trols the VSD to operate the motor at this fre quen cy.
The op ti mum speed is the slowest speed possible that
will avoid compressor surge conditions but still allow
the chiller to meet capacity re quire ments. This speed is
found in an adaptive sense as explained below.
In determining the optimum motor speed, the ACC
Board employs the following:
• Delta P/P – This is the chiller Head pres sure. It
is calculated as (Condenser pressure – Evap o ra tor
pressure) / Evaporator pressure. It ranges from 0.00
to 3.60. The ACC Board cal cu lates this value from
evaporator and condenser pressure values re ceived
from the Microboard via serial communications
link. The ACC Board returns the calculated Delta
P/P value to the Microboard for display over the
same link.
• Pre-rotation Vanes (PRV) position – A po ten -
ti om e ter provides PRV position (0 to 100%) to the
ACC Board. The position is 0% when fully closed,
100% when fully open. This value is sent to the
Microboard via serial communications for display.
The potentiometer must be calibrated by a qualifi ed
Ser vice technician us ing a procedure in the "Sys tem
Calibration, Service setpoints and Reset Pro ce dures"
section of this book. No speed reduction is per mit ted
if this calibration has not been per formed.
• Motor Speed – This is the actual drive fre quen cy
of 30 to 60(50) Hz.
• Surge Map – This contains the Delta P/P, PRV
po si tion and motor speed that existed at the in stant
of each pre vi ous ly encountered surge condition.
These pa ram e ters are stored as a 3-dimenensional
ar ray for each surge. The surge map is stored in the
ACC Board's BRAM battery backed memory. The
fol low ing pro ce dures are detailed in the "Sys tem
Cal i bra tion, Ser vice Setpoints and Reset Pro ce dures
section of this book: The entire Surge Map can be
printed by press ing the "Surge Map Print" keypad
key. Surge Map points can be automatically print ed
to an ex ter nal print er as they are plotted. The Surge
Map can be cleared.
FORM 160.54-M1(503)
12
77
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