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If VFD OPTION is set to VFD and increased capacity is
needed, the control will first try to increase IGV TARGET PO-
SITION if it has not reached the travel limit; if the travel limit
has been reached, the control increases VFD TARGET
SPEED. If decreased capacity is needed, the control first tries
to decrease VFD TARGET SPEED if it has not reached the
minimum VFD speed; if the minimum VFD speed has been
reached, the control decreases IGV TARGET POSITION in-
From the compressor relay closed point to the end of ramp
loading, the VFD TARGET SPEED is the configured VFD
start-up speed. When the chiller is running normally, the capac-
ity control determines whether and how much to change VFD
TARGET SPEED. When the chiller is in the shut-down pro-
cess, VFD TARGET SPEED will be the minimum VFD speed.
NOTE: If the VFD option is set to NO VFD, or the compressor
relay is not closed, VFD TARGET SPEED will be 0.
The guide vane position is determined by the Capacity Con-
trol function under normal conditions and other functions in
abnormal conditions, which include capacity inhibit request or
capacity decrease request. The guide vane actuator is driven by
comparing the guide vane target position and the actual posi-
tion. Guide vane position is limited to a value between zero and
IGV Travel Limit, which is configured from the Service Con-
figuration menu. When the chiller is shutting down or off, the
guide vane is always driven to zero during normal shutdown.
RAMP LOADING — The ramp loading control slows the
rate at which the compressor loads up. This control can prevent
the compressor from loading up during the short period of time
when the chiller is started and the chilled water loop has to be
brought down to CONTROL POINT (Setpoint Table). Ramp
loading helps reduce electrical demand charges by slowly
bringing the chilled water to CONTROL POINT. The total
power draw during this period remains almost unchanged. If
the power outage lasts for more than 3 hours, then Temperature
Ramp Loading will be used regardless of user configuration
and the minimum loading rate (1^F/min) will be used.
Two methods of ramp loading are available: temperature
ramp loading and motor load ramp loading.
Temperature Ramp Loading — Temperature ramp loading
limits the rate at which the controlled water temperature de-
creases for cooling and increases for heating during ramping by
reducing on cooling mode or increasing in heating mode the
PULLDOWN SET POINT (Maintenance Menu
Capacity)
at the configured rate, until the pulldown set point is less than
the cooling mode control point or greater than the heating
mode control point. The PULLDOWN RAMP TYPE (Config-
uration Menu
General Configuration) is configured to 0 for
temperature ramp loading.
Motor Load Ramp Loading — Motor load ramp loading lim-
its the rate at which either the load current percentage or motor
kilowatt percentage increases by incrementing the ramp de-
mand limit at the configured rate. The PULLDOWN RAMP
TYPE (Configuration Menu
General Configuration) is con-
figured to 1 for motor load ramp loading.
If DEMAND LIMIT SOURCE (Configuration Menu
General Configuration) is set to AMPS, then PERCENT
LOAD CURRENT is used for motor load ramp loading. If DE-
MAND LIMIT SOURCE is set to kW, then MOTOR PER-
CENT KILOWATTS is used for motor load ramp loading.
The motor load ramp loading algorithm shall be deactivated
when the Ramp Demand Limit is greater than or equal to the
ACTIVE DEMAND LIMIT (General Parameters). It is also
deactivated when Ramp Demand Limit is greater than or equal
to 80%. There will be a one-minute delay for the compressor to
be uploaded to target load (ramping load target 80% or AC-
TIVE DEMAND LIMIT if less than 80%) after ramping load
demand limit is set to 80% (or ACTIVE DEMAND LIMIT if
less than 80%).
GUIDE VANE
S
AT MAX.
MODULATE VFD
GUIDE VANE
S
VFD AT
S
TART-UP
S
PEED,
MODULATE GUIDE VANE
S
VFD
S
TARTUP:
VFD TO
S
TART-UP
S
PEED,
GUIDE VANE
S
CLO
S
ED
MAX.
S
TART
MIN.
VFD
%
S
PEED
0
TRAVEL LIMIT
100
GUIDE VANE
S
%
PO
S
ITION
Fig. 31 — Guide Vane Position and VFD Speed
a19-2129