Application Guide

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Evaporator 1
Evaporator 2
M
V2
M
V1
Inverter
CC1
CC2
Controller
RST
PG
MON PARREF BTNS
RUN READY FAULT
Honeywel
l
Fig. 46 Application with VFD and Excel Plus
Application Description
If the chillers in the previous example (option 1) are controlled on the flow from the machine and also have Direct Digital
Control or a building management system installed on the air handling and chiller plants, an even more efficient and effective
means of control is available.
The control software is made up of three elements :
— Safety factors, ensuring the minimum design flow rates for each chiller.
— Flow control for supplying the volume of water to suit the needs of the AHU. with the highest demand.
— Load control for sequencing the two machines.
The first element is the low limit control to set the minimum speed of the VFD (minimum design flow) while one machine runs
and then increase the pump speed when both machines run to satisfy the minimum design flow rate (plus a 15% safety factor).
This would act as a 2 position low limit.
The flow rates (pump speed) can then be varied to suit demand. This is achieved by inputting the feedback position of each
AHU. bypass valve into a control loop. Using a predetermined setpoint, e.g. 85% open and P+I control, the output from each
loop is fed into a load selector relay (highest signal). The output from the load selector is the control signal to the VFD. If this
signal is higher than the low limit, the pump motor speed is increased and hence increase the flow rate. Conversely, as the
AHU. with the highest demand begins to close its valve the flow rate would be reduced accordingly.
The third element in the control function is the load control or sequencing of the two machines. With one machine running its
total load can be measured, as the chiller approaches full load e.g. 95% it can signal for the second machine to start. The total
load of the machine can be measured by a current transducer measuring the current drawn by the chiller motor. Another
possibility is by using pressure transducers to measure the differential pressure and by sensing the differential temperature
across the evaporator and then calculating the instantaneous load. This later method is probably the safer as the differential
pressure signal can be used as an input into the low limit flow control. The former is more accurate for starting the second
machine at set load conditions. A variation of this system could be to have more than one chilled water pump and to sequence
them on and off according to load.
Benefits
— Energy savings: Lag chiller will be off for longer periods.
— Reduced maintenance: Few starts and it is unwise to cycle chillers on and off under light load.
— Supply temperature to the building will be more stable.
— Reduced maximum demand: Fewer starts especially during winter.
— Reduced noise.
VFD Features
— Auto restart.
— Speed search.
— Overspeed.
— Overtorque protection.
— Stall prevention.
— Analog or digital control.