ESIE10-01 Functional Control
Part 2 – Functional Description 2–47
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The Stage Down Error Step is added to Stage Down Accumulator once every Stage Down Error Delay
seconds. When the Stage Down Error Accumulator is greater than the Stage Down Error Set Point
another stage of condenser fans is removed.
When one fan is running, a fixed point is used in place of a deadband. When the Saturated Condenser
temperature drops below 20°C, stage down error is accumulated.
When a stage down occurs or the saturated temperature rises back within the Stage Down dead band
the Stage Down Error Accumulator is reset to zero. The accumulator is held at zero after startup until
either the outside ambient temperature is less than or equal to 24°C, or the saturated condenser
temperature is greater than the condenser target less the active stage down deadband.
VFD Condenser pressure trim control is accomplished using an optional VFD on the first fan. This VFD
control should vary the fan speed to drive the saturated condenser temperature to a target value. The
target value is normally the same as the saturated condenser temperature target.
■ VFD State
The VFD speed signal should always be 0 when the fan stage is 0.
When the fan stage is greater than 0, the VFD speed signal should be enabled and control the
speed as needed.
■ Stage Up Compensation
In order to create a smoother transition when another fan is staged on, the VFD compensates by
slowing down initially. This is accomplished by adding the new fan stage up deadband to the VFD
target. The higher target causes the VFD logic to decrease fan speed. Then, every 5 seconds,
0.1°F is subtracted from the VFD target until it is equal to the saturated condenser temperature
target set point. This will allow the VFD to slowly bring the saturated condenser temperature back
down.
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