Operating instructions

IMM AGSB-60 69
circuits. Although fans 5/6 and 7/8 are controlled by one contactor each, more stages are
created by using virtual stages. See the table below:
Table 32, Staging with VFD
Stage Fans On
1 1,2,3
2 1,2,3,4
3 1,2,4,5,6
4 1,2,3,4,5,6
5 1,2,3,5,6,7,8
6 1,2,3,4,5,6,7,8
Staging Up
There are four stage-up deadbands that apply to the fan control stages. Stages one through
three use their respective deadbands. Stage four to eight share the fourth stage-up deadband.
When the saturated condenser temperature is above the Target + the active deadband, a Stage
Up error is accumulated.
The saturated condenser temperature must not be falling for a Stage Up accumulation to occur.
Stage Up Error Step = Saturated Condenser Refrigerant temperature – (Target + Stage
Up deadband)
The Stage Up Error Step is added to Stage Up Accumulator once every Stage Up Error Delay
seconds. When Stage Up Error Accumulator is greater than the Stage Up Error Setpoint,
another stage is added.
When a stage up occurs, or the saturated condenser temperature falls back within the Stage Up
deadband, the Stage Up Accumulator is reset to zero.
Forced Fan Stage At Start
Fans may be started simultaneously with the compressor based on outdoor ambient
temperature. When the compressor starts, a fan stage is forced, based on the following table.
Table 33, Forced Staging
Outside Air
Temperature
Fan Stage At Start
> 75
o
F Forced FanTrol 1 Set Point
> 90
o
F Forced FanTrol 2 Set Point
> 105
o
F Forced FanTrol 3 Set Point
Staging Down
There are four Stage Down deadbands. Stages one through three use their respective
deadbands. Stages four to eight share the fourth Stage Down deadband.
When the condenser saturated refrigerant temperature is below the Target – the active
deadband, a Stage Down error is accumulated.
Stage Down Error Step = (Target – Stage Down deadband) − Saturated Condenser Refrigerant
temperature
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 Setpoint, another stage of condenser fans turned off.
When a stage down occurs, or the saturated temperature rises back within the Stage Down
deadband, the Stage Down Error Accumulator is reset to zero. The accumulator is also held at
zero after startup until either the outside ambient temperature is less than, or equal to 75°F
(23.9°C), or the saturated condenser temperature is greater than the condenser target, less the
active stage down deadband.