Hoffman Controls 890-DSQ Series Complete Installation & Operating Instructions page 5

Figure 2
Traditional Vs Variable Stage Operation
Operating Modes Con't
A traditional sequencer's operation is shown, as a reference, at
the top of Figure 2 (above). The 890-DSQ Sequencer's variable
stage operation is depicted, on it's own, in the center of Figure 2,
while the total sequencer's operation which produces a continuous
output, is shown at the bottom of Figure 2.
When the sequencer's fixed stages are activating, the variable
speed EC motor (or SCR) ramps from a low rpm (or current) to
full speed (or full current) before the next sequencer's fixed stage
turns on. As the next fixed stage turns on, the variable stage's
output immediately drops down the equivalent of one fixed stage.
During stage deactivation, when the previous fixed stage turns off,
the variable stage's output jumps up equivalent to one fixed stage.
As the input continues to decrease, the variable stage's output
ramps down equivalent to one fixed stage just before the, now
current, fixed stage turns off. This combination of fixed stage and
variable stage operation produces a continuously variable (fan rpm
or SCR current) output.
Without hysteresis, the variable stage's airflow capacity or SCR's
KW capacity would only need to be a little larger than a fixed A/C
stage's airflow or a fixed heater element's capacity, due to the elec-
tronic components variances in each fixed stage's turn and turn off
tolerances.
With hysteresis, the EC motor's airflow capacity, or SCR's KW
capacity, needs to be much larger than a single fixed stage's on and
off values. The extra capacity provides enough EC motor speed, or
SCR current variation, to allow continuous increases or decreases
in airflow without short cycling the fixed stages.
When the variable stage ratio is more than 1.0, the 890's micro-
controller uses the variable stage ratio's number as the number of
effective stages to be added to the number of fixed stages selected.
All of these effective stages are activated before the first fixed stage
turns on. The total number of activated, effective and fixed stages
is shown in the LCD Home screen's upper right corner.
Mathematical Formula
A mathematical formula can be used to calculate the size of
the variable stage's ratio number needed for smooth continuous
operation. This ratio number represents the comparison of the
VS (variable stage's) capacity (airflow or KW), to the fixed stage's
airflow or KW capacity. Typical ratios are 1.5 to 2.0 for 4 to 6
stage applications, respectively.
The installer may use the following formula to calculate the
minimum
size of the VS (variable stage) ratio:
Variable Stage Ratio = [1+ (the hysteresis value
Examples:
1) When the hysteresis is 6 °F and the difference between
stage turn on values is also 6 °F, then the
Variable Stage Ratio =[1+(6 °F hysteresis
2) When the hysteresis is 4 °F and the difference between
stage turn on values is 8 °F, then the
Variable Stage Ratio =[1+(4 °F hysteresis
For practical reasons, a VS (variable stage) ratio of 2.0 works
well for A/C condenser banks, containing 4 to 6 fan motors,
or heating systems containing 4 to 6 fixed heating
Use the UP and DOWN buttons to change the VS (variable
stage) ratio's number to match the ratio of the total VS (variable
stage) capacity to that of a single fixed stage. (The range is 1.0
to 12.0). Then press the ENTER button to store this value and
continue to the menu below.
3) Number of used output stages: 6
Use the UP and DOWN buttons to select the number
of relay output stages to be used. The range is 0 to 12
stages. When "0" is selected, only the VS (variable stage's)
EC motors or SCR operation is activated (None of the
fixed relay output stages are used when "0" is selected).
Press the ENTER button when the answer is correct.
5
by the
difference between stage turn on values)]
6 °F )] = 2.0
8 °F)] = 1.5
NOTE:
elements.