8
CMC TECHNICAL REFERENCE MANUAL
1X36003 Version 2.52
1996-1999 Ingersoll-Rand Company
Date of Issue: 18-Oct-1999
Figure 6 shows the
relative valve response
over time for two
combinations of
Pb
and
It
.
As shown, when
Pb
is low
and
It
is fast, valve activity
is significant in both
magnitude and frequency
to obtain the desired set
point. The other scenario,
Pb
is high and
It
is slow,
has relatively little valve
activity, and may never
reach the set point
position.
Proportional Band and
Integral Time are
variables used internally
by the control system to
determine valve response
and direction for a given
compressed air system. Each has an
optimum value based upon the
system's characteristics. Determining
these optimum values is a trial and
error exercise. These set points should
be re-evaluated any time there is a
major change in the compressed air
system.
Up to this point, constant pressure
control has been accomplished with an
analog input (system pressure) and two
analog outputs (inlet valve and bypass
valve position). How is motor current,
the other analog input, used for
constant pressure control? When does
the bypass valve modulate as opposed
to the inlet valve?
Motor Current, MinLoad and MaxLoad
Motor current, in units of power
(normally amps), has two functions in
the CMC. The first is over current
protection for the main motor, and is
referred to as MaxLoad or High Load
Limit (
HLL
). The second function
determines the point at which the
bypass valve begins to modulate for
Time
Valve
Activity
Proportional Band - Low
Integral Time - Fast
Proportional Band - High
Integral Time - Slow
Closed
Opened
Set Point
Figure 6: Proportional Plus Integral Control
Discharge Pressure
Power at Coupling
Capacity - Mass Flow
MinLoad
MaxLoad
T
hot
T
cold
Figure 7: MinLoad and MaxLoad
