Emerson Vission 20/20 Operation And Service Manual page 200

Appendix B • Vission 20/20 Application Procedures
Modbus Register 40502 - Remote Capacity
Control Selection
• 0 = Auto Capacity Control. This selection defi nes
that the 20/20 will control compressor capacity from
its internal Control Setpoints.
• 1 = Pulse Load/Unload. This selection defi nes that
the 20/20 will control compressor capacity from
contents of Pulse Load register 40504 and Pulse
Unload register 40505.
• For correlation between register content and
pulse value, see Table B4.
• 2 = Hold Capacity %. This selection defi nes that the
20/20 will control compressor capacity from con-
tents of Capacity Hold % register 40506.
If Hold Capacity % is selected, then it is typical to write a
Capacity Hold value to register 40506 before the com-
pressor is started, typically 5%, to prevent the compres-
sor from loading immediately.
Modbus Register 40506 - Capacity Hold %
Value = 0-100
Hold Capacity Operation
• Capacity Hold commands defi ne a "target" capacity
slide valve position for the compressor. The 20/20
will position the capacity slide to the "target" posi-
tion. rules of this capacity hold algorithm are;
1. If the new target is < 0.4% (full scale) away from the
current position – then don't do anything – this is
the deadband region.
2. If new target is > deadband region but < 10% (full
scale) away from the current position, then en-
ergize the capacity slide for a time that is propor-
tional to the amount it is away from the new target
position. To say it another way – the further you are
away from the new target position – the longer the
slide is energized in the proper direction. This con-
trol region is called the proportional band region.
3. If new target is > 10% (full scale) away from current
position (greater than the proportional band re-
gion) – then energize continuously.
There are mechanical properties that limit the speed at
which the capacity of the compressor can be changed.
The capacity slide actuators can only turn so fast. The
proportional change to the compressor capacity occurs
at a fi xed rate.
Caphold and Operation with VFD
B - 44
• Using a VFD with a compressor requires considering
the VFD capacity as part of the entire capacity of
the compressor. Typically, a VFD is operated from
50% to 100% speed, therefore the VFD is considered
½ the total capacity and the slide valve movement
is considered the other ½ of total capacity. When
a VFD is employed, the normal control method is
to fi rst move the capacity slide from 0-100% when
additional refrigeration is required. When the capac-
ity slide is at 100% and additional capacity is still
required, then the VFD is ramped up in speed. It
follows then that a Caphold value of 25% will move
the capacity slide to 50% position. A Caphold value
of 50% will move the capacity slide to 100% position.
A Caphold value of 75% will move the capacity slide
to 100% and the VFD to 50% speed... and so on.
Consideration should also be given that when moving
the capacity slide valve from 0-100%, the actual corre-
sponding capacity of the compressor is not changing in
a linear relationship of 0-100%. The last 15% travel of the
slide valve results in a greater change of capacity than
15%. Integrators should realize that the caphold value
sent relates to slide valve "position" and not actual ca-
pacity of the compressor. For most purposes however,
assuming a linear relationship is adequate.
The rate at which the capacity slide moves from 0-100%,
and the rate at which the VFD increases speed from min-
imum to maximum is not the same. So there are two
different capacity profi les that the integrator needs to
consider.
Modbus Register 40507 - Active Setpoint.
This register is used in conjunction with Register 40502
= 0, Auto Capacity Control mode.
1 = Setpoint #1 Active
2 = Setpoint #2 Active (note: you must enable two
setpoints from confi guration screen fi rst).
Sometimes compressors are switched from Suction
Pressure control mode to Process Temp control mode or
vise versa. This can be done via communications using
the following register.
Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 2.0