Metacentre VSD-R 4-20mA Box
Page 3
1.3 General Operation
The VSD-R Box, in conjunction with a
compatible VSD/VFD panel, will control the
speed of an air compressor’s main motor in
response to pressure variations (demand).
The VSD-R Box, VSD Panel and air
compressor combination can function as a
‘stand-alone’ VSD air compressor unit or
operate in conjunction with a management
system unit.
The VSD-R Box performs all pressure
regulation functions and controls the
compressor using load/unload commands (1).
This is achieved by integration of the
compressor unit’s pressure switch with an ‘i-
PCB’ or using the compressor controller’s
remote load/unload input. The air compressor
control system will respond in an identical
manner as if the VSD-R Box was a remote
regulation/sequencing unit.
Run operation of the main motor is derived
from the air compressor controller’s main
motor control outputs (2). The main drive
motor will only operate when the compressor’s
controller is requesting the main motor to run.
This functionality ensures all compressor
controller proprietary and safety features and
function, regardless of how they are
implemented or derived, remain intact.
The VSD-R Box controls the compressor’s
main motor contactor, or star/delta contactor
set, and will operate the contactor(s) in
response to a run request from the
compressor’s controller. The VSD-R Box
monitors the operation of the compressor
contactor(s) and will only operate the VSD
panel (5) to apply power to the compressor’s
main motor (6) when the contactor(s) have
energised correctly (4).
The compressor contactor(s) are used for
safety isolation only and will operate differently
than previous. The contactor(s) will energise
prior to power being applied to the main motor
and de-energise after power is removed from
the main motor. This functionality will
significantly prolong the service life of the
contactor(s) contacts; the contacts are no
longer subjected to high starting current flow
or high current disconnections during a motor
stop.
The compressor controller will be unaware of
the VSD-R Box control of it’s main motor and
will continue to respond as if the motor was
still under normal fixed speed control.
Any star/delta timing implemented in the
compressor controller can remain; the main
motor will use this time to accelerate to
operational speed.
Any motor run-on time implemented in the
compressor controller can also remain;
although in some instances it may enhance
efficiency to reduce this time.
The VSD-R Box monitors the compressor to
ensure that a response to a VSD-R Box load
request is actioned appropriately by the
compressor controller; a fault condition is
generated if correct response is not detected.
1
2
3
4
6
5
C
A
B
A) VSD-R
Box
B) VSD
Panel
C) Compressor Controller
When the compressor is loaded the VSD-R
Box will vary the speed of the compressor’s
main motor is response to pressure variations
(demand). When the compressor is unloaded,
but the compressor controller is still requesting
the main motor to run (run-on time), the VSD-
R Box will run the motor at a pre-defined ‘off
load’ speed regardless of pressure variation.
The VSD-R Box will stop the main motor, in a
controlled manner, when the compressor
controller no longer requests the motor to run.
This will occur when the VSD-R Box has
requested the compressor to ‘off load’ and the
compressor controller’s run-on time has
expired.
In an emergency stop, or trip, situation the
VSD-R Box will remove power from the main
motor and de-eneregise the compressor’s
contactor(s) immediately.
