2: Description 3000PB, 3700B, 4500B, 3000PVB, 3700VB, 4500VB User Manual
02250216-786 R01
18 Subject to EAR, ECCN EAR99 and related export control restrictions.
2.8 Variable speed drive (VSD)
components
The VSD, located in the machine’s electrical enclosure,
works in concert with the controller to allow the compres-
sor to match its output to the current demand of the sys-
tem. The drive’s heat sink extends through the back of
the
enclosure, and its cooled by air flowing through the
compressor enclosure.
2.9 VSD control system, functional
description
Refer to Figure 2-5. The controls consist of:
• a VSD
•
a solenoid valve
• a regulating valve
• an inlet valve
Depending on the model, a compressor can be operated
a
t a setpoint pressure from 60 to 175 psig (4.1 to
12
.1 bar). The controller automatically sets the frequency
r
ange based on the selected pressure. (The compres-
sor's operating range is on its nameplate.)
The following paragraphs apply to a compressor with a
10
0 psig (6.9 bar) operating pressure and a 6 psi
(0
.4 bar) load delta setting.
Compressors with different pressure operating ranges
pe
rform in the same manner.
Start mode—0 to 50 psig (0 to 3.5 bar)
Pressing the controller (Start) pad signals the VSD
to accelerate the motor to fu
ll speed, causing the separa-
tor/sump tank pressure to
rise from 0 to 50 psig (0 to
3.4 bar). At this time both the pressure regulator and
sole
noid valves are closed; the inlet valve is fully open
and the air-end provides a full flow to the separator/sump
tank. A minimum pressure valve set at approximately
50 psig (3.4 bar) isolates the rising compressor air pres-
sure from the service line.
Full load mode—50 to 100 psig (3.4 to 6.9 bar)
When the compressed air pressure rises above 50 psig
(3.4 bar) the minimum pressure valve opens allowing
comp
ressed air to flow into to the service line. From this
point on the controller monitors the line pressure which
controls the VSD. The pressure regulator and solenoid
valves remain closed with the inlet valve fully open, run-
ning at 100 psig (6.9 bar) or lower.
VSD part load control
The service line pressure increases to a value above
100 psig (6.9 bar) if the demand is less than the com-
pressor's rated capacity. In this condition, the VSD slows
the m
otor's rpm which reduces the output to match the
demand. The drive continuously adjusts the motor's rpm
to maintain a 100 psig (6.9 bar) line pressure.The con-
troller maintains the correct fr
equency when the VSD is
operating in this mode.
Modulating mode—100 to 106 psig (7.3 to 6.9 bar)
During low demand periods and with the VSD running at
minimum speed, the line pressure can continue to rise.
When the line pressure reaches 101 – 102 psig (approxi-
mately 7 bar), the regulator valve (Figure 2-5) gradually
opens, directing air pressure to the inlet control valve pis-
ton. This action causes the inlet valve
to partially close,
thereby reducing the air flow entering the compressor
until it matches the demand. The control system func-
tions continuously in this manner between less than
10
1 psig (7.0 bar) to 106 psig (7.3 bar), in response to
var
ying system demand. The pressure regulator has an
orifice which vents a small amount of air to the atmo-
sphere when the pressure regulator controls the inlet
valve. The
orifice also bleeds off any accumulated mois-
ture from the control line. When the discharge pressure
r
ises above 106 psig (7.3 bar), or to a preset unload
pressure, the
compressor unloads.
Unload mode—in excess of 106 psig (7.3 bar)
When there is no demand or it is at a minimal level, the
service line pressure continues to rise. When it exceeds
106 psig (7.3 bar), or reaches a preset unload pressure
valu
e, the WS control system de energizes the solenoid
valve allowing separator/ sump tank air pressure to be
supplied directly which closes the inlet valve. The sole-
noid valve simultaneously sends a pneumatic signal to
the
blow down valve which opens to the atmosphere, and
reduces the separator/sump tank pressure. The check
valve in the air service line prevents line pressure from
back-flowing to the separator/sump tank. The compres-
sor will shut down after the programmed unload time s
et-
ting expires (the default setting is zero (0) minutes for an
im
mediate shutdown upon unload). When the line pres-
sure drops to the low pressure setting of 100 psig
(6
.9 bar) the controller starts the motor and energizes the
sole
noid valve which closes the blowdown valve. The re-
NOTE
The load delta default setting is 10 psi (0.7 bar).
Sullair recommends a setting of 6 psi (0.4 bar)
fo
r the most efficient operation.
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