Hitachi NEXT Series Instruction Manual page 81

8. STANDARD COMPONENTS AND SUBSYSTEMS [Capacity Control System]
① CPCS Motor Speed Control
Based on the pressure signals detected by the pressure
sensor, the built in circuit board's CPU performs PID
operations or calculates the motor speed that meets the
compressed air consumption rate, and then inputs the
calculated results to the inverter. The inverter outputs
the instructions of the corresponding frequency to the
motor. The discharge pressure is controlled within a
targeted range due to the increasing and decreasing
speeds of the motor.
Pressure
② Two-Step Purge Control
Two-Step Purge Control is activated if the following
conditions occur at the same time.
・ T he operating frequency drops to 78 Hz (equivalent to
50% of the air consumption).
・ T he pressure detected by the pressure sensor rises to
a cut-out (purge-start) pressure.
The blowoff solenoid valve (1) operates to start purging
the compressed air (contained between the airend and
the check valve) to atmosphere. The system decreases
the operating frequency to its lowest operating
frequency, 66 Hz, thus saving power. When the pressure
detected by the pressure sensor drops to a cut-in (purge-
stop) pressure, the system increases the frequency to 78
Hz and the blowoff solenoid valve (1) operates to stop
the purging, returning the system control back to CPCS
Motor Speed Control.
(Air-cooled 75 kW/100 psi (0.69MPa) Types)
③ Motor Stop/Restart Control
Motor Stop/Restart Control is activated under one of the
following conditions.
・Air consumption decreases to 5 %
・Purge operating continues for 3 minutes.
The system then automatically stops the motor. For
protection, the motor can only be stopped once every 10
operating minutes. When the pressure detected by the
pressure sensor drops to a cut-out (purge-stop) pressure,
the system automatically restarts the motor, returning
the system control to CPCS Motor Speed Control.
IMPORTANT
(1) Once the motor has stopped, it cannot restart for 15
seconds.
(2) The plant's system pressure may drop drastically
before the motor restarts, 15 seconds later. To avoid
such problems, install an air receiver tank of 400 USgal
(1.24 m ) or larger.
3
CPCS Motor Speed Control by Inverter and CPU
P: proportional
Inverter
I : integral
D: differential
IMPORTANT
PID stands for Proportional, Integral, and Differential.
CPCS stands for Constant Pressure Control System.
Compressed
DSP
Motor
air
Pressure sensor
IMPORTANT
The differential range between the Cut-out pressure and
the Cut-in pressure must be 7 psi (0.05 MPa) or more.
Pressure
①
Cut-in Pressure
Control Pressure
Cut-out Pressure
Q＞50%
or
Hz＞78
Operation Frequency Hz
(Q≦45%)
Q＞5%
T＜3 minutes
① ②
CPCS Motor Two-Step Purge
Speed Control Start/Stop Control
100
①
②
0
The values are for air-cooled, 75 kW, 100 psi (0.69 MPa) type.
Constant Pressure
of the Discharge Air
② ①
Minimum
7psi
Time
Q ≦ 50%
or
Hz≦78
Q≦5%
or or
Air Consumption Q
T≧3 minutes
or
Purging Time T
Number of
N＞1
Motor Stopping N
(for 10 minutes)
N≦1
③
Motor Stop/Restart
Control
② ②
③ ③
Minimum
1psi
79