Hitachi NEXT Series Service Manual page 13

Oil-free screw compressor
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2.3.2 Fix speed type (AUTO Operation)
AUTO operation is the Auto start - stop function in
addition to a standard on-off line, 0% or 100% capacity
2-step control. The CPU on the PC board is continually
calculating a load ratio.
STARTING
Suction throttle valve (STV) closed
START button ON
Airend starts rotation to
compress minor amounts of air
Starts in no load
Changes to full voltage
delta connection
3-way solenoid valve (20Ux)
switched over to connect
Chamber (A) & Chamber (C)
Suction throttle valve (STV) opens
Loading
What is the plant
compressed air piping
Less
pressure ?
than
specified
More than specified
UNLOADING
Air demand decreases
Discharge pipe pressure rises
Pressure sensor (PS) operates
3-way solenoid valve (20Ux)
switched over to pressurize
Chamber (A)
Suction throttle valve
(STV) closes
Blowoffvalve opens
Internal air in discharge
pipe is exhausted to
atmosphere
How long has
motor been
running ?
10 minutes or more
Load rate ?
Less than 30%
Has motor stopped and
restarted 3 times or more for
90 minutes ?
NO
Motor stops automatically
How long has
motor been
stopped ?
15 sec or
more
Less than
15
seconds
Air-Cooled
Water-Cooled
When the load ratio is less than 30%, CPU stop the main
motor. And automatically restar t when the line
pressure is dropped to cut-in pressure.
RELOADING
STOPPING
Air demand increases
STOP button ON
3-way solenoid valve (20Ux)
Discharge pipe pressure drops
switched over to pressurize
Chamber (A)
Pressure sensor (PS) operates
Suction throttle valve
3-way solenoid valve (20Ux)
(STV) closes
switched over to pressurize
Chamber (B)
Blowoff valve opens
Suction throttle valve
(STV) opens
Internal air in discharge
pipe is exhausted to
atmosphere
Blowoff valve closes
Compressor stops
Loading
Less than 10 minutes
30% or more
YES
If the third stop is done after 70
minutes or more has passed
after the first stop, the motor
c o n t i n u e s t o o p e r a t e f o r 2 0
minutes after restart of the air
compressor, and then return to
the initial cycle.
11
Load-Unload Operation
AUTO Operation
STV: Suction Throttle Valve
20Ux: 3-Way Solenoid Valve

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