Nederman 40103102, User Manual

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VAC 20
3 Description
VAC 20 is a series of vacuum units that operate for air-
flow up to the level specified in
Section 3.3 Technical
data .
Units are delivered with different capacities, voltages,
and frequencies. The motor is a 3-phase asynchron-
ous motor. The motor power matches the unit’s ca-
pacity. The vacuum source is a belt-driven high-pres-
sure fan. The power consumption of the fan increases
with increasing airflow. It is necessary to minimise the
power requirement during Y/D - starting. This is done
by restricting the airflow while the motor is operating
in Y-mode.
VAC 20 units have a start-up valve at the fan inlet. The
valve is, apart from a small leakage flow, closed when
the unit is at standstill and during start-up in Y-mode.
The valve will open when the motor switches over to
full power in D-mode. The valve is controlled from the
unit’s start and control unit.
See Section 4.6 Bearing temperature switches re-
garding the bearing overheating cut-out on VAC 20.
See
Section 4.7 Optional: Compressed air switch re-
garding compressed air switch.
3.1 Anti-surge control
A high-pressure centrifugal fan operating with too
small an airflow will run in a surge. This means that
the operation is not stable. A characteristic ‘pumping’
or ‘breathing’ sound can be heard and the airstream at
the fan outlet will be uneven. The vacuum generation
is unstable and this can, under certain circumstances,
cause the ducting to move in rhythm with the pump-
ing.
The motor current is closely related to the airflow
through the fan. By monitoring the current by means
of a current transformer in the start and control unit,
it is possible to determine whether the flow is small
enough to cause a surge. If this is the case, a valve in-
side the vacuum unit will gradually open to let some
more air into the fan.
There are two versions of VAC 20 with the ASC func-
tion. One is controlled with the current transformer
and the current sensing relays. The other version is
controlled by the start and control unit and has the
text ASC by PLC on the base label.
3.2 Pressure drop diagram
See Figure 14 .
A VAC 20 - 1500
B VAC 20 - 2500
C VAC 20 - 3000
D VAC 20 - 4000
EN
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