Anti-Surge Control; Main Components; Overview; Connections - Nederman VAC 20 Manual De Instrucciones

with increasing airflow. It is necessary to minimise the
EN
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
garding the bearing overheating cut-out on VAC 20.
See Section 4.7 Optional: Compressed air switch
garding compressed air switch.
3.2.1 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.
4 Main components
4.1 Overview
Figure 1 shows the main components of the VAC 20
unit. These are as follows:
1 Acoustic enclosure.
2 Motor.
3 2-stage high-pressure fan.
4 Start-up valve. SUV 200 which also serves as a
backflush valve. 24 V DC is supplied as standard
but a solenoid for 24 V AC is also enclosed.
5 Anti-surge valve. (TVS76, Control motor and silen-
cer).
6 VAC 20-1500/2500/3000: Flow restrictor FR 160,
see also Figure 4.
7 Manual reset thermal switch for the fan bearings.
8 Belt transmission.
The optional inlet silencer is shown in Figure 2.
16
VAC 20
1 Optional: Inlet silencer for extra low noise levels
indoors.
4.2 Connections
Figure 5 is a schematic diagram of the normal connec-
tions to a VAC unit.
tions to VAC 20. These are as follows:
1 Exhaust duct.
2 Vacuum pipe from the dust collector.
3 Dirt and water separator for compressed air. The
re-
separator is supplied with the unit.
4 6 mm (1/4") tube airline to start-up valve. The line
re-
is supplied with the unit.
5 Control cable for start-up valve, thermal switches
and ASC damper.
6 Optional: Control cable for when not using ASC by
PLC.
7 Motor power supply.
8 Optional: Maintenance switch. This is required in
most countries.
9 Start and control unit normally with Y/D-starting.
Direct starting is also an option.
10 Terminal box
11 Optional: ASC terminal box for when not using ASC
by PLC. Sold as an accessory.
4.3 Start-up valve
Figure 6 and Figure 7
control of the start-up valve. The solenoid valve V1
is engaged only after the start and control unit has
switched over to D-mode. The valve needs com-
pressed air to work. See the start- and control unit's
wiring diagram for connecting the start-up valve.
4.4 Flow restrictor FR 160
A flow restrictor, FR 160, is mounted close to the fan
outlet on most VAC 20 units, see Figure 4. The re-
strictor protects the motor from overloading by gradu-
ally closing a valve restricting the airflow.
The flow restrictor is fully mechanical. It consists of
a valve blade, Item 1, welded to a shaft, Item 3. The
shaft turns in ball bearings fitted to the housing, Item
2. The spring, Item 6, holds the blade in the normally
open position.
The spring keeps the blade fully open when the flow
is lower than the restrictor setting point. At the set-
ting point, the blade starts turning and closes more
and more as the flow increases. This result in a flow
restricted to a value corresponding to the nominal mo-
tor power. The spring is correctly adjusted before de-
livery of the unit. See
justment of the spring (if necessary).
The flow restrictor is fitted with a damper, see
ure 4 Item 4, to prevent the flow restrictor from self-
oscillating. It consists of a cylinder filled with oil. In the
Figure 5 shows the normal connec-
show a circuit diagram for the
Chapter 8 Maintenance for read-
Fig-