Electrical connection between the EXT250M and the EXC300M Controller is by a 19-way
connector and a pump-to-controller cable (supplied).
The EXT250M may be cooled by natural convection to the surrounding air, by forced-air cooling
(with an optional air-cooler accessory), or by water cooling (by the flow of cooling-water through
the water-cooler). The water-cooler has two riffled hose connectors for connection of your
cooling-water supply and return pipelines. A thermal sensor monitors the temperature of the
motor and the pump.
1.3
Vent options and vent control
To maintain the cleanliness of your vacuum system, we recommend that, whenever you switch
the pump off, you vent the pump (or vacuum system) when the speed of the EXT pump is
between full rotational speed and 50% of full rotational speed. At and above 50% of full
rotational speed, the rotor spins fast enough to suppress any backstreaming of hydrocarbon oil
from your backing pump.
If you vent the pump when it is at full rotational speed, you must limit the rate of pressure rise to
avoid damage to the safety bearings. A VRX10 vent-restrictor is fitted to the EXT pump to limit
the rate of pressure rise and allow you to open the manual vent-valve when the pump is at full
rotational speed. The VRX range of vent-restrictors (see Section 7.4.8) have fixed orifices, so
venting time will depend on the volume of your vacuum system. If you use a VRX vent-restrictor
to vent the pump, the time required to vent to atmospheric pressure may be long, particularly if
your vacuum system has a large volume. You can change the VRX vent-restrictor to suit the
volume of your vacuum system to minimise venting time: refer to Section 3.4.
If you wait for the EXT pump speed to fall to 50% of full rotational speed before you open the
manual vent-valve, you do not need a vent-restrictor. If you cannot fit a vent-restrictor (for
example, if you vent through a suitable flange on your vacuum system), you must only open the
vent-valve after the speed of the EXT pump has fallen to 50% of full rotational speed. If you use a
TAV vent-valve controlled by the EXC Controller, configure the Controller to select this option:
refer to Section 3.4 for more information. The EXC Controller is factory set to vent when the EXT
pump is at 50% of full rotational speed.
1.4
Safety bearings
The safety bearings of the EXT250M pump are dry-lubricated and have a limited useful life. In
normal use, the safety bearings will survive several thousand operational cycles, including
Start/Stop, electrical supply failure, external shocks and controlled venting (see Section 4.3).
If one of the accidents listed in Table 1 occurs, this will reduce the remaining life of the safety
bearings
4
EXT250M Turbomolecular Pump
