1.2
General description
The EXT250M Turbomolecular Pump is a multi-stage axial-flow turbine, optimised for
operation in molecular flow conditions. The internal structure of the EXT250M Turbomolecular
Pump is shown in Figure 1. In the following description, item numbers in brackets refer to items
shown in Figure 1.
The multi-stage light alloy turbine rotor (22) is machined from one piece to form rows of angled
blades fitted to a central shaft (3). The blades of the rotor rotate between the stator blades. The
stator assembly has bladed half-discs (21) separated by spacer rings (20). The blades are angled
so that the gas from the vacuum chamber is compressed and is transferred to the pump-outlet.
The rotor and stator blades have an open structure at the pump-inlet and a more closed structure
at the outlet. This configuration gives an optimum combination of pumping speed and
compression when the pump is operated with gases of both high and low molecular mass.
The rotor is driven by a high-efficiency, brushless d.c. motor. The motor has a magnetized rotor
(7) fitted onto the shaft, and a wound stator (19) located in the pump-body. For the blades to be
effective, their speed must be close to the thermal velocity of the gas molecules. The rotor is
therefore rotated at up to 60000 r min
-1
. during pump operation.
The rotor assembly is supported radially at both ends of the shaft by frictionless, passive
magnetic bearings (4, 13). Both bearings incorporate resiliently-sprung mass dampers (12). An
active electromagnetic bearing (14, 17), controlled by a drive circuit in the EXC300M Controller,
provides axial support and stability. Sensor coils (16) detect any axial movement of the rotating
armature (15) and cause the drive circuit to modulate the current in the drive coils (10, 18) to
counter the movement.
The pump has safety bearings (5, 8). These safety bearings are precision ball-bearings which
limit the displacement of the rotor assembly if excessive mechanical shocks occur when the
pump is in operation. The bearings also support the rotor when the EXC300M Controller is
switched off and in emergency operation caused by a vacuum accident or disconnection of the
electromagnetic bearing drive.
The EXT250M is available with either an ISO collar or a CF inlet-flange. An inlet-screen (2) is
fitted in the bore of the inlet-port. The inlet-screen protects you from the sharp blades and also
prevents the entry of debris into the pump.
The pump has a vent-port which you can use to vent the pump and your vacuum system to
atmospheric pressure. The vent-port introduces the vent gas part way up the pump rotor to
ensure maximum cleanliness. The pump is supplied with a manual vent-valve (6) and a VRX10
vent-restrictor (23) fitted to the vent-port. As described in Section 3.4, you can remove or replace
the vent-restrictor and you can replace the manual vent-valve with a TAV solenoid-operated
vent-valve (available as an accessory: see Section 7).
The EXT250M pump has a purge-port (Figure 3, item 4) in the motor and bearing housing
chamber. You can introduce an inert purge gas through the purge-port to protect the bearings
and the motor from chemical attack by the gases pumped.
You can fit an optional
purge-restrictor to the purge-port to control the flow rate of the purge gas and to filter the gas
supply (see Section 7). The purge-port is fitted with a VRX10 vent-restrictor; this prevents
accidental venting by the purge gas. You can remove the vent-restrictor as described in
Section 3.5.1.
2
EXT250M Turbomolecular Pump
