2.8.1 Design / Operating principle
The turbomolecular pump was developed and patented at Pfeiffer Vacuum in 1957 by Dr. W. Becker. Turbomolecular pumps belong to the category of kinetic vacuum pumps. Their design is similar to that of a turbine. A multi-stage, turbine-like rotor with bladed disks rotates in a housing. Interposed invertedly between the rotor disks are bladed stator disks having similar geometries.
Mounting the rotors by means of two ball bearings would be problematic, since the lubricants require that both bearings be arranged on the backing-vacuum side, and the rotor, with its high mass, can only be supported by bearings on one side (floating).
Hybrid bearing support offers advantages in this regard with respect to rotor dynamics. In this case, an oil-lubricated ball bearing sits on the end of the shaft on the backing-vacuum side, and the high vacuum side is equipped with a maintenance- and wear-free permanent magnetic bearing that centers the rotor radially. A small dry safety bearing is arranged within the magnetic bearing stator. During normal operation, a journal rotates freely within this bearing. In the event of strong radial shocks, it stabilizes the rotor and rotates only briefly. If the rotor is out of balance, the bearings on both ends of the shaft will generate significantly lower bearing-stressing vibration forces than in the case of a floating bearing. Moreover, this enables the larger of the two bearings on the drive shaft, whose size allows only limited rotor speeds, to be eliminated.
So-called 5-axis magnetic bearings are used in large pumps. The rotor is levitated axially through digital electronic control via distance sensors and electromagnets, and in two radial directions each at both shaft ends. Electronic balance compensation and the absence of mechanical contact between rotor and housing keep the vibration generated by the pump low. In addition to the absence of oil on the backing-vacuum side, as well, freedom from wear and maintenance is another advantage. In the event of a power failure, the magnetic bearings are supplied with electricity through the rotational energy of the pump.
This enables power failures to be easily bridged for several minutes. Should the power failure be of longer duration, the rotor will safely come to a stop at only very low speed through the use of an integrated safety bearing. If the electronics are defective or if there is significant ingress of air, the rotor will be braked without damage with the aid of safety bearings.
Motors / Drives
Brushless DC motors that afford rotational frequencies of up to 1,500 Hz (90,000 rpm) are used to drive the rotors. This enables the blade velocities that are necessary for pumping the gases to be achieved.
Today, the drives are typically attached directly to the pumps. Power that is generated by external power supply units is supplied at 24 VDC or 48 VDC. In the case of large pumps, the drives are supplied directly from the rectified mains voltage.
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