Wet runner

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As a wet rotor will pump units (pump plus drive) denotes whose rotating parts inclusive of the rotor or the rotor of the driving electric motor to rotate in the liquid conveying medium which can be used for cooling and storage. The prerequisite for this is a separating pipe in the gap between the rotor and stator. Such drives therefore do not require any shaft seal. Wet runners or canned motors are available based on the principle of the asynchronous motor (often shaded pole motors ) or as so-called magnetic motors based on the principle of the reluctance or synchronous motor . Motor concepts that require a directly energized rotor, such as the single-phase series motor , are ruled out.

They are used z. B. as a circulation pump and drain pump ( washing machine , dishwasher ), but also as a pre-feed pump in fuel tanks . The great advantage of the lack of a seal or the hermetic separation from the pumped medium also led to applications as a pump for chemicals and food. Rotary drives in a vacuum are also related, for example for rotating anodes of X-ray tubes .

The wet runner was invented by Benjamin Graemiger , who applied for a patent in 1914.

The main problem was initially to find a suitable non-magnetic material for the canned can of the shaded pole motor. Stainless, non-magnetic steel was only available from the 1930s. It was launched in 1935 by two Swiss companies.

In the case of a magnet motor, a permanent magnet rotor located on the shaft rotates in a plastic tube enveloping the liquid between the poles of an alternating current operated stator magnet, usually with only two poles (recognizable by a latching every 180 ° when it is turned by hand). No direction of rotation is preferred for these magnetic motors, so the pumps must be designed with symmetrical rotation. The run-up takes place spontaneously by swinging back and forth until the runner takes a step. The efficiency is significantly better than with the shaded pole motor, but it has a higher bearing wear.

There are pumps with point bearings and pumps with slide bearings . In pumps with point bearings, the shaft is clamped between two journals, whereby the bearing slowly wears out and has to be readjusted. In hydrodynamic plain bearings, the shaft floats on a film of water so that no wear occurs. Both versions require very hard bearing materials such as hard metal or oxide ceramics , since mixed friction with increased wear occurs at low speed, for example when starting up.

Wet rotor circulators are a special type of robust asynchronous motors with rotors in the form of water-cooled squirrel-cage rotor. Because of the larger compared to standard motors gap distance, the rotational speed of a pump with a two-pole motor instead of 2900 falls when loaded heavily, so that, for example, min -1 a nominal speed of 1850 min −1 .

Once modern wet runners are wet, they should stay wet. If the gap between the shaft and the plastic pipe dries, impurities in the gap can stick the shaft and the pipe so that the shaft cannot rotate again the next time it tries to pump.

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