Eureka March 1998 Cover Story

BEARINGS - MAGNETIC

Attractive alternative to needles and rollers

Replacing steel balls or rollers with a controlled magnetic force offers a radical option to designers of bearing systems. Tom Shelley reports

A new generation of pumps and motors that use inherent magnetic bearings, suspending their rotors in free space, may never need to be replaced.

Unlike other magnetic bearings, these motors naturally suspend their rotors in their centres, with no separate bearings at the ends. Pump rotors can be suspended in their working fluids, like a space station in empty space; the absence of wear means they can last virtually forever.

The first applications are in the gentle pumping of fluids containing biological organisms and fluids too fragile to survive passage through mechanical bearings. They might also replace rotors in pumps in the wider field, especially those involving abrasive and corrosive fluids.

In conventional electric motors, one effect of the rotating magnetic flux is to pull the rotor towards the outer stator. If the rotor starts to approach the stator, this force increases. Hence, unsupported motor rotors are inherently unstable, and quickly end up rubbing against the insides of their stators.

Motors that keep their rotors centralised by magnetic force need active electronic control to counteract the natural instability. Development of this concept began at the Federal Institute of Technology in Zurich in 1987. In January 1995, Sulzer Electronics, together with the Institute and German company Lust Antriebstechnik started a further project to develop the technology for industrial applications.

In its present form, the motor has separate coils to turn the rotor round and maintain its central position. For motors driving pumps, the power electronic system comprises three conventionally switched, three-phase power converters and one special magnetic bearing converter. The conventional units are normal drive converters and are used on the one hand for the motor and on the other hand for the two radial magnetic bearings. The axial magnetic bearing is powered by the special magnetic bearing converter. Control is by two signal processors, one VeCon chip set for the drive system, and one Texas TMS320C50 80MHz processor for the whole bearing system.

The 540V DC links of the power stages are all connected to make use of the kinetic energy of the motor during power loss. If power fails, the motor behaves as a generator and maintains power to the magnetic bearings and the control unit until the motor stops.

In order to maintain control, it is necessary to measure both rotor position and magnetic flux. Inductive, eddy current and Hall effect distance sensors have all been tested for position measurement. Five are used in a pump system. To achieve immunity to radial rotor displacement, magnetic flux has to be measured differentially. Four sensors are needed with this method.

The first serious bearingless pump application is a canned pump unit for heart-lung machines. The pump element has to be removable and disposable. This is easily achieved by having a pump element which consists of only two parts, a disk rotor and a plastic casing.

The blood pump looks conventional, but the latest design to come out of Sulzer is radically different, taking advantage of the further possibilities of magnetically suspended impeller design.

In a pump for a bioreactor, the pump impeller consists of a ring running just inside the bioreactor casing. The magnetic driving and suspension coils are on the outside.

One of the main functions of a bioreactor is to feed growing cells with oxygen. For a cell density of 10million cells/ml, it is necessary to have an aeration rate of 150mg of oxygen per hour. Studies show that bubbles bursting in the foam region lead to cell death. The Institute of Process Engineering of the Swiss Federal Institute of Technology in Zurich has developed a bubble bed reactor. Instead of air being admitted at the bottom, and bubbling up to the top, the working fluid of water and cells is circulated downwards in the centre of the reactor, so that the downward flow of fluid counteracts the tendency of the bubbles to rise upwards.

Compared to conventional systems, the bubbles spend much more time in contact with the fluid, and oxygen uptake is increased more than 10 times.

If the reactor had a conventional pump impeller at its base, the flow field in the down comer would be likely to be relatively inhomogeneous, releasing some of the bubbles. An axial upwards flow round the periphery, on the other hand, leads to a more homogenous flow field. Mounting and driving a ring shaped impeller from a central shaft causes problems, but if the impeller is driven and suspended magnetically, there is no need for a drive shaft.

Previously, Sulzer had only made magnetic bearing pumps with 45mm diameter impellers, but the reactor required a rotor with a bore 160mm across.

The prototype motor rotor has an outer diameter of 188mm and 24 rotor poles. Its total weight is 2.2kg. The stator has an inner diameter of 194mm, an outer diameter of 266mm and a height of 48mm. It has 4 poles. The reactor itself has a diameter of 200mm without the motor, a height of 720mm and contains about 18 litres. The system is able to capture bubbles in the bed at less than half the maximum speed of 300 rpm and requires a torque of less than 0.3Nm. The magnetic bearing system can handle axial loads of up to 53N and radial loads of 35N, much greater than is required in normal operation. The passive axial magnetic bearing has an axial stiffness of 22N/mm and tilting stiffness of 1.1Nm/deg. The power consumption of the whole system is 78W at the operating point.

The company has so far tested the system for more than half a year. During this time, the drive system has showed no failure or operating problem. So it has designed a range of flat motor/bearing systems, ranging up to a largest size with a rotor outer diameter of 500mm, and an inner diameter of 440 or 420mm. Maximum axial load for this size is calculated to be 220N; maximum radial load, 300N and maximum torque, 12Nm.

The complexity of the motor and bearing control system means that cost is higher than for conventional motors and pumps. The rapid rate of advance in electronic technology means it should soon be possible to put all the control logic on a single chip and build all the electronics into a single module.

Sulzer Electronics

Design Pointers

* Using magnetic suspension, it is possible to make motors and electric pumps with no mechanical seals

* Such pumps and motors should last virtually for ever, and are highly suited for pumping both delicate fluids, such as those containing living organisms, and those that are corrosive or abrasive

* Though relatively expensive due to their complexity, technical advance and higher volume sales are likely to make the products price competitive with conventional designs in several years

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