EUREKA NOVEMBER 1997 COVER FEATURE

Shuttling magnet ensures efficient gas compression

Tom Shelley reports on a low cost combined linear motor and gas compressor

An electric linear motor, whose single moving component that doubles as both armature and piston, promises radically improved gas compression efficiency.

Its simple construction incurs no side loads or friction penalties and requires no lubrication.

The prototype designs hope to replace present day conventional compressors with new types that can cope with 'greener' working fluids. But the basic principles can be applied to any kind of compression operation, including that on air.

In conventional refrigerator compressors, a rotating electric motor drives a reciprocating pump through a crank. The whole arrangement of motor and pump resides within a sealed enclosure and requires oil splash lubrication. The oil is recovered by condensation processes downstream within the refrigerator and runs back into the pump. While CFC and HCFC refrigerants require only low pressures, alternatives such as ammonia, propane and carbon dioxide require working pressures beyond the reach of conventional small compressors. Some recently developed non-CFC refrigerants work within the range of the conventional small compressors but are not compatible with conventional lubricants. So to make greener refrigerators, it is necessary to deliver a higher pressure or be able to work without lubrication.

Wei-Min Zhang, a patent agent living in North London, and Dr Ivor Day, at Cambridge University's Whittle Laboratory, have spent some time developing the concept of a very simple linear motor and compressor. The basic idea, as patented by Mr Zhang is simple enough: a permanently magnetised piston shuttles back and forth between two electromagnets powered by the 50Hz mains. But optimising it for maximum efficiency at a sensible production cost is another story. Several linear motor compressor designs are known for cooling systems in highly specialised applications, such as cooling infrared sensors in satellites. Such compressors are usually very small in capacity and not suitable for mass production.

The Zhang/Day concept has so far been through a series of designs, prototypes and modifications, aided by a 1996 DTI Smart Award. The latest design produces a maximum pressure of 9.5 bar at no flow, from an internal two stage compression, and delivers a maximum gas flow in excess of 3 litres/minute at 1 bar.

In its simplest embodiment, a piston magnetised as a bar magnet, with a North pole at one end and a South pole at the other, will happily move to and fro between two electromagnets energised by an alternating supply.

The inertia of the piston at the end of each stroke is absorbed by compressing either gas or a spring. This means that the applied frequency must coincide with the natural frequency of the piston, its return springs and gas cushions, damped by gas being drawn in and pumped out through non-return valves. If the applied frequency is the 50Hz mains, as in the version shown to Eureka, then the rest of the system must have a similar natural frequency.

In larger pumps, a lower excitation frequency might be used to match a lower frequency, or the arrangement can be made self controlling. In such a case, the motor could be driven by DC, switched on and off and reversed in response to the position of the piston detected by Hall Effect or other sensors, in the same way as in a brushless DC motor. High frequency DC powered units might find particular favour in motor vehicle air conditioning and refrigeration systems. The other aspect of the design, into which much effort has been put, is to make it electrically efficient by minimising magnetic flux leakage.

The armature is magnetised so that at its ends, one pole is in the centre and the opposing pole is around the outside, matching a similar arrangement on the faces of the electromagnets.

In the original design, the outer parts of the armature/piston, were to have been tapered, fitting over cone shaped outer pole piece sections of the electromagnets. This was designed to tackle the problem caused by the contradictory requirements of a long stroke for good gas compression and a small air gap for good electromagnetic performance. Such an approach would certainly be efficient, if special kit could be built for mass production of the magnet parts. Another idea seriously considered w

Mr Zhang has already considered how the design could be scaled up. For larger units, he thinks there would have to be at least two separate units coupled side by side, or end to end, working in counter phase in order to reduce vibration. For pumps running on a three phase supply, it might be possible to have three units, each running on a separate phase pair, mounted along the sides of a triangle.

The development has reached the stage where the refrigerator-sized prototype demonstrates what can be done, but shows some limitations concerning seal life and other matters. The team is still improving the design, while two major UK manufacturing companies have expressed support. But the next stage, of finely optimising the design for maximum efficiency and manufacturability, is the expensive one. For this reason, the team wants to talk to companies interested in licensing the technology, especially those that can help to commercialise the product by investing time and money.

Zhang and Day

Design Pointers

* Combined pump and motor design has only one main moving part which acts as both linear motor armature and compressor piston

* There are no side loads and no need for oil lubrication

* The basic simplicity of the design suggests that eventual manufacturing costs should be low

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