Fisher & Paykel Limited

John Henry Boyd

Gerald David Duncan

The present invention provides an improved design of linear motor as well as an improved control strategy. The design allows for a shorted stator, where the armature magnets are controlled to reciprocate to a greater maximum displacement that for a equivalent conventional linear motor. The control strategy is such that a minimum of external sensors are required. The linear motor is driven at its resonant frequency ensuring optionally efficient operation. A determination of the maximum current is made based on a relationship with the resonant frequency and the evaporating temperature/pressure of the vapor entering the compressor. The current is then limited to control the maximum displacement to avoid damage.

Linear motor

An electric linear motor for driving a reciprocating load includes a stator, a coil, an armature and a commutation circuit. The stator has a magnetically permeable core with an air gap. The coil is wound around a portion of the stator and energized with a non-constant voltage for producing a non-constant magnetic flux in the stator and the air gap. The armature supports a permanent magnet having a substantial portion located in the air gap. The interaction of the magnetic field of the magnet and the non-constant flux in the air gap produces a force on the armature. The armature is connected to the load and reciprocates with respect to the stator. The circuit controls the coil such that at least one end of the magnet passes outside the region of substantially uniform flux density within the air gap during a portion of the motion of the armature.

The present invention provides an improved design of linear motor as well as an improved control strategy. The design allows for a shorted stator, where the armature magnets are controlled to reciprocate to a greater maximum displacement than for a equivalent conventional linear motor. The control strategy is such that a minimum of external sensors are required. The linear motor is driven at its resonant frequency ensuring optionally efficient operation. A determination of the maximum current is made based on a relationship with the resonant frequency and the evaporating temperature/pressure of the vapor entering the compressor. The current is then limited to control the maximum displacement to avoid damage.

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Holland, MI, United States of America

John Henry Boyd