Study on Iron Loss in Two Kinds of Moving-magnet Linear Motors

Linear motors are widely used in cryogenic fields to drive Stirling-type cryocoolers. Generally the Joule heat loss associated with copper lead is considered as the main loss in motors while the iron loss is ignored at a lower frequency operation, not only because it’s regarded as small enough but also it’s difficult to measure or calculate. This paper introduces an experimental method to measure the iron loss for linear motors. Two kinds of movingmagnet linear motors have been tested, one is known as single coil and magnet linear motor, and the other as double coils and magnets linear motor. Dependence of iron loss on frequency and displacement are investigated. Results show that there is a 1.6 power exponent relation between iron loss and frequency in the single-coil movingmagnet linear motor, and its iron loss come to be the same as copper loss at 75 Hz, it even grows to 1.65 times of copper loss at 100Hz. As for the double coils and magnets linear motor, there is a 1.3 power exponent relation between iron loss and frequency, and its iron loss percentage is only 15% at 120 Hz compared with 68% in the single-coil linear motor. Iron loss is proved to be significant in linear motors especially at higher frequency.