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A cost effective power meter for lift systems based on a novel definition of total power factor

Most lift systems are fed by a 3-Phase-3-Wire (3P3W) supply with unbalanced line currents and slightly unbalanced line voltages, as experienced by the authors. Then, power meters available in the market may not give a consistent total power factor (TPF), one important electrical parameter to describe how well the supply feeds its load for lift systems because existing definitions have not been developed to take care of such situations. The main problem is related to the estimation of the apparent power, S, of an unbalanced 3P3W system. A new definition mainly for 3P3W lift systems was previously proposed from the supply perspective, with a new method to convert the measurable three line voltages into hypothetical 3-phase voltages of such 3P3W system. This definition was recently adopted by the Hong Kong Special Administrative Region government in the latest building energy code. In this article, the following contributions are made. The method of conversion was shown to bear a special meaning in geometry, where the sum of magnitudes of the three hypothetical phase voltages is minimum. A new definition of TPF from the load point of view was proposed and compared with that from the supply point of view. For the sake of statutory implementation of the definition in Hong Kong, a new digital power meter specialized for a lift or escalator system was developed. Such power meter is of low cost and optimal design due to the use of only four channels of data sampling versus the six channels of meters commonly available in the market.