On the total power factor of lift systems

摘要

The term "Total Power Factor" defined as the ratio between the active power and the apparent power (consisting of active, reactive and distortion power) of an electrical system has been used for years to describe how good an electrical supply system is utilized. To a single-phase system, it is well defined. To a balanced three-phase supply system, there is no problem in evaluating it because any voltage between two lines and any line-current could be used to find out the apparent power. To an unbalanced three-phase supply without a neutral wire, like that fed to a modern variable voltage variable frequency lift drive, an issue has arisen. This issue was raised by the first author of this article some 16 years ago. The active power can always be accurately measured by the famous "two-wattmeter method" but which line-voltage and which line-current should be chosen to estimate the apparent power becomes a big issue. The issue is now considered significant, because it was recently required in the mandatory Building Energy Code enforced in Hong Kong. In this article, various existing definitions have been studied and a couple of new definitions have been proposed. Based on cases of computer simulation, one final recommendation has been made, which is simple, straight forward, practical for use on-site and reasonable. It is hoped that this article could solve the problem raised by the first author published in year 2000 in BSER&T and practitioners may find it easy to implement this new definition. It is agreed that such a problem is mainly faced by the Building Energy Code assessors in Hong Kong. Practical application: The measurement of total power factor is a statutory requirement in Hong Kong and is a useful parameter used around the world. Based on site measurements, it is an inherent fact that the currents of the three phase lift systems are not perfectly balanced, whereas the voltages are more or less balanced. And owing to the absence of a neutral wire of the three phase supply to a lift system, it is impossible to measure such total power factor by using the conventional definition. If a neutral wire exists, it is not a problem anymore as the total power factor of each phase can be independently measured. Various definitions have been reviewed and one new definition is suggested that could solve this problem. Lift engineers could easily do the job by adopting this new definition, which is verified by simulation and site measurement.