Comparison Of Two- And Four-Pole Vsd Motors Up To 4000 RPM

Abstract

Electric motors were originally developed for Directly-on-Line (DOL) operation. The speed of a motor was determined by the number of poles. A two-pole motor was used to reach 3600 rpm with 60 Hz grid frequency, and the lower speeds were achieved by higher pole numbers. Later on, the frequency converters were introduced to offer variable speed operation with Alternating Current (AC) motors. This was realized by a frequency converter with rated output frequency around 60 Hz and a slightly modified DOL motor. Even today, this original approach has remained as predominant practice with electric drives. However, this is not necessarily the optimal solution for Variable Speed Drives (VSD). One alternative solution for speed ranges of traditional two-pole motors is a four-pole motor. The larger number of poles is com- pensated by the higher supply frequency. Today, this two-times-higher frequency can be achieved by most Low-Voltage (LV) and increasingly by standard Medium-Voltage (MV) frequency converters. The main aim of this tutorial is to compare the two- and four-pole motors in VSD applications up to 4000 rpm. The comparison starts by describing the characteristic features of these motors. After that, the pros and cons will be dissected based on the physical features and example motors. Finally, the pros and cons of the four-pole design are evaluated from the system point of view using the traditional two-pole concept as the reference. The comparison indicates strongly that the four-pole motor concept is superior in VSD applications. The most remarkable advantages are obtained in large MV electric motors. Thus, the increased application of the four-pole concept seems to be dependent on the end-user approval and open-minded thinking by motor and converter manufacturers. In essence, this new optimum is based on a step-wise modification on both sides.