A STUDY ON CURRENT RIPPLE OF PWM INVERTER-FED MULTIPHASE AC MOTORS

• Main Author: Muqorobin, Anwar
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/34863
• Institution: Institut Teknologi Bandung

Multiphase motor is an ac motor with number of phase more than three. It can be divided into two types, i.e. prime phase and multiple three phase motors. Until now, only multiple three phase motor that has been applied in industry because it can be supplied by several conventional three phase inverters. Besides that, the three phase sets configuration can be optimized to obtain some advantages i.e. minimum inverter input current ripple, stator current ripple, and rotor current ripple. Inverter input and rotor current ripples are more important compared to stator current ripple because the stator losses are easier to handle. For that reason, optimization in this dissertation has an aim to minimize the inverter input current and rotor current ripples. Inverter-fed ac motor can be operated under either squarewave or PWM modes. The PWM mode is used for low speed operation and squarewave mode for high speed operation. The optimal configuration that could minimize the rotor current ripple under squarewave operation is well known in the literature. Asymmetrical configuration is optimal for even phase motor. On the other hand, both asymmetrical and symmetrical configurations are optimal for odd phase motor. Furthermore, this dissertation shows that besides all the optimal configuration could minimize the rotor current ripple, it also minimize the inverter input current ripple. Further investigation is conducted to find the optimal configuration under PWM operation. In the inverter input current ripple analysis, sinusoidal output currents are assummed. For the output current ripple analysis, the stator currents are transformed by using special transformation to separate the current components that are useful for rotor current production and the current components that only circulate in the stator and produce additional losses. Both the current ripples are evaluated during switching period and then the rms of the current ripples are calculated over the fundamental period. The resulted current ripple expressions are optimized to obtain the optimal phase displacement. Based on the analysis, it is symmetrical configuration that is optimal for all multiple three phase motor. For dual mode operation, the best choice is odd multiple three phase motor with symmetrical configuration. This dissertation also explore the optimal phase number for multiphase PWM inverter. The input current ripple is used for the performance indicator. It is found that the input current ripple can be reduced by increasing the phase number. However, the reduction is not significant when the phase number is more than nine. So, more than nine phase number is suitable for applications where reliability and switching device rating are considered as important