7상 BLDC 전동기의 리플전류 감소 연구

Abstract

Motors are indispensable components in the industrial field, and they are used in various fields such as transportation, industrial equipment and electric equipment. DC motors have been used in many fields because of their speed and torque characteristics. But there are some problems of maintenance and repair due to the life degradation that is occurred by the structural nature of the brush.

Brushless DC(BLDC) motors are simple in structure and high in efficiency, and their use is increasing due to the development of permanent magnets and power electronic devices. The BLDC motor has excellent torque characteristics because it can make uniform torque by the product of square wave phase current and trapezoidal counter electromotive force. In addition, since the constant field flux is supplied to the rotor by using the permanent magnet, the efficiency and the power factor are superior.

And because BLDC motors maintain high efficiency over a wide operating range, they are well suited for applications requiring variable speed drive. Generally 3 phase BLDC motors are limited in high speed due to torque ripple and speed fluctuation.

In order to improve the torque ripple, it is necessary to increase the number of phases, poles and slots of the motor.

Multi phase BLDC motors have advantages such as reduction of torque ripple and increase of efficiency compared with 3 phase BLDC motors and can operate even if one or two phases fail. These characteristics make multi phase BLDC motors widely used in military application products and special fields. Particularly, 7 phase BLDC motors are widely used as propulsion motor for military application products and underwater applications.

7 phase BLDC motor has higher phase usage rate than 3 phase BLDC motor, which is advantageous in achieving high output power within a limited space. In order to achieve high output power, 7 phase BLDC motor operates at a high speed of about 20,000rpm. The reactance of the phase winding increase due to the high speed operation, and the reactance value becomes larger as the speed of the motor increases. The influence of the reactance in the high speed operation is delayed in the rise of the phase current, making it impossible to make a sufficient phase current magnitude necessary for the output power, and the output power is lowered. In order to improve this phenomenon, the advance angle method and the 7 phase winding excitation method are applied..

However when the advance angle method and the 7 phase winding excitation method is applied, the ripple current and ripple voltage at the power input terminal increase, which may be a main cause of electrical noise.

In order to reduce the ripple current and ripple voltage for the 7 phase BLDC motor operated to achieve high output performance within the limited space, analyze the ripple current and ripple voltage generated at the power input terminal of the 7 phase BLDC motor. And apply the DC Link Capacitor to check how the ripple current and ripple voltage change. This paper proposes a mothod to select the DC Link Capacitor with the minimum and optimum capacity according to the motor characteristics.