Research On Modulation Technologies Of Multi-Leg Switching Power Amplifiers For Magnetic Bearings

In order to popularize the active magnetic bearings (AMB), low costs power amplifier is needed. This paper studied on the four-leg and six-leg switching power amplifiers, which could respectively drive three and five coils of the AMB. The amounts of the switches in two multi-bridge amplifiers were respectively reduced 1/3 and 2/5 comparing with H-bridge power amplifiers. The complexity of the power amplifier is reduced, and the I/O source in the controller is saved. The objective of this paper was to research the current methods with low total harmonic distortion (THD), high stability and strong adaptability to loads, and to resolve the interaction of the output currents for all loads were connected with the neutral leg.A node potential control method in four-leg power amplifier based on current PID regulating, which has the merits of three-state power amplifiers, was presented. It needs little algebraic operation, so it is easy to be implemented for the duty cycle of each bridge can be obtained directly from each potential. Three typical methods of distribution of the potential were summarized and their influence on current ripple and switch times were compared, the dead-time compensation of the snubber circuit was analyzed. A over-modulation algorithm was proposed and it eliminated the phase interference in the four-leg power amplifier. This method can be extended to control arbitrary n-bridge converter.The three-dimensional (3-D) space vector pulse width modulation (SVPWM) was simplified and its performance was compared with the node potential control method. The relationships, whether in abc orαβ0 coordinates, between three-phase voltages and the following elements: the tetrahedron in which the reference vector resides, three adjacent active switching vectors (or three switching states) and their duration times, were analyzed. Based on the analysis, the calculation of turn-on time of each switch in the actual implementation was simplified, and it was the same with the node potential control method. The implementation of common discontinuous PWM (DPWM) methods in existence were also simplified. In addition, two more DPWM methods peculiar to 3-D SVPWM were presented.The over-modulation was researched to resolve the phase interference in the four-leg amplifier. Over-modulation of 3-D SVPWM was changed into 2-D problem by separating the zero components from three-phase voltages. Two simple algorithms of single-mode and two-mode over-modulations in 2-D SVPWM were presented, and it is no need to identify the sector in which the voltage vector locates or to lookup tables. That makes the 3-D over-modulation can be implemented by the simplified SVPWM algorithm. A hybrid control method for four-leg amplifier was studied to overcoming the shortcomings in PID control. In this method, the neutral leg, which duty cycle is continuously adjusted, is controlled separately from other legs according to three current errors and three phase bridges are controlled by sample-hold control used in H-bridge amplifier to enhance dynamic performance and to enlarge the load range.The performances of various current control methods for six-leg power amplifier were compared. A maximum current error preferred sample-hold control method was proposed. Five currents all can be regulated in one sampling period by modified control method of the neutral leg. This method and the hybrid current control mentioned above were similar in the performance and spectrum of the output currents.The experiments were carried out with two multi-leg amplifiers using DSP and CPLD as the core controller and the experimental results verified the feasibility and effectiveness of these current control methods. The study of this paper has significance for reference in controlling the multi-leg inverters for multi-phase motors.