In a single-phase grid-tied inverter, the direct current (DC) offset error included in the measured grid side phase current has various causes, such as a non-ideal current sensor, unbalanced power supply of an operational amplifier, and nonlinear features of analog components in interface circuits, etc. If the DC offset error is included in the measured current, it causes the secondary harmonic of fundamental frequency and the DC component in grid phase current which result in degradation of inverter performance. In this paper, a theoretical detection method of the secondary harmonic of the fundamental frequency and a DC component in grid phase current for a proportional-resonant (PR) current control system is introduced. Based on the detection method, an algorithm for compensating DC offset error is also presented for single-phase grid-tied inverters. Simulation results and experimental verification of the DC offset error compensation algorithm are shown in this paper.

This paper discusses an inverter-driven single-phase brushless direct-current (BLDC) motor assembled by a housing for a cordless vacuum cleaner. Air gap in the single-phase BLDC motor is asymmetrically designed to satisfy starting and continuous torque by considering voltage fluctuation in a battery. By varying both advance and conduction angles in response to the change of battery voltage, the proposed single-phase BLDC motor with asymmetric air gap is able to maintain sufficient output power. The system efficiency of a vacuum cleaner driven by the proposed motor assembly is estimated by means of fluid dynamics in air watt, and it is also verified experimentally. From the results of the condition of 100,020 r/min, it was confirmed that the motor efficiency was in good agreement with the estimated efficiency, and air flow efficiency of 45.7% and system efficiency of 41.8% were achieved.