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Analysis and Modeling of a Single-Phase Brushless DC Motor Drive System

Brushless dc motors are becoming widely used in low-power applications such as blower motors, computer disk-drive spindle motors, and in copiers and laser printers. For these applications, the brushless dc motor offers the following advantages: small size, reliability, no carbon dust from brushes, precise speed control, and potentially high efficiency. The most commonly used brushless dc motors are twoor three-phase permanent-magnet ac machines driven by a dc-to-ac inverter. The three-phase machine combined with a six-pulse full-bridge inverter often represents the best trade-off of machine iron and copper utilization with the cost of the inverter. Because of the prevalence of two-and three-phase devices, they are the brushless dc motors which have been analyzed the most extensively. On the other hand, there has been little investigation into the operation of the single-phase brushless dc motor. Single-phase motors, in general, suffer from slower starting characteristics, less efficient utilization of machine iron and copper, and higher losses. Therefore, single-phase motors are limited to low-power applications where rapid response and high efficiency are not required. The primary advantage of single-phase devices is the simplified source requirements. For example, a single-phase brushless dc motor requires only one-third the number of transistors and position sensors needed by a three-phase motor and can be powered by a single dc voltage source. Thus, in applications where cost is of greater importance than performance, the single-phase motor may be a superior alternative to a two-or three-phase motor.