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Chapter 12: Brushless DC Motors

Tài liệu học ngành cơ khí-chế tạo máy, phần động cơ điện 1 chiều không cổ góp. Tài liệu bằng tiếng Anh dùng để tham khảo. | Chapter 12. Brushless DC Motors Topics to cover 1. Structures and Drive Circuits 3. Performance 2. Equivalent Circuit 4. Applications Introduction Conventional dc motors are highly efficient and their characteristics make them suitable for use as servomotors. However their only drawback is that they need a commutator and brushes which are subject to wear and require maintenance. When the functions of commutator and brushes were implemented by solid-state switches maintenance-free motors were realised. These motors are now known as brushless dc motors. In this chapter the basic structures drive circuits fundamental principles steady state characteristics and applications of brushless dc motors will be discussed. Structures and Drive Circuits Basic structures The construction of modern brushless motors is very similar to the ac motor known as the permanent magnet synchronous motor. Fig.1 illustrates the structure of a typical three-phase brushless dc motor. The stator windings are similar to those in a polyphase ac motor and the rotor is composed of one or more permanent magnets. Brushless dc motors are different from ac synchronous motors in that the former incorporates some means to detect the rotor position or magnetic poles to produce signals to control the electronic switches as shown in Fig.2. The most common position pole sensor is the Hall element but some motors use optical sensors. Fig.1 Disassembled view of a brushless dc motor from Ref. 1 p58 Fig.4.1 48531 EMS - Chapter 12. Brushless DC Motors Fig.2 Brushless dc motor Permanent magnet ac motor Electronic commutator Although the most orthodox and efficient motors are three-phase two-phase brushless dc motors are also very commonly used for the simple construction and drive circuits. Fig.3 shows the cross section of a two-phase motor having auxiliary salient poles. Comparison of conventional and brushless dc motors Although it is said that brushless dc motors and conventional dc motors are similar in their