| Variable Speed Drives (VSDs) are increasingly being used in the industry due to the demand for high efficiency. Cost minimization is still the key to their large volume manufacture and application. Significant cost savings can be obtained by minimizing the component count in the power electronic converter. This is the primary motivation for studying the unipolar excitation of Brushless DC (BLDC) motors and Switched Reluctance (SR) motors for VSDs. This research proposes solutions to the machine design as well as the converter and control aspects of the drive in order to improve the performance of unipolar-excited drives. A method to minimize the ripple torque in unipolar BLDC motor drives by matching the motor back-emf widths and conduction pulse widths is proposed. Machine design choices to enhance the fault-tolerance of the BLDC drive are also discussed. A new minimum-switch converter topology is proposed for the unipolar operation of BLDC and SR motors. It has the ability to boost the available input voltage, making it a good choice for low-voltage applications. For applications requiring operation from an ac supply, the use of the proposed converter enables power factor correction without a separate preregulator. For low power levels, the converter is designed to operate in the discontinuous conduction mode. The input current naturally follows the input voltage waveform, eliminating the need for a current control loop. For higher power levels, operation in the continuous conduction mode with multiplier control is possible. These operating modes are analyzed and simulation and experimental results are presented to verify the operation of the proposed scheme. In addition, the implementation of position sensorless control for the unipolar BLDC motor is discussed, which results in a cheaper and more reliable solution. |
