Efficiency and Power Losses in PM BLDC Motor with Variable Bridge/half-bridge Structure Electronic Commutator

In practice we often meet with electrical drives, where it is necessary to operate with higher speed, while load torque is less than rated torque. This type of operation is called "constant power operation" and may be defined as second range of natural motor torque-speed characteristic. In case of motors excited with permanent magnets magnetic flux is constant, and while supply voltage remains constant, motor speed is also (approximately) kept constant. In practice, different solutions are attempted in order to achieve increased rotating speed of brushless motors at decreased torque. One of the methods, which make possible PM BLDC motor operation at increased speed and decreased torque, is use of electronic commutator with switchable bridge-half-bridge structure. The investigation presented in the paper has been aimed at estimation of losses and efficiency in the drive consisting of brushless motor operating with variable structure commutator. Results of theoretical analysis, computer simulations and laboratory tests have shown that application of electronic variable structure commutator, which makes possible operation in the second speed range without increasing motor power and supply source power, causes an insignificant decrease in overall drive efficiency.