The disadvantages of the Brushless DC Motor over the AC motors are:
higher maintenance (due to brushes)
more expensive
In addition to the above the Brushless DC Motors have the same advantages over AC motors but they also have more advantages over Brushed DC Motors
lower maintenance (no brushes)
even higher starting torque ( Due to rare earth magnets)
even higher efficiency (over 90%) on sub fractional motors
even more compact design
better heat dissipation
lower noise
This paper describes the design, construction, and testing of a thyristorized speed control unit for a separately excited dc motor. The motor is fed from a three-phase six-pulse fully controlled thyristor bridge. A speed loop with a proportional plus integral controller maintains the desired speed irrespective of the load variations on the motor. An inner current control loop protects the thyristors from overcurrents. This loop also provides fast response overcoming the effect of disturbances such as variations in supply voltage. The design aspects of the control loops are discussed, and experimental results are given.
