This application note discusses the advantages and disadvantages of using BLDC motors to drive brushed DC motors. For BLDC controllers using Hall-effect devices, there are six steps in the commutation sequence to make the motor rotate one revolution.

This application note discusses the advantages and disadvantages of using BLDC motors to drive brushed DC motors. For BLDC controllers using Hall-effect devices, there are six steps in the commutation sequence to make the motor rotate one revolution.

The Hall device tells the BLDC motor controller which winding is energized at any given time. Therefore, the BLDC controller only activates one H-bridge configuration from the complete three-phase bridge at a time. As shown in Figure 1, a complete three-phase bridge consists of three half-bridges connected together. Two half bridges are needed to create the H bridge in order to power one winding at the right time.

About the advantages and disadvantages of BLDC motor drive DC brushed motor
Figure 1 The three-phase bridge consists of three half bridges. Two half bridges working together will create an H bridge.

The top Transistor is energized in the half-bridge connected to the winding terminals of the A-phase motor, connecting A to V+. Powering up the bottom transistor in the rear half-bridge connected to the B-phase motor winding terminals will connect B to V-. This will complete the circuit of the A-phase winding and force the current to flow in one direction (positive direction). Turning off the pair of transistors and turning on the opposite pair of transistors will force current through the winding in the opposite way (backwards). See Figure 2.

About the advantages and disadvantages of BLDC motor drive DC brushed motor
Figure 2 is an H-bridge configuration that shows the complete circuit and the current flow through the motor windings in each direction.

In a brushed DC motor, the motor also contains multiple windings, which can only be energized at a time. The biggest difference is that the motor “self-commutates” through the commutator on the armature and will control which winding to open at any given time. Only two terminal connections to the two brushes in the commutator are brought out to connect to V+ and V-. To drive a brushed DC motor, only one H bridge is needed to make the motor current flow forward or backward. In order to utilize MSK’s unique BLDC torque amplifier characteristics when driving brushed motors, designers only need to lock the controller to one of six sequences to use one H bridge in the BLDC controller.

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