How does a Brushless DC motor work

How does a Brushless DC motor work

1. Brushless DC Motor Principles

Brushless DC motors use permanent magnets similar to motors, but are constructed “from back to front.” Magnets are fixed to the rotor, and the stator is wound around a specified number of poles. By using an electronic commutator, the current is reversed around the stator poles. That sets up a rotating field in the stator which the rotor magnets will follow, thereby turning the rotor, as shown in Figure 1. It is essentially a synchronous machine, with the rotor speed controlled by the frequency of commutation between stator coils.

Figure 1: Brushless DC Motor Types

Brushless DC Motor Types

Electric switches control the commutation between stator coils. Control of this commutation requires feedback on the rotor position relative to the stator coils. That may be by shaft encoder or Hall effect sensors, but for smaller motors, most commonly inferred by measuring the back- induced in the undriven stator coils. That now enables soft starting, direction, and torque control.

The number of stator coils will be a multiple of 3 and will be interconnected in series or parallel to form three groups, A, B, and C.

Advantages:

Brushless motors have several advantages over brushed DC motors, including

  • Higher torque to weight ratio
  • More torque per watt (increased efficiency)
  • Increased reliability, reduced noise, longer lifetime (no brush and commutator erosion), and elimination of ionizing sparks from the commutator.
  • With no windings on the rotor, they are not subjected to centrifugal forces, so very high-speed designs are possible.
  • Because the windings are composed of shells, they can be cooled by conduction without airflow cooling inside the motor, which in turn means that the motor’s interior can be completely enclosed and prevent dust or other foreign matter from entering.

Variation: Figure 1(a) shows one type of Brushless DC motor, called the inner rotor design or in runner design, where the rotor is on the inside and the stator on the outside. A variation is shown in Figure 1(b), called the outer rotor design or outrunner design. Here the rotor is on the outside and the stator on the inside.

2. Control of Brushless DC Motor

Brushless DC motor can not run without an electronic controller. That can commutate the current through the stator coils and can be used to provide speed control. Figure 2 shows a simplified circuit diagram of such a controller.

Figure 2: Brushless DC motor controller using Hall effect sensors

Brushless DC motor controller using Hall effect sensors

Figure 2 shows what is essentially a three-phase inverter bridge, using IGBTs as the semiconductor switching devices. Smaller low voltage motor controllers are more likely to use MOSFETs for cost and performance reasons.

3. Sensorless Control of BLDC Motor

The commutation of output current between motor windings is controlled by the transitions in the Hall sensor feedback. It can be shown that this corresponds approximately to a zero-crossing point in one of the back-e.m.f., waveforms. Thus it is possible to detect commutation points by monitoring these back e.m.fs. That makes it possible in some motors and applications to eliminate the Hall sensors and auxiliary magnets. That simplifies and reduces the cost of the motor. Figure 3 shows a block diagram of a sensorless BLDCM control scheme.

Figure 3: Sensorless brushless DC motor controller

Sensorless brushless DC motor controller

The zero-crossing point is usually different from the ideal commutation point, so the controller needs to be able to compensate for this. At low speed, the back-e.m.f. is a very low amplitude, so it cannot be measured, and the motor needs to run in an open-loop. Therefore, sensorless control is not suitable for applications with high dynamic performance or low speed and high torque.

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