What is a BLDC Motor?
A BLDC motor (brushless dc electric motor) is an electric motor, powered by a DC voltage supply and is commutated electronically, usually by hall sensors, instead of brushes. The development of these type of motors was possible by the development of semiconductor electronics in the 1960s. Since then the BLDC motor has been increasing its popularity as a result of its brushless design.
Typically a BLDC motor is a permanent magnet synchronous motor. This means that permanent magnets are used in the rotor. And that the rotor turns synchronously with the rotating magnetic field, created by the windings in the stator.
How does a BLDC motor work?
The mechanism that drives the BLDC motor is the same as that of the brushed DC. The magnetic field in the copper windings in the stator, either attract or repel the permanent magnets in the rotor. This creates an axial force, or torque, causing the rotor to start rotating. However, to keep the motor rotating, the magnetic field in the stator need to rotate as well. This is done by continuously switching the DC current through the windings on (+), off (0) or in the opposite direction (-). This is called commutation. For BLDC motors this is not done by brushes, but by using sensors that measure the position of the rotor and a small circuit board that functions as a switch. The input signals of the sensors are processed by the circuit board and it accurately times the right moment to commutate as the rotor turns. In most BLDC’s these sensors are hall sensors. But for more accuracy a rotary encoder can also be used.
Although single phase and 2-phase BLDC exist, generally a BLDC motor is a 3-phase motor, with 3 sets of windings. As for most applications this results in the best performance. 3-phase BLDC’s also have 3 hall sensors. The commutation is done in 6 steps that are continuously repeated. This is called “6 step commutation”.
Advantages and disadvantages of a BLDC motor
The advantages of a BLDC motor over a brushed DC motor is that they have a far longer lifespan, without the need for maintenance as there are no brushes. Also BLDC motors don’t release small particals, caused by the wear of brushes, making them suitable for the use in electronics, like harddiscs. Also BLDC’s provide a higher level of control and a higher power to weight ratio.
There are also some disadvantages. The main disadvantage is that because of the electronics, they are more complicated and expensive. But because of all the benefits, BLDC motors are likely to keep being favoured over brushed DC motors.
A BLDC motor is also often compared with an AC permanent magnet synchronous motor, as the design is very similar. But there are differences. Mainly the windings are different as the drive current is either AC or DC. As a result the AC permanent magnet motors runs more smoothly than BLDC motors, with less audible noise. But they generally are more expensive, because of the more complicated drive design.