Brushless DC Motor (BLDC Motor) – how it works

Brushless DC Motor (BLDC Motor) – how it works


Finally a toy not only for children! At the drives for these rotors high demands are demanded. High rpm but also low maintenance. High efficiency so that the battery pack is not immediately discharged. In fact brushless DC motors are used here. The traditional DC motor has a high rate of wear and tear due to its carbon brushes. That is why this motor is replaced more and more by the BLDC motor. Since the commutation is done electronically, the BLDC motor can perform high rotational speed with high efficiency. Let us dismantle this drone and have a closer look. The rotor, the moving part of the motor consists of permanent magnet. What you see here is the outrunner type. The rotor will rotate synchronous to the rotating magnetic field of the stator because of its permanent magnets. To have a better understanding we should go to the top view and have a closer look at the stator itself. You should know that when the stator coils are energized, they become an electric magnet. Also, you know that opposite stator and rotor poles attract each other. The rotor is attracted by coil a right at the point when it reaches coil a coil B is energized. Reaching coil B. Coil C is energized. After that coil a is energized with opposite polarity, this sequence goes on and is repeated after a full 360 degree turn. But sorry, this operation is too simple to be effective. Because only one coil is energized at one moment and two coils are dead. You would get small torque. We need another solution. Let’s arrange the stator coils this way you see and watch how the coils are energized over time. At every commutation moment two coils are energized with different polarity resulting in a positive and negative current. Therefore you can connect the ends of these coils together to a neutral point and the wiring can be simplified. To get higher torque, you can energize the coil behind the rotor in the way that it pushes the rotor. Having attraction and repulsion on the rotor has another effect: you get almost constant torque. To get high efficiency and high torque, it is necessary to commutate the stator current at the right point in order to get the rotor position. Usually Hall sensors are used. So when the rotor spins around, the hall sensors give feedback to the electronics. To avoid high costs for wiring and Hall sensors, sensor less BLDC motors are offered. Here the rotor position and speed is determined via the so-called back electro-motive force or back-EMF. To explain back EMF in a few words: When the rotor passes a stator coil, a voltage is generated within this coil. The back EMF is proportional to the motors rotational speed. It’s up to the electronics to evaluate these signals. Sensorless BLDC motors have only three wires. But how does the BLDC motor behave when overloaded and no longer can rotate? Well, there is no more commutation due to rotor position detection. This makes this motor different from a stepper motor. Internally the magnets of out runners are on the outside while the coils are in the inside. The inrunners have the coils on the outside while the magnets are on the inside. In general out runners have more torque, run cooler and can swing a bigger load. In runners have higher rotational speed and must use a gearbox for high load. ESC stands for electronic speed control. In most cases you use a BLDC in conjunction with an ESC, controller and a battery. The ESC consists of a network of field transistors which change the speed of the motor on the principle of pulse width modulation. You use the ESC together with a controller. How to program a controller is not subject of this video. Brushless DC motors offer significant advantages over standard brushed motors, especially at high rotational speed. With its high efficiency, high reliability and less wear, this motor is used for examples for ventilators, tools, model kits… The BLDC motor can be characterized as a synchronous DC motor. Subscribe to our Channel and never miss a new video!

10 Replies to “Brushless DC Motor (BLDC Motor) – how it works”

  1. why are these motors still considered DC when the these look closer to some type of AC? i may be misunderstanding but I thought DC current didn't 'reverse'?

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