The brushless DC motor is often powered by direct current from batteries but is not a simple direct current machine with carbon grinders, but uses control electronics to convert the direct current into suitable three-phase current for operation as a three-phase synchronous machine with excitation by permanent magnets. The three-phase winding is controlled by a suitable circuit in such a way that it generates a wandering magnetic field, which pulls the permanent-magnet rotor along with it. The control behavior is largely similar to that of a DC shunt machine.
BLDC motors are used in drives for hard disk drives, PC fans, quadcopters and model airplanes. Automation technology is also a wide range of applications, especially for actuators in the form of servo motors, in joints of industrial robots, and drive systems for machine tools such as lathes.
Since the beginning of the 2020s, there has been a shift from brushed machines to brushless ones (BL) in the field of battery-powered battery platform hand tools such as cordless drills, hand-held circular saws, cutting wheels, etc. This is where maintenance-free, more compact design, slightly better energy efficiency and higher power density come into play. Currently, brushless devices are still slightly to significantly more expensive, but the differences are starting to level out. Brushed devices are currently still widely available, but are increasingly being phased out by well-known manufacturers.
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How Brushless DC Motor Works
In BLDC motors, the rotor is equipped with permanent magnets and the fixed stator encloses the coils. In addition to the internal rotor, the design as an external rotor is also frequently used, and the special shape as a disc rotor can also be realized. The winding is usually designed as a three-phase system and, depending on the speed range, with a low to very high number of poles. For small axial fans, systems with only one phase and sensor are also known.
The star replacement circuit diagram corresponds to the synchronous machine, but there are differences in the pole shoes and in the winding design. Ideally, a BLDC motor generates a trapezoidal generator voltage (counter voltage) when rotating. There are also BLDC motors with sinusoidal generator voltage, which show variations in torque during block commutation. They are commutated with sine wave oscillation generated by pulse width modulation and then show smoothness.
In the case of sensor-controlled block commutation, the BLDC motor also contains three magnetic sensors (Hall sensor) for detecting the rotor position. In order to realize block commutation, a bridge circuit is required for the BLDC motor, which in the case of a three-phase BLDC motor consists of a bridge circuit with three push-pull stages.
Commutation
A characteristic feature of the BLDC motor is its commutation, which in a three-phase motor consists of six blocks per rotational field pass, each of which differs from the switching state of the bridge circuit. Particularly striking in the table of commutation blocks is that only two push-pull stages of the bridge are active at any one time and one “floats”. The voltage at this bridging point is defined by the circuit network according to the star replacement circuit diagram. The bridge control ensures that the motor phase is always floating, which – in the case of trapezoidal counter voltage – is in the process of changing polarity.
Since the bridge control system automatically switches on in a DC motor, similar to the commutator, the stator field is always located in the block with the optimal magnetic flux change (maximum generator voltage). The motor revs up until its generator voltage matches the supply voltage. The supply voltage does not necessarily have to change for speed control, but the bridge circuit performs a pulse width modulation. A distinction is made between unipolar and bipolar PWM.
In unipolar PWM, the push-pull stage, which is clamped on supply voltage, repeatedly switches briefly to ground, so that the average value of the voltage at the motor changes. The floating motor connection is temporarily negatively clamped to ground by transistor protection diodes, which is not efficient but is accepted.
In bipolar PWM, the two active push-pull power amplifiers change their switching state. The advantage here is that high braking torque is possible even at low engine speeds up to a standstill. This is important, for example, for a robotic arm that is supposed to hold its position.
Due to the magnetization and demagnetization of the motor phases at each commutation step and the non-ideal trapezoidal generator voltage, the BLDC shows more or less pronounced torque ripples at each commutation step.
Sensor-controlled commutation
In this case, sensors detect the current rotor position and this information is used to control the commutation. Hall sensors are used, which determine the current rotor position by detecting the magnetic flux. According to this position information, the control electronics control the power drivers and the windings that generate torque in the rotor. The advantage is that the sensor-controlled commutation also works at very low speeds or when stationary. Usually, not all phases are energized at the same time in this commutation. In the case of three-phase motors, one phase is usually de-energized at any given time.
There is also sensorless commutation, vector control, multiphase systems to name a few.
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