Brushless DC Motors (BLDC) Technology – short and to the point
maxon BLDC motors stand out for their long service life with maintenance-free operation. Depending on the version, either a high torque density or an extremely wide speed range of up to 120,000 rpm is available. Many product lines can be configured online and are available in sterilizable versions. The operation of brushless DC motors requires commutation electronics, which are integrated into the motor controllers.
Bearings and service life The long service life offered by the brushless design can be achieved with preloaded ball bearings. The bearings are designed for several 10,000 hours. The service life is influenced by maximum speed, residual imbalance, and bearing load Hall sensor circuitry The open-collector output of the Hall sensors typically does not include a pull-up resistor, as this is integrated into the maxon controllers. Exceptions are described in the respective motor data sheets. Circuit diagram for Hall sensor Hall sensor supply voltage
different conductive phases, depending on the sensor information. The current and voltage supply are block-shaped. The swit- ching position of each electronic com- mutation is offset 30° from the respective torque peak. Properties of block commutation – Simplified electronics – No encoder needed – Torque ripple of 14% – Controlled startup with high starting torques and acceleration possible – Servo drives, start-stop operation – Positioning tasks – The data of the maxon EC motors are determined by means of block commutation Sensorless block commutation The rotor position is determined by the progression of the induced voltage. The electronics detect the zero crossing of the induced voltage and commutate the mo- tor current after a speed-dependent delay (30°e electrical after the zero crossing). At standstill and low speeds, the voltage signal is too small, and the zero crossing cannot be detected accurately, or at all. Therefore, special startup algorithms are required. Properties of sensorless commutation – Torque ripple of 14% (block commutation) – No defined startup – Not suitable for low speeds and dynamic applications – Continuous operation at higher speeds (fans, milling tools, drills)
motor-specific parameters (speed constant and torque constant) meet the requirements.
Delta circuit
Star circuit
W 1
W 1
U 1-2
U 3-1
U 1-2
U 3-1
W 2
W 2
W 3
W 3
U 2-3
U 2-3
For more detail, see page 190 or refer to the book “The selection of high-precision microdrives” by Dr. Urs Kafader.
R Pull-up
Electronic commutation
Control circuit
Hall sensor output
GND
The three phases of the BLDC motors are powered depending on the rotor position. The rotor position is detected via Hall sen- sors, encoders, or other feedback sensors. Sinusoidal commutation The commutation electronics generate si- nusoidal motor currents in the three partial windings, which have phase offsets of 120° to each other. Sinusoidal commutation or field-oriented control (FOC) provides very smooth and precise motor operation with highly accu- rate control. Properties of sinusoidal commutation – Field-oriented control (FOC) – In addition to Hall sensors, usually requires an encoder as a higher- resolution feedback sensor – No torque ripple – Very good synchronous running even at very low speeds – Approx. 5% higher continuous torque than with block commutation – Highly dynamic servo drives – Positioning tasks Block commutation with Hall sensors The rotor position is reported by three Hall sensors integrated into the motor, which deliver six different switching combinations per magnetic pole pair. The three partial windings are powered in six
The power consumption of a Hall sensor is typically 4 mA (if Hall sensor output = HI). Winding The wire cross-section and number of windings have the following effect: Low connection resistance – Low-ohm windings – Thick wire, few windings – High startup currents – Motor with high specific speed (speed per volt) High connection resistance – High-ohm windings – Thin wire, many windings – Low startup currents – Motor with low specific speed (speed per volt) The maximum permissible winding tempe- rature is 125°C or 155°C for high-temperature versions, otherwise 100°C or 85°C. The three partial windings of the BLDC motors can be connected in two configura- tions: “star” or “delta.” As a result, speed and torque change inversely by a factor of 3. The winding circuitry is not decisive for motor selection. It is important that the
Currents in sine and block commutation
Sinusoidal phase currents
Block-shaped phase currents
300° 0°
60° 120° 180° 240° 300° Turning angle
maxon 73
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