Brushless DC motor
The brushless DC motor ( english Brushless DC motor , abbreviated BLDC or brushless motor, as well as electronically commutated motor , short EC motor ) is not based, contrary to the naming on the operating principle of the DC machine , but is structured as a three-phase synchronous machine with excitation by permanent magnets . The (often three-strand) three-phase winding is controlled by a suitable circuit in such a way that it generates a moving magnetic field that pulls the permanently excited rotor with it. The control behavior is largely similar to a direct current shunt machine.
BLDC motors are used in drives for hard disk drives , PC fans , quadrocopters and model airplanes . A broad area of application is also automation technology, especially for control devices in the form of servomotors , in joints of industrial robots , through to drive systems for machine tools such as lathes .
functionality
In BLDC motors, the rotor is equipped with permanent magnets and the stationary stator encompasses 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 implemented. 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 . The exception are PC fans, which for cost reasons only have one phase and sensor.
The star equivalent circuit corresponds to the synchronous machine , but there are differences in the pole pieces and in the winding structure. Ideally, a BLDC motor generates a trapezoidal generator voltage (back EMF, back EMF ) when rotating . Because of the simpler construction, BLDC motors with a sine-wave generator voltage are also widespread, but they show higher torque fluctuations during one revolution . The only difference between the BLDC motor and a synchronous machine is that it is controlled with block commutation.
As with the DC shunt machine, the motor generator voltage is proportional to the motor speed , and the current through the motor corresponds to the torque output .
With sensor-controlled block commutation, the BLDC motor also contains three magnetic sensors ( Hall sensor ) to detect the rotor position.
To implement the 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 .
Star equivalent circuit diagram for three-phase BLDC motor
Generator voltage and bridge voltage through commutation
Torque fluctuations in the sinusoidal BLDC motor
Three-phase bridge circuit with transistor output stages
Commutation
A characteristic of the BLDC motor is its commutation , which in a three-phase motor consists of six blocks per rotating field run (" motor revolution"), each of which differs from the switching state of the bridge circuit. What is particularly noticeable in the table for the commutation blocks is that only two push-pull stages of the bridge are active and one is "floating". The voltage at this bridge point is defined by the circuit network according to the star equivalent circuit diagram. The bridge control ensures that the motor phase that - in the case of a trapezoidal countervoltage - is changing polarity (dotted in the voltage diagram) always floats.
| 1 | 2 | 3 | 4th | 5 | 6th | |
|---|---|---|---|---|---|---|
| V1 / V2 | 0 | z | +1 | +1 | z | 0 |
| V3 / V4 | +1 | +1 | z | 0 | 0 | z |
| V5 / V6 | z | 0 | 0 | z | +1 | +1 |
z = floating, 0 = ground, +1 = supply voltage
Since the bridge control switches automatically, similar to the commutator in the DC motor, the stator field is always in the block with the optimal change in magnetic flux (maximum generator voltage). The motor revs up until its generator voltage corresponds to the supply voltage. To control the speed, the supply voltage does not necessarily have to change; a PWM signal can also be fed into the bridge circuit . A distinction is made between unipolar and bipolar PWM.
With unipolar PWM, the push-pull stage , which is clamped to the supply voltage, repeatedly briefly switches to ground, so that the mean value of the voltage at the motor changes. The floating motor connection is temporarily negative and clamped to ground by transistor protection diodes, which is not efficient but is accepted.
With bipolar PWM, the two active push-pull output stages change their switching status. The advantage here is that a high braking torque is possible even at low speed and even at a standstill. This is essential for a robotic arm that is supposed to hold its position.
By magnetizing and demagnetizing the motor phases with each commutation step and with the non-ideal trapezoid generator voltage, the BLDC shows more or less pronounced torque ripples with each commutation step.
Sensor-controlled commutation
In this case, sensors detect the current rotor position and this information is used to control the commutation. Are used Hall sensors which determine the actual rotor position by detecting the magnetic flux, some optical sensors used in the field of the stator is used. In accordance with this position information, the windings that generate a torque in the rotor are activated by the control electronics via suitable power drivers. The advantage is that the sensor-controlled commutation also works at very low speeds or when stationary. With this commutation, not all phases are usually energized at the same time. In three-phase motors, one phase is usually de-energized at any one time.
Sensorless commutation
With sensorless commutation, no separate sensors are used to record the current rotor position, but the information is obtained indirectly by measuring electrical parameters on the coils. There are several methods available for this purpose, such as the detection of the rotor position via the counter voltage triggered in the coils of the stator , which is evaluated by the electronic control circuit. However, a certain minimum speed is required to evaluate the counter voltage, and until the minimum speed is reached, blind switching must be performed with this method. The star or delta connection of the motor is used, in which exactly six different currents can flow. A current is used for the drive and a small current 60 ° el. In front of and behind the drive current is influenced by the rotor magnet. Commutation, i.e. further switching by 60 ° el., Occurs whenever the magnet axis coincides with the axis of the driving current (° el. Denotes the angle in the space vector representation . It equals the mechanical angular position of the rotor, multiplied by the number of pole pairs ). This can be measured because there the difference between the two lower measuring currents has a maximum.
In order to avoid the disadvantages of blind commutation at low speeds, there are also more complex sensorless methods which measure the rotor position by measuring the current inductance . The inductance depends, among other things, on the current rotor position, but is also dependent on various other factors such as the structure of the motor and is more difficult to evaluate. As a rule, the control electronics must be matched to the respective EC motor. In this method, the control electronics send short current pulses through the individual windings to measure the inductance, which means that the rotor position can also be detected when the motor is stationary and at very low speeds. The strength and duration of these current impulses are selected to be so short that they do not cause the rotor to move. The rotor position is then usually evaluated and obtained using digital signal processing , tailored to the respective type of EC motor .
Vector control
BLDC motors can also be operated with vector control , which is used for demanding tasks in drive technology. In this case, switching to the next commutation block is advanced or delayed. This compensates for the time it takes to remagnetize the motor phases and depends on the speed and torque.
Multiphase system
In a multi-phase system, several strings together form a pole. This means that adjacent strands have the same magnetization and form a partial pole. To rotate the magnetic field, only one of the strands of the pole is commutated and not the entire pole jumps. This results in more commutation steps per rotation and thus fewer torque discontinuities.
| 1 | 2 | 3 | 4th | 5 | 6th | 7th | 8th | 9 | 10 | |
|---|---|---|---|---|---|---|---|---|---|---|
| V1 / V2 | 0 | z | +1 | +1 | +1 | +1 | z | 0 | 0 | 0 |
| V3 / V4 | 0 | 0 | 0 | z | +1 | +1 | +1 | +1 | z | 0 |
| V5 / V6 | z | 0 | 0 | 0 | 0 | z | +1 | +1 | +1 | +1 |
| V7 / V8 | +1 | +1 | z | 0 | 0 | 0 | 0 | z | +1 | +1 |
| V9 / V10 | +1 | +1 | +1 | +1 | z | 0 | 0 | 0 | 0 | z |
z = floating, 0 = ground, +1 = supply voltage
Other brushless machines
In addition to the EC motor with electronic commutation, there are a number of brushless machines such as the asynchronous machine (squirrel cage rotor), the synchronous machine (internal pole machine with brushless excitation or permanent excitation ) or the cascade machine . These machines can be operated as a motor or as a generator and are operated with multi-phase alternating voltage.
Web links
literature
- Hans-Dieter Stölting, Eberhard Kallenbach: Manual electrical small drives . 3. Edition. Hanser, 2006, ISBN 3-446-40019-2 .
- Roland Büchi: Brushless motors and controllers . 1st edition. Verlag für Technik und Handwerk, vth, 2011, ISBN 978-3-88180-427-1 .
Individual evidence
- ↑ Patent DE10102235A1 : Brushless DC machine. Published on August 14, 2002 .
- ^ ON Semiconductor: FAN Motor Driver, Single-phase. Retrieved May 13, 2018 .
- ↑ a b Texas Instruments: Trapezoidal Control of BLDC Motors Using Hall Effect Sensors. Retrieved May 13, 2018 .
- ↑ Vol. 2, Special Issue 1, December 2013. International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering, accessed July 20, 2018 .