Brushless DC electric 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 opposition to the naming on the functional principle of the DC machine, but is structured as a three-phase synchronous machine with excitation by permanent magnets. (Often a tri ) three-phase winding generates a rotating magnetic field which pulls along the permanently excited rotor. By a suitable drive circuit, a control action is obtained which corresponds to the behavior of a DC machine largely. To this end, a simplified AC drive is used with block commutation, in which the intermediate circuit can be fed directly to a DC voltage of variable amplitude and in this way controls the speed of the motor.

Areas of application of EC motors are in the range of drives as small fans, drives in disk drives, compressors, video recorders and model airplanes, parking facilities in the form of servo motors to drive systems for machine tools such as lathes.

Construction

Usually in EC motors, the rotor having a permanent magnet is achieved, the stationary stator includes coils which are driven with a time delay of an electronic circuit to give rise to a rotary field, which causes a torque on the permanent-magnet rotor. The vast majority of EC motors (like the larger three-phase motors) designed with three phases. For smaller EC motors with low expectations as fans, there are also two-phase systems and single-phase systems, in which the cogging torque permanent magnet generated replaces the second phase. To minimize angle-dependent torque fluctuations are sometimes higher phase numbers are used. Characterized the running properties are improved when driven by a square-wave alternating voltage with respect to three-phase systems.

Commutation

If the commutation of the EC motor regardless of the position, speed and load torque of the rotor, is in principle just before, a conventional synchronous motor, or a shape of the stepping motor. For EC motors is the most significant difference, the possibility that electronic commutation of the rotor position, the rotor speed and torque to be addictive. This represents a form of direct feedback, so that the frequency and, for some systems, the amplitude in function of the position and speed of the rotor can be changed. The electronic commutation becomes a controller, and the method of production of the rotating field so that essentially determines the characteristics of the EC motor.

To detect the rotor position and speed are different possibilities:

Sensor-controlled commutation

In this case, there are sensors such as Hall effect sensors for detecting the magnetic flux of the rotor, or optical sensors, in the field of the stator. Corresponding to this position information, the coils are driven through suitable power drivers of the control electronics, which produce a torque in the rotor. The advantage is that the sensor-controlled commutation works even at very low speeds or when stationary. Not all phases are energized at the same time the ordinary for this commutation. In the three-phase motors usually one phase is energized at all times.

Sensorless commutation

In sensorless commutation, the detection of the rotor position of the induced in the coils of the stator voltage, which is evaluated by the electronic control circuit is carried out. However, a certain minimum speed is necessary for the evaluation of the offset voltage. Sensorless EC motors therefore have to be switched blindly to reach the minimum speed as synchronous motors, stepper motors.

In the meantime, however, there are methods by which an EC motor is below this minimum speed is not blind controlled. To short current pulses are sent at a standstill, while not moving the engine, but can be influenced by the magnetic field of the rotor. The magnetic field reduces or enhances the flow of current, thereby changing the time required for a pulse of current to exceed a threshold. These times are measured, and allows you to determine the rotor position already at standstill.

The starting of the motor to a rotational speed at which the reverse voltage can be evaluated, can then be controlled using a special process. This one uses the Star or delta connection of the motor, in exactly six different currents. A stream is used for the drive, and in each case a small electric current 60 ° before and after the driving current is affected by the rotor magnets. Commutes, thus connected by 60 ° electrical on, is, whenever the magnetic axis coincides with the axis of the driving current ( ° el indicates the angle of the space vector representation. It is similar to the mechanical angular position of the rotor divided by the number of pole pairs ). Can be measured, since there the difference between the two smaller measuring currents at a maximum.

Vector control

EC motors can also be operated with a vector control, which finds application in a range of demanding tasks of drive technology. In contrast to operating on three-phase power must be in the vector control not the rotor position passively adjusts the voltage and torque, but the voltages of all phases are adapted active the rotor position and the desired torque. In the steady state current and voltage still form but of a symmetrical three -phase system.

Other brushless machines

In addition to the EC motor with electronic commutation exist a number of brushless machines such as the induction ( squirrel cage), the synchronous machine ( with revolving field brushless permanent arousal or excitement) or the cascade machine. These machines can be operated as a motor or as a generator and to be operated with polyphase AC voltage.