For applications in which wear needs to be avoided – for reasons of purity in the field of medical or semiconductor technology or because long service lives are absolutely essential – "brushless", i.e., "electronic" commutation is suitable. With brushless motors, it is not possible to establish an electrical connection through sliding contacts. The winding therefore forms part of the stator, in which the permanent magnet rotates. With this type of commutation, the application of voltage to the contacts is electronically controlled. Here, the current flow in the stator must be precisely matched to the position of the armature. The position of the rotor is usually determined with the help of sensors. To actuate brushless motors, an electronic commutation signal is therefore produced using sensors and a controller. There are a number of different processes here. These are described in the following:
With very small motors, there is not sufficient space for Hall sensors or encoder discs. The volume of the sensor system would exceed that of the drive itself. This is remedied by means of so-called sensorless commutation. With brushless DC-motors, the back-EMF that is back-induced in the winding by the magnetic field is often processed and evaluated for this purpose. It is thereby possible to obtain both signals that are necessary for the commutation of the motors as well as signals that contain the speed information. This principle only functions above certain speeds, however, as the voltage signal is weak at low speeds – and is actually zero when at a standstill, causing the rotor position information to be interrupted.
In block operation, so-called digital Hall sensors only provide information about which strands are switched on or off next. As a result, the sensor signals are rectangular. With three strands, there are, thus, six possible switching combinations. These relatively simple electronics ensure low wear and also facilitate high starting torques and high speeds.
Hall sensors can be used just like encoders to generate a sinusoidal control voltage or a sinusoidal current in the windings. While a very high and constant current would be ideal, as this allows a high torque to be produced, asymmetries can cause in interruptions during the reversal of current. These result in torque fluctuations. Because the sudden change in current is associated with the induction of a voltage (back electro-motive force, or back-EMF), interactions with the magnetic field can occur that produce forces in the bearings. This means increased wear and, under certain circumstances, can lead to additional noises. This effect can be avoided by using a sinusoidal current. Current reversal occurs at the moment the current would be passing through the zero line anyway. Sine commutation therefore results in especially low noise and minimal torque fluctuations. For motors that employ the bell-type armature principle, these properties are of great significance due to the typically very low inertia. They make an especially positive impression at low speeds.