The electronically commutated motor are the characterized especially by their favorable torque characteristics, high power, extremely broad speed range and of course by their unsurpassed service life.

The differences between a DC motor having a mechanical commutation system and a BLDC motor are mainly found in: ■The product concept ■The commutation of phase currents. From the user's point of view, DC brushless motors follow the same equations as those with brushes: torque is proportional to current, speed depends on the voltage and the load torque.

In the conventional DC motor commutation takes place mechanically through the commutator-and-brush system. In a BLDC motor, commutation is done by electronic means. In that case the instantaneous rotor position must be known in order to determine the phases to be energized. The angular rotor position can be known by: ■Using a position sensor (Hall sensor. optical encoder. resolver) ■Electronically analyzing the back-EMFof a non-energized winding. This is called sensorless commutation.

In general, BLDC motor have three-phase windings. The easiest way is to power two of them at a time, using Hall sensors to know the rotor position. A simple logic allows for optimal energizing of the phases as a function of rotor position, just like the commutator and brushes are doing in the conventional DC motor.

The rotor position may also be known by use of an encoder or resolver. Commutation may be done very simply, similar to the procedure with Hall sensors, or it may be more complex by modulating sinusoidal currents in the three phases. This is called vector control, and its advantage is to provide a torque ripple of theoretically zero, as well as a high resolution for precise positioning.

A third option requiring no position sensor is the use of a particular electronic circuit. The motor has only three hook-up wires, the three-phase windings are connected in either triangle or star. In the latter case, resistors must be used to generate a zero reference voltage. With this solution the motor includes no sensors or electronic components and it is therefore highly insensitive to hostile environments. For applications such as hand-held tools, where the cable is constantly moved, the fact of just three wires is another advantage.