A brushless DC motor has the rotor and stator positions interchanged compared to a conventional, permanently excited DC motor. The winding is located in the outer stator where permanent magnets are normally found, and the permanent magnets are in the rotor. This construction is similar to that of a permanently excited synchronous motor. The commutator supplies the stator winding with a pulse-modulated direct current, reducing the cost of electronic commutation as the stator winding typically consists of only three or four bundles of windings. These bundles are adjusted in such a way that the flux density of the stator and rotor is approximately phase-shifted by 90°, which securely fixes the rotor’s position. Hall probes, an optoelectronic system, or a magneto-resistive system are usually used to achieve this.

Brushless DC motors not only have additional power electronics windings, but also utilize new permanent magnetic materials, such as neodymium-iron-boron and samarium-cobalt, although the latter are still relatively expensive.

The latest advanced solution comes from the Magnet Motor Company, which boasts simple construction, excellent electrical parameters, and a compact weight and size. This motor belongs to the electronic commutation group of synchronous motors with permanent excitation. For all electric motors, the achieved torque is proportional to the magnetic induction in the air gap, the axial length of the rotor, and the square of the air gap radius. The outer rotor design is advantageous because the moment depends on the air gap radius squared. It is composed of pressed laminates with separate, tangentially magnetized magnets of alternating polarity (neodymium-iron-boron), resulting in no rotating electrical parts. Inside there is a stator, which is composed of pressed electrical sheets and forms high-pole coil carriers. The coils are connected to the output of the power electronics, which commutates the currents into the stator winding so that the motor behaves like a DC motor with external excitation. It is the so-called electronic commutation. Regulation is simple and flawless across the entire speed range up to n = 0. To handle the tenfold increase in power compared to conventional electric motor designs, the stator winding is cooled by liquid. Additionally, the motor is up to four times lighter than conventional design, and smaller.


left: rotor – rotor bandage, laminates, magnet

middle: magnetic flux, transformer plates, magnet – power electronics – stator – coil, carrier, laminates

right: 1-wires, 2-stator, 3-rotor, 4-bearings