Brushless Motor PM Contruction

In the case of the brushless motor the location of the magnets and windings are swapped. The magnets are mounted to the rotor, and the windings to the stator. Because of this the need for a mechanical commutator declines so one does not have the pitfalls of a brushed-commutator. The pitfalls are a limited livespan caused by wear and the sensitivity for leading through peak currents and large powers. The sensitivity of the brushed motor comprises the creation of sparks and the burning of grease on the commutator surface that leads to interruptions in the current supply and thus a failing drive. Something that is avoided when a brushless motor is used.

Electrical commutation

Instead of a mechanical commutation the commutation is done electric and is being done by an inverter. This inverter knows the position of the rotor (the position of the magnets w.r.t. the windings) so the the correct voltage (voltage inverter), or the correct current (current inverter) can be applied to the windings. There are three forms of electrical commutation:

• Block commutation with Hall sensors

The rotor position is reported by three in-built Hall sensors arranged in an offset of 120°. These sensors provide six different switch combinations per revolution. The three partial windings are supplied in six different conducting phases in accordance with the sensor information. The current is block-shaped and the induced voltage (back EMF) is sinusoidal which means that the three phase approach suffers from an intrinsic 13% torque ripple.

• Sinusoidale commutation

The 13% torque ripple is avoided when a sinusoidale current is applied. This is achieved by using a high resolution position signals from an encoder or resolver for generating sine-shape motor currents in the electronics. The currents through the three motor windings are dependent on the rotor position and are shifted at each phase by 120 degrees (sinusoidal commutation). This results in the very smooth, precise running of the motor. The electronics are more expensive than block commutation.

• Sensorless block commutation

In this concept the rotor position is determined using the progression of the induced voltage. The electronics evaluate the zero crossing of the induced voltage (EMF) and use that to commute the motor at the right time. The amplitude of the induced voltage is dependent on the speed. At low speeds the voltage signal is too small to precisely determine the zero crossing so that is why special algorithms are required for starting. Sensorless commutation is unsuitable for servo-applications which typically require the load to be accelerated and decelerated a lot.

Sometimes the inverter is integrated in the motor design so that the motor can be driven like a brushed motor (2 wires).

• Stator with iron teeth
• Ironloss stator

Stator with iron teeth

The stator is constructed with iron teeth that have a small airgap with the rotor. Around these teeth the windings are wound (as is done in case of an iron core motor). Because of this construction the brushless motor will endure cogging and iron losses. The stator, the non rotating part of the motor, can be on the outside or on the inside. When the stator is on the outside, the teeth point inwards and the rotating magnet is positioned in the center of the motor. This type is called an inrunner.
When the stator is on the inside, the magnets are positioned on a rotating bell. This type of brushless motor is called an outrunner. This type generally has more torgue with a lower rotation speed.

Ironloss stator

This construction of the brushless motor looks like that of a coreless motor and does not have cogging or ironlosses. Cogging is the hard stops you feel when rotating the motor by hand. The windings are wound in the shape of a tube and glued together with epoxy, and mounted to the stator. The magnets are contained inside the tube and mounted to the rotor. The disadvantage of 'this' particular ironless construction compared to the ironless brushed motor is that the inertia is larger because the magnet is rotating instead of the much lighter windings.