Brushless motor

brushless motor , or ” motor brushless ” or synchronous machine self-driven permanent magnet is an electric machine of the category synchronous machines 1 , the rotor consists of one or more permanent magnetsand provided with origin of a sensor rotor position ( Hall sensor , synchro-resolver, incremental encoder …).


Seen from the outside, it works in direct current . Its name ( Brushless English ) comes from the fact that this type of engine contains no rotating collector and therefore no brushes. On the other hand, an electronic control system must ensure the switching of the current in the stator windings 2 . This device can be either integrated in the motor for small powers, or external in the form of a power converter (inverter). The role of the more electronic sensor control unit is to ensure the self-driving 3 of the motor, that is to say the orthogonality 3 of the magnetic fluxrotor with respect to the stator flux 3 , a role formerly devolved to the collector-broom assembly on a DC machine 3 .

Evolutions in relation to the DC machine

This type of electric motor eliminates all the disadvantages of the conventional DC motor : switching problems at the collector, unloading, inertia, cooling (the joules losses being located at the stator they are easier to remove), substantially greater power density , geometry, life time; in particular, the degree of protection (IP) can be increased compared with DC machines because of the absence of brushes.

At equal performance, its performance is always better, this being due in part to the absence of mechanical and electrical losses associated with the brushes (especially during low loads). But also most of the time at its inertia significantly reduced – especially for models using samarium-cobalt or neodymium-iron-boron magnets – compared to a machine equivalent to direct current, this parameter being preponderant in many applications, in particularly in the acceleration phases.

Always at equal performance, the brushless motor is lower cost than the DC machine due to the replacement of the collector and brushes by an electronic sensor at a very low cost. For small powers, this sensor provides the two functions of rotor position detection and current switching. In this case, the operation is identical, seen from the outside, to a DC machine: it is sufficient to vary the supply voltage to vary the speed of rotation and in many uses this does not require the recourse to an electronic speed variator (small fans for example, the Hall effectsensors incorporated in the stator also ensuring the switching of the phases).

For the vast majority of applications requiring electronic control and control of torque , speed and / or position, the advantages of the brushless motor are such that it has completely replaced the DC machine and, in conjunction with the progress of power electronics (eg IGBTs ), the cost of these solutions has been reduced at the same time as their performance has been significantly improved.


The engines brushless are widely used in industry, especially in servo mechanisms of machine tools and robotics 4 , where they removed the DC machines . Such motors are found for couples from a few new meters up to several hundred Nm and powers from a few hundred watts to hundreds of kilowatts.

In particular, they equip hard disks and DVD burners .

A simplified and popular form of these technologies is used in fans providing cooling of electronic systems, including microcomputers . In this case, the stator (coil) is inside and the rotor (with the magnets) on the outside.

In the field of transportation, the electric motors that equip hybrid vehicles such as the Toyota Prius and the Honda Civic IMA to ensure, among other things, low speed operation are brushless motors. These engines also equip recent electric cars , such as the Chevrolet Volt or the Nissan LEAF . Type engines brushless are also used for ventilation systems / auto air conditioning since the 1990s ; one of the main advantages in this case is their silence.

They also equip power-assisted bikes, bikes that are pedaled as on a conventional bike but where a motor comes to help the effort. Some scooters on the market also use this engine for low speeds or full replacement of the engine is the case of the BMW C-evolution scooter .

They are also very used in model making to move model aircraft, helicopters ( model aircraft ) and small drones . They are less noisy than the motors with brushes and their weight / power ratio is very favorable to their use in this area.

They are also found in the motorizations of satellite dishes .

Notes and references

  1. ↑ ( in ) P. Zimmermann, ” Electronically commutated DC Feed Drives for Machine Tools “, Robert Bosch GmbH – Geschäftsbereich Industrieaurüstung, Erbach, Germany, p.  69-86, Proceding of PCI Motorcon , September 1982, p.  72.
  2. ↑ P. Zimmermann, op. cit. p.  81.
  3. ↑ a , b , c and d P. Zimmermann, op. cit. p.  78-81.
  4. ↑ P. Zimmermann, op. cit. p.  69.


: document used as source for the writing of this article.

  • In ) P. Zimmermann, ” Electronically commutated DC Feed Drives for Machine Tools “, Robert Bosch GmbH – Geschäftsbereich Industrieaurüstung, Erbach, Germany, in Proceding of PCI Motorcon , September 1982, p.  69-86.

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