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how does a brushless DC motor works

In this video we will talk about a DC motor which is the ideal choice for many applications where it’s needed:

low maintenance
less noise
speed control
and low cost

In the specific, we are going to see how does a brushless DC motor works.
We can assume already by its name, that this type of electric motor has no brushes.
As we have already seen in our previous video about the working priciple of DC motors, the brushes are sliding devices which allow the electricity to flow into the copper coil of the engine.

The brushless motor have no sliding elements on the collector, so, this completely solves the problem of maintenance and replacement of brushes and other electrical contacts.

JAES, in its catalog offers a wide range of brushless motors of the major manufacturers.

Unlike the common DC motors, that use brushes for their functioning, brushless motors do not produce sparking or friction noises within them.
This is a very interesting feature of brushless motors, becouse allows them to be suitable for use in environments where sparks are dangerous. For example in environments where ignitable gases are present.
This type of motor has a rotor with permanent magnets, it could not be otherwise, since there are no contacts on the rotor, as we already said.
Its stator has electromagnets excited by windings in coils.
This is an important difference compared to the traditional brushes DC motor, since the latter can have both stator and rotor composed of electromagnets, to increase the magnetic fields and therefore have an higher mechanical power density by unit of weight of the motor.
The torque of the brushless motor is due to the magnetic interaction imposed by the electromagnets on the permanent magnets.
There is a precise sequence of excitation of the electromagnets which imposes to the permanent magnets an angular movement, which is always in the same direction, and it is as constant as possible under the same excitation.
When electricity flows into coil 1, the opposite poles of the rotor and stator are attracted to each other.
When the rotor is getting closed to the coil 1, the electricity flows in the coil 2. When the rotor is getting closed to the coil 2, the electricity flows in the coil 3. Later in the coil 1 the electricity will flow again, but with opposite polarity. This process is repeated continuously inside the engine ensuring a constant rotation of the rotor.
Obviously the sequence is cyclical and the rotor continues its rotation until it is powered, repeating the sequence, from time to time.
To be more specific: the sequence of excitation of the electromagnets inside the stator is such that the magnets of the rotor are attracted to the electromagnets without never reaching them, as the excitation is always and only of those coils "not yet reached".
An example to better understand this principle is represented by the dog race in which a hare is placed in front of the greyhounds at such a distance to be seen but never reached.
In this metaphor, the dogs represent the rotor that follows the magnetic flux generated in exact sequence in the coils of the stator.
In the case of more coils, always multiples of 2, it makes sense to use a sequence that uses them all, to increase the magnetic flux to which the permanent magnets are subjected, and turning off only the opposite electromagnets.
This simple foresight increases the torque, that is the power that the motor can supply, without substantially modifying its design criteria.
The greater the number of coils, the smoother is the rotation of the rotor, which will thus have a constant torque.
But how does our brushless motor know the precise moment in which to excite the correct sequence of electromagnets which allow continuity of the rotary motion?
For this purpose, an electronic controller is typically used. This controller has a sensor to detect the exact position of the rotor magnets and thus decide which coil energize by following the right excitation sequence.
The modern brushless motor sensors uses the Hall effect.
In this animation we can observe the configuration of a standard sensor.

This is a simple 2-pole device that can drive 2 or 4 or 8 or more electromagnets in the just seen sequence.
This, is instead the representation of a classic circuit layout with protection diodes. The presence of diodes is necessary to protect the sensor from negative self inductance, in case we need to suddently block the power supply from one of the coils.
There are basically 2 families of brushless motors. Those with external stator and those with internal stator.
We can summarize the main strength points of the brushless DC motor
• Brushless motor is noiseless
• It is reliable and has a very low MTBF parameter (which is the predicted elapsed time between its inherent failures)
• It provides low energy costs
• Does not require any maintenance.
• It can also be used in flammable environments
• It has a more competitive price than traditional brush motors of the same power.

However, the brushless motor has also its limitations, like:
• Requires an electronic driver in order to control the right excitation sequence.
• Using permanent magnets, the specific power is generally lower than the conventional motors equipped with stator and rotor.