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After the decision to go with a brushless motor is made and the motor model is selected, one final decision needs to be made: sensored or sensorless?
The sensors on a brushless motor are usually Hall Sensors. These devices detect change in magnetic fields (a shift from North to South and vice versa). They are used to detect the position of the permanent magnet rotor inside the motor housing. Depending on how they are positioned inside the motor and how the rotor of the motor is designed, they may be detecting the actual rotor magnets or additional magnets (very small) inserted in the back end of the motor at a 90 degree angle with the actual magnets. With Aveox motors, these small magnets resemble small pills and are called ‘pill magnets’.
Before any explanation of the sensors is made, a fundamental understanding of motors needs to be developed. A motor functions on Faraday’s principle which states that when an electric current runs through a coil of wire it creates a magnetic field perpendicular to the coil. Based on this principle, you can essentially create a magnet out of a coil of wire. The advantage of this type of magnet over a permanent magnet is it can be turned on and off. Another variation of Faraday’s law states that when a magnet moves through a coil of wire, it creates an electrical current in that wire. This is how a generator works.
Now, imagine if you had a regular, permanent magnet sitting next to a coil of wire in such a way that the magnet’s south pole was closest to the coil. If an electrical current is passed through the coil in such a way that a north/south pole is created with the north pole being closest to the permanent magnet, then the magnet would move towards the coil of wire due to the attraction of the opposing poles.
In a permanent magnet motor, the permanent magnets are placed on the rotor and the coil wire (the stator) is inside the housing of the motor. In order to get the rotor to turn, we need to create a magnetic field inside the coil which attracts the poles. If the magnetic poles of the rotor are not aligned properly with the poles of the "created" magnetic field, then nothing will move. Therefore, it becomes necessary to know where the rotor is with respect to the stator. One way of knowing is to place sensors inside the motor which detect the position of the rotor with respect to the stator. Once this information is known, the motor controller can place the current in the correct coils of wire (motor phases) and create the desired motion. Once the rotor is in motion, the controller begins switching the current from phase to phase to create a rotating magnetic field which is always aligned with the rotating permanent magnet rotor.
There are other techniques of "‘sensing" where the rotor is without sensors. Three of them will be discussed here. One way is to arbitrarily place current into any two phases. There is no guarantee that the created magnetic field will be in the correct direction or position. However, it will be enough to cause the rotor to slightly move. The movement of the permanent magnet rotor in the stator, creates a back emf (electro motive force) on the stator wires. This back emf can be measured by the controller. Based on the value measured, we can calculate where the rotor is. This is at best a guess. If you have ever turned on a sensorless system and observed that the rotor moves erratically at first, you are observing the controller trying to get a reading on the position of the rotor.
Another method is to slowly start cycling the current through the motor phases. The slow speed allows the rotor to begin moving and to eventually fall into synch with the rotating magnetic field in the stator.
The third method employs the knowledge that when a magnet is placed inside a coil of wire, it affects its inductance. By knowing the inductance of the stator phases and measuring the change in them, the position of the rotor can be determined. This method works with a particular type of motor and is not very versatile.
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