What is the difference between brushed and brushless electric motor?

Answer 1

To move, an electric motor must have one magnetic field slightly ahead of another. the magnetic attraction causes a pull that makes the magnets get closer together. To keep it moving, you need to move the magnet further away so that it never reaches the other magnet.

Ok. So how do you keep moving it ahead? Well, you use an electro-magnet rather than a permanent magnet. You put multiple electro-magnets in a circle around the shaft of the motor. You put a permanent magnet on the outside of the shaft separated by an air gap. Then you switch on the power to the electromagnet it causes the shaft to rotate so that the electro and permanent magnets line up. To keep it going, you have to switch to the next electro-magnet on the shaft that is slightly off alignment with the permanent magnet. Every time one of the shaft's multiple electromagnets try to line up you switch over to the next one so that they are always pulling toward the fixed permanent magnets.

So, how do you do the switching? One way is to have the electromagnets connection wires stick out of one side of the shaft so that the set of electromagnets that are close to the magnet are connected. The contacts that aren't rotating are called brushes. The inner wires that connect different electro-magnets for different rotations of the shaft are called a commutator. The brushes are always in contact with the commutator and so they wear down after a while. One strike against brushes. There is another strike that has to do with when you put a big load on the motor. The magnetic field ends up rotating such that when you switch poles there is still current in the wires. Because the electromagnet is inductive a sudden change in current causes a huge voltage spike (think lim Ldi/dt with t -> 0). That voltage spike causes sparks that cause the commutator and brushes to wear even faster.

So, brushles -- get rid of the brushes. Put the permanent magnet on the shaft and put the electro-magnets in a circle around the shaft. Also - put some tiny magnetic sensors around the motor. To switch the right electro-magnet on you connect the wires to the power source using a semi-conductor switch. The switches are generally Metal Oxide Field Effect Transistors (MOSFETS) which have built in diodes for that over-voltage Ldi/dt thing.

Now you have a microprocessor or some specially designed electronics connected to the magnetic sensors (Hall effect devices) and to the semiconductor switches. When the magnetic sensor turns on because the motor is at an angle in which a certain electro-magnet should be turned on, the microprocessor or electronics switch on the appropriate semiconductor switch (MOSFET) connecting the correct electromagnet to the power source. So, no bruches. Just semiconductor switches, sensors, and a control circuit.

Hope this is helpful,

Al

I hope this makes sense and it really