How Electric Motor Works?
Electric motors are found in most electrical and electronic appliances: Fans, drillers, electric engines, DVD players, washing machines etc. creating rotary motion. They are also used in saw mills and cutters where these motors are mechanically designed to create linear motion.
Electric motors are based on the principle of electromagnetism. The mechanical construction uses two main components; a Stator and a Rotor. 
Stator (as in stationary) is a stationary component of the motor and rotor is a rotating component. Stator generally has permanent magnets (in some cases electromagnetic windings), and rotor has electromagnets (in some cases permanent magnets) attached. Ideally one of the two is a permanent magnet and the other is an electromagnet. When electric current passes through the electromagnet, it creates a magnetic field generating a magnetic force between the stator and the rotor. The attractive and repulsive forces between the electromagnet and the permanent magnet make the rotor to turn and rotate.
To get the rotor to turn, the electromagnets create a repulsive force at one end (pole) which attracts the permanent magnet towards the other end of the electromagnet.Once the rotor does a half-turn, the poles of the electromagnet flips (North to south or south to North) pushing the rotor further towards the other end. The process continues creating a circular motion. (Image source: Wikipedia)
The force created by the magnetic field between the rotor and the stator determines the torque of the motor. Number of times the rotor rotates in one minute determines the velocity of the motor, measured as Revolutions per Minute (RPM). Hence whenever you intend to buy a motor, always watch out for RPM and torque of the motor. Installing gears can increase either torque or velocity, but reducing the other.
AC motors and DC motors
AC Motor: Motors are designed to run on Alternate current (AC) or Direct Current (DC). However, it is very uncommon to use AC motors in our mobile robots as they are harder to use, and most of our robots and circuits are DC powered. Therefore use of AC powered motor is limited to stationary and industrial robots.
DC motor: DC motors are very easy to implement and most commonly used actuators in robots. There are different types of DC motors available and in the next section, we will discuss the various DC motors available.
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