Principle and theory of motor operation

The discovery that led to the invention of the electric motor was simply this:
 A current carrying conductor will experience a force when placed in a magnetic field. The conductor can be any metal—iron, copper, aluminum, and so on.

The direction of the force is perpendicular to both the magnetic field and the current .

 A demonstration of this principle is easy to perform with a strong magnet, flashlight battery, and a wire and is highly recommended! Place the wire between the magnet poles and alternately connect and disconnect the wire from the battery. Each time you complete the circuit, you should feel a little tug on the wire.
The magnitude of the force on the wire can be calculated from the following equation:

 F = IBL sin θ 
where
F = force on the conductor (in Newtons)
I = current through the conductor (in amperes)
B = magnetic flux density (in gauss)
 L = length of the wire (in meters)
θ = angle between the magnetic field and current

Advantage of AC motor over DC motor

AC motors have some advantages over DC motors:
AC motors tend to be smaller, more reliable, and less expensive. However, they generally run at a fixed speed that is determined by the line frequency.

 DC motors have speed-control capability, which means that speed, torque, and even direction of rotation can be changed at any time to meet new conditions. Also, smaller DC motors commonly operate at lower voltages (for example, a 12-V disk drive motor), which makes them easier to interface with control electronics.