38 A permanent magnet is a magnet whose magnetic field is generated by its internal structure and is always present. Search the Internet to find out who invented the first DC and AC motors. ONLINE RESOURCES Sustainable cities brush: spazzola conveyor belt: nastro trasportatore torque: coppia, torsione ELECTRIC MOTORS An electric motor is a device used to convert electricity into mechanical energy which is the opposite of what an electric generator does and operates using the principles of electromagnetism. How an electric motor works Every motor has a stationary part, called the stator, which is a permanent magnet , and a rotating part, called the rotor, made of a coil of wire wound around a cylinder, the armature, and placed inside the stator. When current is applied to the coil of wire of the rotor, this becomes a magnet and starts rotating between the two poles of the permanent magnet. The rotation creates a torque, which is the turning force of the motor. This rotation spins a shaft, a pole connected to the armature, which transmits the mechanical energy to the device run by the motor. DC Motor Diagram Armature core Pole shoe / Face Stator Field winding Bearing Shaft Armature winding Commutator Brush 94 ELECTRICITY DC and AC motors Motors can work either on alternating current (AC) or on direct current (DC). DC motors were the first type of motors produced. As the direction of the current is constant, a commutator is needed to reverse the current each time the rotor does a half turn. The oldest types of DC motors have brushes to connect the commutator to the rotor, while the more recent types have substituted the commutator and the brushes with electronic circuits to control the motor s speed and direction. DC motors are the most suitable types of motor for applications requiring a high starting torque, controllable speed and precision; they are normally used to run elevators and industrial robots. The brushless version is preferred for drones, electric vehicles, and toys. In AC motors, the direction of the current is automatically reversed, so this type of motor does not need either a commutator or brushes. AC motors have a constant speed; therefore, they are commonly used for applications where little changes in speed are needed, such as washing machines and fans, or in industrial machines, such as pumps or conveyor belts.