electromagnetic clutch
Electromagnetic clutches operate electrically, but transmit torque mechanically.

Construction
In a clutch, (B1) when powers applied, a magnetic field is created in the coil (A2 blue). This field (flux) overcomes an air gap between the clutch rotor (A2 yellow) and the armature (A2 red). This magnetic attraction, pulls the armature in contact with the rotor face. The frictional contact, which is being controlled by the strength of the magnetic field, is what causes the rotational motion to start. The torque comes from the magnetic attraction and also and the friction between the steel of the armature and the steel of the clutch rotor.



Basic operation
field, rotor, armature, and hub (output) are the main parts of the clutch. When voltage is applied the stationary magnetic field generates the lines of flux that pass into the rotor.The flux pulls the armature in contact with the rotor (the armature is connected to the component that requires the acceleration), as the armature and the output start to accelerate. Slipping between the rotor face and the armature face continues until the input and output speed is the same.

Engagement time
The first time consideration is the time it takes for a coil to develop a magnetic field, strong enough to pull in an armature.It depends on the amount of ampere turns in a coil, which will determine the strength of a magnetic field. The second one is air gap, which is the space between the armature and the rotor. The second factor detarmining the engagement time of the clutch is that which involves calculating the amount of inertia that the clutch needs to accelerate. This is referred to as time to speed. In reality, this is what the end-user is most concerned with. Once it is known how much inertia is present for the clutch to start then the torque can be calculated and the appropriate size of clutch can be chosen.

For more details, visit:
http://en.wikipediawiki/Electromagnetic_clutch