Types of electromechanical relay based on contact state
Electromechanical relays are basically divided into two main groups; SPDT and bistable relays.
The contacts of SPDT DC or AC relays automatically return to the idle state when the exciter is switched off. The contacts of bistable relays remain in the current switch position when the exciting current is switched off.
Types of electromechanical Relay based on working principle.
electromagnetic attraction, electromagnetic induction.
Electromagnetic attraction relays operate by virtue of a plunger being drawn into a solenoid, or an armature being attracted to the poles of an electromagnet. Such relays may be actuated by d-c or by a-c quantities.
Electromagnetic-induction relays use the principle of the induction motor whereby torque is developed by induction in a rotor; this operating principle applies only to relays actuated by alternating current, and in dealing with those relays we shall call them simply “induction-type” relays.