What are different types of principles of operation of electro-mechanical relays?

What are different types of principles of operation of electro-mechanical relays?
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Answer: Electro-mechanical relays operate by two principles. Electro-magnetic attraction and electromagnetic induction. In electromagnetic attraction relay plunger is drawn to the solenoid or an armature is attracted to the poles of the electromagnet. In case of electro-magnetic induction, principle of operation is similar to induction motor. Torque is developed by electromagnetic induction principle
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