Principles and characteristics of electromagnetic relays

An electromagnetic relay is a switch that uses an electromagnet to control the on/off state of a working circuit

(1) Structure: The main components of an electromagnetic relay are electromagnet A, armature B, spring C, moving contact D, and stationary contact E. (As shown in the figure)

(2) The working circuit can be divided into two parts: the low-voltage control circuit and the high-voltage working circuit. The low-voltage control circuit includes the electromagnetic relay coil (electromagnet A), low-voltage power supply E1, and switch S; the high-voltage working circuit includes high-voltage power supply E2, motor M, and the contacts D and E of the electromagnetic relay

(3) Operating Principle - When the switch S in the closed low-voltage control circuit is closed, current flows through the coil of the electromagnet A, generating a magnetic field that exerts an attractive force on the armature B, causing the moving and stationary contacts D and E to make contact, closing the working circuit and enabling the motor to operate. When the low-voltage switch S is opened, the current in the coil disappears, and the armature B, under the action of the spring C, causes the moving and stationary contacts D and E to disconnect, breaking the working circuit and stopping the motor  

As long as a certain voltage is applied across the coil, a certain current will flow through the coil, generating electromagnetic effects. The armature will be attracted to the iron core by electromagnetic force, overcoming the tension of the return spring, thus driving the armature's moving contact to engage with the stationary contact (normally open contact). When the coil is de-energized, the electromagnetic attraction disappears, and the armature returns to its original position due to the spring's reaction force, releasing the moving contact from the original stationary contact (normally closed contact). This engagement and release process achieves the purpose of making and breaking electrical circuits. For the 'normally open and normally closed' contacts of a relay, they can be distinguished as follows: the stationary contact that is in the disconnected state when the relay coil is not energized is called the 'normally open contact'; the stationary contact that is in the connected state is called the 'normally closed contact'.