AC relays work by utilizing an alternating current (AC) to energize a coil, creating a magnetic field that actuates a mechanical switch. When voltage is applied to the coil, it generates a magnetic field which attracts the armature. This movement either opens or closes the contacts, thereby controlling a separate circuit.
Key Components and Functionality
To understand how an AC relay functions, it's essential to look at its primary components:
- Coil: A winding of wire that generates a magnetic field when AC voltage is applied.
- Armature: A movable component that is attracted by the magnetic field generated by the coil.
- Contacts: These are the switches that open or close when the armature moves. There are typically three types:
- Normally Open (NO): The circuit is open until the relay is energized.
- Normally Closed (NC): The circuit is closed until the relay is energized.
- Common (COM): A terminal that connects to either the NO or NC contact, depending on the relay's state.
The Operational Process
Here's a step-by-step breakdown of how an AC relay operates:
- AC Voltage Application: When an AC voltage is applied to the relay coil, an alternating magnetic field is generated.
- Armature Attraction: The alternating magnetic field attracts the armature. Since the AC current changes direction periodically, the magnetic field does as well. To ensure consistent attraction, AC relays often use a shading coil. This helps maintain a minimum holding force, preventing the armature from vibrating at the AC frequency (e.g., 60Hz in the US).
- Contact Switching: The movement of the armature causes the contacts to either close (if they were normally open) or open (if they were normally closed), thus completing or interrupting the circuit connected to the contacts.
- De-energization: When the AC voltage is removed from the coil, the magnetic field collapses. A spring then returns the armature to its original position, and the contacts revert to their normal state.
Why Use AC Relays?
AC relays are beneficial in applications where the control signal is AC, or where the load being switched is AC. They provide electrical isolation between the control circuit and the load circuit, enhancing safety and preventing damage.
Considerations for AC Relays
- Coil Voltage: Ensure the applied AC voltage matches the relay coil's voltage rating.
- Contact Rating: The contacts must be rated to handle the voltage and current of the circuit they are controlling.
- Frequency: AC relays are designed to operate at specific frequencies (e.g., 50Hz or 60Hz).
