A switching device is an element capable of opening or closing electrical power circuits in a controlled manner.
We have reached the end of the electromagnetism module with a practical question:
If we have a microcontroller or a PLC with low-power signals, how do we control an industrial 400 V motor drawing tens of amps?
We cannot connect the motor directly to the chip; we would vaporize it. We need an intermediary. A device that accepts a weak command and executes a strong action.
That device is the Electromechanical Switching Element. And its greatest exponents are the Relay and the Contactor.
The Relay
A Relay is essentially a mechanical switch, but instead of moving it with your finger, we move it with an electromagnet.
Its operation is directly based on what we have seen in previous articles (Oersted + Solenoid):
- Control Circuit (The Coil): We apply a small current to an internal coil.
- Magnetism: The coil becomes an electromagnet and attracts a movable metal piece (armature).
- Power Circuit (The Contacts): When it moves, the armature pushes metallic contacts, closing or opening the high-power circuit.
Galvanic Isolation
The beauty of the relay is that there is no electrical connection between the coil and the contacts. They are physically separated. The command is transmitted via magnetism.
- This protects your control circuit. If the 230V motor explodes and there is a short circuit, your microcontroller on the other side is safe.
Types of Contacts
A switched relay usually has three contact terminals. It is important to understand them:
- COM (Common): This is the moving contact that alternately connects to NO or NC.
- NO (Normally Open): Like a light switch. At rest, it is open (off). When the relay is activated, it closes (turns on).
- NC (Normally Closed): At rest, it is connected to the common terminal and opens when the coil is activated.
The Contactor
If a relay is great, why do contactors exist? The difference is not just size: the contactor is designed to handle power loads, often motors, with specified utilization categories and durability.
- Relay: Usually used for signaling, automation, and moderate loads. Its capacity depends on the load type, voltage, and required electrical life.
- Contactor: It is prepared for repeated switching of power loads and usually has main poles, auxiliary contacts, and accessories for protection and interlocking.
Important Differences
Beyond size, the Contactor has features that the relay does not:
- Arc Extinguishing Chamber: When you separate contacts carrying high current (especially in inductive motors), a spark (electric arc) jumps. The contactor has a special internal design to “blow out” and extinguish that spark before it melts the metals.
- Auxiliary Contacts: The contactor usually has power contacts (for the motor) and additional small contacts (auxiliary) to tell the PLC if it has closed or not.
- Are you going to turn on a light bulb or an electronic board? Use a Relay.
- Are you going to turn on a three-phase motor, a pool pump, or an entire house circuit? Use a Contactor.
Solid State Relays (SSR)
Although the title says “electromechanical”, we cannot ignore modern evolution. SSRs (Solid State Relays) do the same thing as a relay, but without moving parts. They use semiconductors (triacs, transistors) and light (optoisolators) internally.
- Advantages: They are silent, have no mechanical wear, and some models with a transistor allow fast switching.
- Disadvantages: They exhibit voltage drop, heating, and leakage current. AC SSRs with a triac are not suitable for just any PWM, and the common failure mode of many semiconductors is a short circuit. Isolation depends on the model.
Up until now, in almost all examples we have assumed direct current. In the European grid, alternating voltage completes 50 cycles per second and changes polarity twice in each cycle.
