Understanding the Basics of Electrical Controls

Every home has circuit breakers to protect electrical devices and the house from damage, fires, or electrocution. Breakers trip (that is, shut off) when they see too much current coming through a circuit.

Students have plenty of experience using appliances like torches and mobile phones that depend on electric circuits to work. But they often need clarification about what happens when a battery switches on and lights up a bulb.

Relays

Relays are electromechanical devices that convert low-voltage signals of electrical controls, either AC or DC, across their terminals into a pulling mechanical force that operates a set of electrical contacts. Relays are used to assemble logical circuits that perform more complex functions.

Large relays can handle high currents and are often employed to switch motors or other heavy loads. More miniature reed relays are also helpful in controlling analog signals from sensors or connecting to other controls.

A common type of control relay is a single-pole, single-throw (SPST), which has two terminals that can be connected or disconnected, including one for the coil. Some relays have a normally open, timed-close contact, which closes when the coil is powered but only after it has been continuously powered for a specific amount of time.

This type of contact is sometimes referred to as Form C or “transfer” contact (“break before make” functionality). Other relays may have different configurations of poles and throws, such as double-pole double-throw (SPDT), which has five terminals, including two for the coil.

Switches

Switches, those seemingly mundane devices found in every home and industry, play a vital role in the operation of modern technology. These electromechanical marvels act as gatekeepers, controlling the flow of electric current within circuits. From simple light switches illuminating our rooms to complex industrial switches regulating intricate processes, they contribute significantly to our daily lives.

Beyond functionality, the electric equipment manufacturer also considers aesthetics and user experience. Switches come in various sizes, styles, and finishes to complement different environments and user preferences. From sleek and modern designs to classic and traditional styles, there’s a switch to match every taste and application.

The contacts of a switch can be in one of two states: “closed,” allowing electricity to flow between the contacts, or “open,” which disconnects the contacts and stops the current. A mechanism (often a knob or actuator) is used to transition the switch between its two states.

The switch’s power rating is the amount of electric current it can safely handle. Switches operating in high-power applications must be specially constructed to ensure minimal power loss, smooth transition between the on and off positions, and protection against destructive arcing between open and closed contacts.

A switch’s number of poles and throws tells its precise function; for example, a single-pole, double-throw switch is an internal polarity reversing device. Contact and terminal plating materials also affect a switch’s responsiveness, reliability, cost, and durability.

Contactors

Contactors are heavy-duty electromagnetic switches commonly used to control motors and large lighting loads. They work similarly to relays but handle higher current and voltage rates.

They have a power contact and an auxiliary contact connected by a contact spring. These contacts must have stable arc resistance and high welding resistance. They also must be able to withstand mechanical stress and erosion.

The power contact is stationary, while the auxiliary is movable. They connect when the coil is energized and disconnect when it’s de-energized. The power contact is the only one from which the full load current will flow.

The auxiliary contact is usually labeled with two-digit numbers on the device’s body; the first number indicates the order, and the second shows the function. Contactors have a built-in arc chute to suppress the electrical arc that may form during switching. This reduces wear on the contacts and extends their lifespan.

Loads

A load is a device that consumes power. It can be an appliance, a light, or any electricity equipment. Electrical engineers often discuss the load a device consumes and its role in power systems.

There are three basic types of electrical loads: resistive, capacitive, and inductive. Resistive loads contain a heating element that converts electrical energy into thermal energy. Examples include light bulbs, toasters, and electric water heaters. In this type of load, the current waveform is in phase with the voltage waveform. They reach the maximum peak at the same time and also reach zero peaks simultaneously. The power factor of this load is unity.

Capacitive loads rely on capacitors to store electrical energy. Their current waveform leads to the voltage waveform. This leads to a power factor that is less than unity and either leading or lagging. Inductive loads have wire coils that create individual inductive fields—their current waveform peaks immediately after the voltage sine wave peaks.