How Many Types of Relays Are There? Their Construction, Operation & Applications

Relays are electrically operated switches that use an electromagnet to mechanically operate a switch. They are used to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal.

There are many different types of relays, each designed for specific applications. Understanding the different relay types, how they are constructed and operate, and their common applications is important when selecting a relay for a particular purpose.

Electromechanical Relays

Electromechanical relays are the most common type. They consist of an electromagnet, a movable armature with one or more contacts, and a spring to provide tension. When the electromagnet is energized, it attracts the movable armature, which either makes or breaks a contact with the stationary contact(s), switching the relay on or off.

Key characteristics of electromechanical relays include:

• Use electromagnets to mechanically operate switches
• Contacts are electrically isolated from the coil
• Can switch AC or DC signals/loads
• Relatively slow operation (5-50ms operate/release time)
• Susceptible to physical shock and vibration
• Mechanical wear limits lifespan (millions of cycles)
• Commonly used in automotive, industrial control, test equipment applications

Major electromechanical relay sub-types include:

Solid State Relays (SSRs)

Solid state relays perform the same function as electromechanical relays but have no moving parts. They use power semiconductors like thyristors, triacs or MOSFETs to switch loads on and off. The control signal is optically isolated from the load circuit.

Key characteristics of SSRs include:

• No moving parts or mechanical wear
• Very long lifespan
• Fast switching speeds (as low as 1ms)
• Silent operation
• Higher cost than electromechanical relays
• Increased power dissipation and heat generation
• Commonly used for heating, lighting, motor, and industrial process control

Major SSR sub-types include:

Reed Relays

Reed relays are a special type of electromechanical relay. They contain reed switch contacts sealed inside a glass tube filled with inert gas. The contacts are made of magnetic material. When a magnetic field from the relay coil is applied, the reeds become magnetized and attract each other, closing the contacts.

Key characteristics of reed relays include:

• Hermetically sealed contacts
• Very small size
• Fast operation (as low as 0.5ms)
• Ability to switch very low level signals
• Limited current carrying capacity
• Susceptible to external magnetic fields
• Commonly used in test & measurement, telecommunications, medical and security systems

Contactors

Contactors are heavy-duty electromechanical relays designed to handle high power. They are used for switching electric motors, lighting, heating loads and power distribution.

Contactors typically have:

• High current carrying capacity (10A to 1000s of amps)
• Ability to withstand high inrush currents
• Robust construction for frequent operation
• Double-break contacts for higher breaking capacity
• Magnetic arc blowouts for extinguishing arcs
• Auxiliary low current contacts for control interlocking

Contactors are commonly used for:

• Motor starters and reversers
• Lighting control
• Capacitor bank switching
• HVAC control
• Starter generator control

Overload Relays

Overload relays are protective devices, rather than control devices. They are designed to prevent damage to motors due to overheating from extended overload operation. They contain either bimetallic strips or solder pot constructions that deform with heat and mechanically trip the relay if safe temperatures are exceeded.

Overload relays are usually used together with contactors to provide complete motor branch circuit protection. When specifying overload relays, key considerations include:

• Motor full load current
• Trip class (length of time before tripping)
• Ambient temperature compensation
• Manual vs automatic reset
• Single-phase vs three-phase sensing

Time Delay Relays

Time delay relays introduce an intentional time delay into the switching process. This delay can be on energization (on-delay), on de-energization (off-delay), or a combination of both. The time delay is accomplished using electronic timing circuits, pneumatics, or dashpots.

Applications for time delay relays include:

• Sequenced start-up or shut-down of equipment
• Allowing time for pressure to build up or dissipate
• Allowing time for motors to come up to speed before applying load
• Overriding momentary input fluctuations
• Staggering electrical loads to minimize peak demand

Time delay relays can have fixed or adjustable time delays, normally open or normally closed contacts, and operate on-delay, off-delay, or both. Typical time delay ranges span from fractions of a second to many minutes.

Frequency Relays

Frequency relays respond to changes in the frequency of an AC supply. They contain circuitry that measures the frequency and trips the relay output if it goes above or below preset thresholds.

Frequency relays are used for protection of equipment sensitive to over or under frequency conditions, such as:

• Generators and turbines
• Motors
• Transformers
• Electrical transmission networks

Specifying a frequency relay requires understanding:

• Nominal system frequency (50Hz or 60Hz)
• Under/over frequency trip points
• Operating time at set points
• Voltage and auxiliary supply requirements

Differential Relays

Differential relays compare current flowing into and out of a protection zone and trip if the difference exceeds a set threshold. This difference indicates a fault within the zone. They are very fast acting and highly selective. Common applications include protection of:

• Transformers
• Generators
• Buses
• Transmission lines
• Large motors

Modern differential relays use electronic sensors and microprocessor logic to perform the comparison and trip determination. Key setting parameters include:

• CT ratios and configurations
• Relay sensitivity (pickup)
• Time delay
• Harmonic restraint (to avoid tripping on inrush)
• CT circuit supervision

Buchholz Relays

Buchholz relays are specialized protection devices for oil-filled transformers. They are located in the pipe connecting the conservator to the main tank. Under normal conditions this pipe contains only oil. If a fault develops inside the transformer, gases are produced that flow up into the relay, displacing the oil.

The Buchholz relay contains two sets of contacts - one actuated by slow accumulation of gas, the other by a sudden oil surge caused by a severe fault. The former is usually arranged to give an alarm, the latter to immediately trip the transformer.

Frequently Asked Questions

What is the difference between a relay and a contactor?

The main differences are that contactors are designed for higher current loads and more frequent operation. Relays are typically used for lower power applications with less frequent switching. However, the terms are often used interchangeably, especially in Europe.

What is the difference between a solid state and electromechanical relay?

Solid state relays have no moving parts. They use semiconductors to perform the switching function, while electromechanical relays use physical contacts operated by an electromagnet. SSRs switch faster, last longer, and operate silently, but dissipate more heat and cost more.

What does SPDT and DPDT mean in relation to relays?

These abbreviations describe the contact arrangement in the relay. SPDT stands for Single Pole Double Throw, meaning it has one common terminal and two switchable paths. DPDT stands for Double Pole Double Throw and has two switchable poles, each with their own common terminal and two throws.

How do you wire a relay?

Relays have two separate circuits - the coil circuit and the contact circuit. The coil terminals get connected to the signal that will control the relay, observing the coil voltage and polarity. The contacts then connect in series with the load being switched. It's important that the contact ratings exceed the load current and voltage.

What is the purpose of a flyback diode across a relay coil?

When the current through an inductive load like a relay coil is suddenly interrupted, the collapsing magnetic field induces a large voltage spike in the opposite polarity. This can damage switching contacts and electronics. A flyback diode across the coil gives this induced current a safe path to dissipate, protecting the circuit.