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What is a Control Relay? Its types, function, difference between contactors and price.

March 23, 2020 | POSTED IN: Articles

What is a Control Relay? Its types, function, difference between contactors and price.

What is Control Relay?

A Control Relay is also known as a Relay, is a switch, an electromagnetic switch. A control relay allows electrical current to flow through a conducting coil that opens or closes a switch. It also protects the circuit current. With a control relay, users do not need to manually turn the switch to isolate or change the state of an electric circuit.  Currently, control relays play a crucial role in today’s electronic devices. They are electronic parts that power electronic parts such as motors, power plants, power supply systems, transistors, and many more. 

Different Types of Control Relays

There are different types of Control Relays depending on the operating principle and structural features.   Solid State Relays – It uses solid-state components to perform the switching operations without moving any parts.   Contactor – A large relay used to switch a large amount of electrical power through its contacts.   Electromagnetic Relays – Constructed with electrical, mechanical and magnetic components and have operating coils and mechanical contacts. Hence, when the coil is activated by a supply system, the mechanical contact is either open or close. The supply system has 2 types of AC and DC. Thermal Overload Protection Relay – works on the principle of the thermal effect of electrical energy. When excessive current flows through the circuit, the circuit opens due to the bimetallic strip experiencing an increase in temperature.

Understanding Contact Type of a Relay

Every Control Relay has a Contact type such as SPST-NO but what does it mean? 

Poles represent the number of circuits controlled by a switch. Throws represent the number of positions the switch can adopt.  

Single Pole Single Throw, SPST, has two terminals that can be connected and disconnected. Including two for the coil, such a relay has four terminals in total.   Single Pole Double Throw, SPDT, has a common terminal that connects either one of two others. Including two for the coil, this relay has five terminals in total. Regardless of whether the coil is active or inactive, either ‘A’ or ‘B’ is always resting while the other needs to be the coil to be powered.

Double Pole Single Throw, DPST is equal to two SPST activated by a single coil. Including two for the coil, this relay has 6 terminals in total. Double Pole Double Throw, DPDT is equal to two SPDTs activated by a single coil. Including two for the coil, this relay has 8 terminals in total.  

Difference between Normally Open (NO) and Normally Closed (NC) contacts

NO contacts allow current when the relay is energized. This means when there is voltage, the contact closes and allows current to flow through. NC contacts allow current when the relay is not energized. Opposite to NO, NC contact opens and interrupts the current flow.   *Change Over (CO) is the same as Double Throw (DT) relay.  

Differences between Control Relay and Contactors

Both of these electrical devices perform the same task of switching a circuit and even Contactors is a term for large relays.

Does this mean it is okay to use either Control Relay or Contactors? No, and here is why?   

  • Load Capacitor – Control Relays are classified as carrying loads of 10 Amperes or less. Whereas, Contactors will deal with loads of more than 10 Amperes. Contacts - Contactors are mostly designed to operate NO while Control Relay can operate on either NO or NC.   
  • Auxiliary Contacts – Contactors are often fit with Auxiliary Contacts which are used to perform additional functions but Control Relay doesn’t.   
  • Safety Features – Since Contactors are operated to carry high loads, it is common for them to equip Safety Features such as Spring-Loaded Contacts, Arc Suppression and Overloads.   
  • Applications – Contactors are commonly built and used in 3 phase applications but a relay is more commonly used in single-phase applications.   How do I know whether I should get a Control Relay or Contactor?  

To summarize which electrical device to pick: 

Control Relay Contactor
10 A and below 9A and above
Max Voltage 250V Max Voltage 1000V
1 phase 1 or 3 phase

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