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Function of Relays in Generators

Digitally Controlled Electromagnetic Relays in Generators

Electromagnetic Relay
Electromagnetic relays are components that control circuits using an electrical signal. They are used in applications where it is necessary to control one or more circuits with one electrical supply circuit. The supply may or may not be isolated from the relay circuit. In many applications, relays are the link from low power input to high power output.

Each switch in the relay is called a pole. The poles can be Normally Open (NO) or Normally Closed (NC). Poles can be single or double throw. Throws indicate the number of contacts that open or close when the relay is energized.

Sequence of Operation Example
In this example, a relay is installed in a water treatment control panel. The relay coil is a low voltage design. Contacts are rated at 120 VAC 10 Amps. Relay has constant 120 VAC supplied to Pin 3 of relay. F01 is installed in the circuit in case of overload or short circuit. The sequence of events are as follows:
  1. The controller sends a 5 VDC signal through the coil pin 2 to pin 1. Current flow generating electromagnetic force.
  2. Throw moves and opens contact 3 to 4 and closes contact 3 to 5.
  3. Valve 1 solenoid moves from the closed to open position and chemical additive is injected into the system.
  4. Controller stops sending signal to coil (timer input). No electromagnetic force.
  5. Throw moves and opens contact 3 to 5 and closes contact 3 to 4.
  6. Valve 1 solenoid moves from closed to open position to stop additive flow.
Many automated industry control panels use this type of arrangement. Often there are banks of relays in the control panel that serve as an interface between the output of the controller and the component. However, relays are not commonly used on the input of the controller. Inputs are generally 5 VDC components that communicate directly with the controller.

Electromagnetic Relay Logic Circuits

Relay Logic Circuit
Relay logic circuits are the building blocks of the digital world. For example, digital gates such as AND/OR/NOR can be duplicated without the use of a digital controller.

In this circuit design, multiple relays are used to control outputs. The bank of relays are commonly located in a control panel. This panel resembles any other control panel, but it does not have a controller or associated digital components. 

Industries with digital control and interface systems often do not employ relay logic. This circuit application is found in older industrial machine applications. Often, these circuits have an isolated supply to one relay. This relay will energize/de-energize other relays when it changes states.

Sequence of Operation Example
Relays generate heat during operation. In large control panels ventilation is required. Some require cooling fans to maintain internal temperature. This circuit uses two relays to start the cooling fan, extinguish Stop Lamp and illuminate the Run Lamp. When the panel is energized 120 VAC is routed through Relay 1 pins 3 and 4, illuminating the Stop Lamp. When the Start Switch is pushed:
  1. Voltage is supplied to the Relay 1 coil (pin 1) and Relay 2 contact (pin 3). 
  2. Relay 1 contacts 3 to 4 open extinguishing the Stop Lamp and contacts 3 to 5 close.
  3. Relay 2 receives coil power on pin 1.
  4. Relay 2 is a double throw relay. Contacts 3 to 5 and 3 to 6 close. Run Lamp and Cooing Fan energize.
  5. When Start Switch is pressed again and opens, Relay 1 coil is de-energized closing contacts 3 and 4, energizing Stop Lamp.
  6. Relay 2 coil is de-energized opening contacts 3 to 5 and 3 to 6, extinguishing Run Lamp and shutting down Cooling Fan.
This example is a simple logic circuit and only a portion of what a control panel using this application would use. Relay logic circuits are complicated to troubleshoot. The addition of multiple pins and one relay relying on the other make the process more complicated to troubleshoot. 

Solid State Relays (SSR)

Solid State Relays
Solid state relays are an electronic switching device just as their counterparts electromagnetic relays. Electromagnetic relays have moving parts (throws). Solid state relays do not have any moving parts. 

SSRs use electronic components for switching operations. These components are comprised of dielectric materials and doped to desired levels. The construction of the SSR determines the switching capabilities. These relays receive a small input voltage (3 to 32 volts) and can control high voltages (up to 240 VAC).

Circuit cards solely employ the use of solid state relays. Common uses are Light Emitting Diodes (LED) and providing switched inputs to other low voltage electronic devices. Advanced control panels use these relays because:
  • Smaller, slimmer profile allowing for more streamline installation
  • Silent switching operations
  • Longer lifetime. No moving parts fail or contacts to pit
  • Output resistance constant regardless of amount of use
  • No sparking and can be used in explosive environments
  • Less sensitive to shock, vibration, humidity and external magnetic fields
Some disadvantages when comparing to electromagnetic relays are:
  • Has voltage and current characteristics of semi-conductor instead of contacts (generates heat when closed)
  • When open has lower resistance. Can have some reverse leakage measured in the micro amp range
  • Some designs have polarity-sensitive output circuits. Not present in electromagnetic relays


Relays are a big factor in the electronic switching world. Each relay and circuit design has its own advantages and disadvantages and provide varying degrees of protection. Having basic switching knowledge can aid when describing issues to technicians. Often, replacing the relay with a ready spare can resolve issues. If designing a new system or upgrading, give Diesel Service & Suply a call and our expert staff can assist you with any question that you may have.

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