Automatic Transfer Switch (ATS) Using Classic Control Circuit

Automatic Transfer Switch (ATS) is one of the most important systems at any Gas station or factory, this system provides a reliable means of transferring essential load connections between primary (Main supply) and alternate (Power Generator) sources of electrical power.

Automatic Transfer Switch (ATS)

Automatic Transfer Switch
Figure 1

Data centers, hospitals, factories, and a wide range of other facility types that require continuous or near-continuous uptime typically utilize an emergency (alternate) power source such as a generator or a backup utility feed when their normal (primary) power source becomes unavailable.

There are many ideas to implement such a system but today we are going to make a simple one using Classic Control Circuit.

Circuit Components List

As we said above there are many circuits to implement this idea but we are going to illustrate the most simple and basic one and here are the required components:

SymbolNameFunctionOperating Voltage
K1ContactorControlling the incoming power from the main source (Power Network).220V AC
K2ContactorControlling the incoming power from the secondary source (Generator).220V AC
MCB1Main Circuit BreakerConnecting/Disconnecting power.380V AC
MCB2Main Circuit BreakerConnecting/Disconnecting power.380V AC
F1Protection FuseProtect the Main line control circuit.220V AC
F2Protection FuseProtect the Pump & Marsh control circuit.12V DC
F3Protection FuseProtect the Generator control circuit.220V AC
R1RelayIndicates the Main power outage.220V AC
R2RelayIndicates the Generator power outage.220V AC
R3RelayOperating the fuel pump.12V DC
R4RelayHandling the Generator marsh.12V DC
T1ON Delay TimerDelaying the Main power suppling.220V AC
T2ON Delay TimerDelaying the Generator supplying.220V AC
T3Flasher Timer (ON/OFF)Handling the marsh operation.12V DC

The ATS circuit consists of four major circuits:

  • Incoming Power Circuit
  • Main Power Control Circuit
  • Generator Control Circuit
  • Operation of Generator (Marsh & Fuel pump)

Incoming Power Circuit

Here in Fig. (2) you can see the main power circuit that contains the main switching contactors (K1, K2).

Incoming Power Circuit of Automatic Transfer Switch
Figure 2

Notice that besides the electrical interlock between the two contactors in the control circuit, there has to be a mechanical interlock also as mentioned in Fig. (2) for more safety.

The electric load is connected to both of the power supply sources (Main Network, Generator) so in case of failure of anyone, the other one will cover the load.

Main Power Control Circuit

First of all, this circuit is supplied from the “Main Power Supply” line by 220V AC.

As you can see in Fig. (3) “R1” is directly operated by the main power so in case of power failure the relay automatically would be OFF and would flip its contacts.

Main Power Control Circuit of Automatic Transfer Switch
Figure 3

The timer “T1” gives us a delay time between the switching of the two contactors (Main, Generator) this time could be from 5 to 10 seconds.

After the timer succeeds in turning the main contactor “K1” on, the contractor will disconnect the timer and make a latch on itself.

Then the main power supply will be in service.

Generator Control Circuit

Here is shown in Fig. (4) the control circuit that handles the connection and disconnection of the generator.

This circuit is supplied from the “Generator Power Outage” by 220V AC.

Generator Control Circuit of Automatic Transfer Switch
Figure 4

As you can see “R1” contact is controlling the operation of all the circuit components so this is used to indicate the disconnection of main power then it will energize the timer and the contractor for the generator outcome.

The timer “T2” gives us a delay time between the switching of the two contactors (Main, Generator) this time could be from 5 to 10 seconds.

After the timer succeeds in turning the main contactor “K2” on, the contractor will disconnect the timer and make a latch on itself.

Then the Generator power supply will be in service.

Operation of Generator (Marsh & Fuel pump)

 Here is the most important one, this circuit is responsible for the operation of the generator.

It is operated by a backup battery (12V) as when it should be operated there will not be any power from the Mainline or the Generator.

Operation of Generator (Marsh & Fuel pump)
Figure 5

First of all, when the Main power fails the relay “R1” will return to its normal state (NC).

As a result, the fuel pump relay will be energized and starts to inject fuel.

At the same time, the Flasher timer “T3” will starts to turn the marsh on and off till the Generator is turned on and generates the required power.

“R2” will be energized (see Fig. 4) and disconnect the marsh timer and relay to protect them.

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About the author

Instrumentation Engineer at TAQA Arabia | + posts

I am Hussein Junior Mechatronics Engineer, I have specialized in Automation Industry and I have made many projects that are related to PLC Programming - Instrumentation Installation - Networking - Circuit Design - Motion Control.

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Automatic Transfer Switch (ATS) Using Classic Control Circuit

 
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