Introduction to Industrial Automation

Figure 2.7  Typical internal structure of power relay.

Figure 2.8  Typical external view of a power relay by Siemens.

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Hardware Components for Automation and Process Control 

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with power contacts and a block of auxiliary contacts on its right side. Usually, two to four maxi-

mum blocks of auxiliary contacts, whose supply is optional, can be adapted to the body of a relay, 

in order for the required mechanical coupling to be achieved. The auxiliary contacts of power 

relays are rated up to 15 A, because they are used only in automation circuits and don’t supply any 

kind of load.

The possibility of selecting the coil’s nominal voltage is also valid for the case of the power 

relays. The coil nominal voltage should be the same as the operation voltage of the whole automa-

tion circuit. It should be also noted again that the coil nominal voltage is quite independent of the 

power circuit voltage. Therefore, it is possible with a coil of a nominal voltage (e.g., 24 V DC) to 

control the operation of a power circuit (electric motor of a machine) of a nominal voltage (e.g., 

660 V AC).