Control the Contactors with PLC


A typical application for contactor of DC operated is the direct control from the PLC. Hereby the contactor of a semiconductor output is supplied directly with control voltage. Typical semiconductor outputs of PLC’s provide a power of 500 mA at 24 Volts. Contactors up to 32 A can be operated directly by semiconductor output of this type without the additional coupler relays need.

The engineering cost of power DC supplies can be reduced by the use of electronics. The power supply no longer needs to be engineered as with conventional contactor coils according to the sum of the powers pick up. The power supply is engineered according to the considerations as following: the sums of the switch on powers of all contactors which pick up simultaneously are added to the sum of all holding the powers of all contactors which is switched on simultaneously. As the holding powers are very low the power supply can be smaller significantly.

The direct control of contactor coils from the PLC reduces the coupler relays costs and reduces the wiring effort involved. For this the output must provide both the holding and pick up power for the contactors coils. PLCs with different outputs are available on the market. There are also semi-conductor outputs are offered with 0.1 A or 0.5 A and the most common are the semi-conductor outputs with 500 mA, in addition to expensive relay outputs.

A significant difference between PLC’s for control of safe PLC’s and normal operational functions is the cyclic monitoring of the PLC outputs in safety PLC’s. The outputs of master drives are typically controlled and cyclically briefly interrupted in safety PLC’s. The interruption is in the order of just 10 – 5 ms, so that actuators of connected (valves, contactors) do not shut off. During the interruption it is monitored if the voltage of output drops below a defined level.


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