PLC Relay Inrush Current Load
Alex Karpov
Dec-2nd, 2020 8:51

Unitronics use relay data from relay manufacturer data sheet.

Unitronics cannot add more information than relay manufacturer provide.

I will refer to PLC models using TYCO relays (for example , SMxx-J-R20, etc.).

See following Technical Specifications and Installation Guide:

Relay Outputs:

Relay type is Tyco PCN-124D3MHZ or compatible.

Following is link to TE Application Note “Relay Contact Life”:

From Google you can find manufacturers data sheet:

This manufacturer data sheet does not include any reference for inrush current.

Only resistive load parameters provided.

In this case we are looking for other sources about how to calculate relay load current for different type of load.

Relay contacts maximum inductive load and actually also the life expectancy are depended on the time constant (Tau) of a DC inductive load.

In most of cases manufacturer of the inductive load doesn’t provide this information, and on the other hand, also the relay manufacturer doesn’t provide the maximum load and life expectancy depending on the Tau.

However, there is a rule of thumb, if the maximum rated current is 3A for resistive load the maximum current for inductive load will be about 0.6 to 1.2A (between 20% to 40% of the maximum rated current).

This rule of thumb is assuming that a suppressor circuit is used to protect the contacts of the relay.

In accordance to PLC datasheet, maximum inductive load current may be between 0.6 A to 1.2 A.

For more information I will refer you to Digi-Key document “Relay Technical Information”:

It explains different relay terms, parameters, and relation between load type and other parameters.

Page 11 of this document show examples of inrush current, dependent of load type.

Final decision about fitting relay parameters and selected load type should be taken by Customer.

Attached referred files.

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