Motor protection relay
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Description
The FB_Motor Protection function block protects motors from overload by monitoring a measured current value ( #Current ) against a maximum permissible threshold ( #IMax ). If the current is exceeded, it triggers after a defined delay time ( #PT ). The trigger is held (latch) and can be reset manually via #Reset or optionally automatically ( #AutoReset ) as soon as the current returns to the permissible range. Feedback is provided via normally closed and normally open contacts; additional simulation allows testing without a real load.
Note: This logic serves as a template. Before productive use, it must be integrated, tested, and approved by qualified personnel. Incorrect integration may impair the protective effects or lead to unintentional shutdown.
How it works
Manual reset
With #Reset the trigger state is reset and the internal timer is stopped so that the motor is again considered not overloaded.
Disabled
If #Enable is not set, outputs are disabled, the timer does not run and the block reports “disabled” without errors.
simulation
In simulation mode, the behavior is simulated based on the comparison of #Current > #IMax and the delay. The trigger is set via #tripped . Contacts reflect this state: the normally closed contact ( #NO_13_14 ) is active as long as the trigger is not triggered, and the normally open contact ( #NC_21_22 ) is active when the trigger is triggered. If #AutoReset is active and the current falls below the threshold, a reset occurs.
Normal operation
The motor current is continuously compared with the limit threshold. If the current exceeds #IMax for the set delay time #PT , the overload condition is set and maintained. If #AutoReset is active, an automatic reset occurs as soon as the current returns to the permissible range. The outputs are only passed through if the corresponding feedback inputs are set:
-
#NO_13_14(normally closed) indicates “no overload” and is only activated if#Input_13is set. -
#NC_21_22(normally open) indicates “overload” and is only activated if#Input_21is set.
Errors are reset, the status shows OK.
Technical details
-
Inputs:
#Enable,#Reset,#SimMode,#AutoReset,#Current,#IMax,#Input_13,#Input_21 -
Timer:
#tripTimer– Switch-on delay for overload triggering -
Internal state:
#tripped– held trip -
Outputs:
#NO_13_14(normally closed),#NC_21_22(normally closed) -
Status & Diagnosis:
#Error,#Status,#DiagnosticCode - Modes: Manual reset, automatic reset, simulation
- Logic: Time-delayed overcurrent tripping with self-maintaining and optional reset
Possible applications
- Motor protection in drive and control systems with delayed overload release
- Simulation and validation of protective behavior without real load
- Integration into higher-level diagnostic systems for utilization monitoring
- Flexible reset behavior (manual or automatic) to adapt to process requirements
Note: A complete technical inspection and approval by qualified personnel is required before productive use.
compatibility
- Can be used in PLC systems with SCL support (e.g. Siemens TIA Portal)
- Modular integration into protection, process and diagnostic architectures
- Simulation enables safe pre-tests without real triggering
- Safe basic state when deactivated
Scope of delivery
You will receive:
- An SCL file with the implemented function block FB_Motorschutz
- Complete declaration of all inputs and outputs, timers and states
- Briefly commented structure for quick adaptation
- Short guide for integration into PLC projects
The download link will be provided after purchase.
Support
I am happy to support you with:
- Adjustment of limit and delay times
- Integration into higher-level control or HMI processes
- Auto/Manual Reset Configuration
- Analysis of trigger events and advanced protection logic
Contact is possible via shop message or email. Individual extensions are available as optional customizations.