This article is part of a guided learning series on building and analyzing User-Defined Function Blocks (UDFBs).
Canonical Article: Building a User-Defined Function Block (UDFB) Using Siemens TIA Portal
Learning Companion (Q&A): Explore All Questions
You are reading: Question 14
Modify the state machine to report a thermal overload
Clarification
First, determine if the existing state machine detects an overload condition.
Second, modify the state machine to include a fault code with text that specifically states that the motor starter is in an overload condition. This will require a second motor starter feedback line to monitor the status of the motor starter’s thermal or solid-state overload.
Be sure to add a second input derived from the motor starter overload block’s normally closed contacts.
Answer
This is left as a design exercise for advanced students.
For clarification:
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An overload event is indirectly detected as part of the closed state. Recall that the UDFB enters a fault timGlitch seconds after the auxiliary contact feedback signal is lost.
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Most motor starters thermal overload blocks include a normally open and a normally closed set of contacts:
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The normally closed contacts are wired in series with the coil. Upon overload the coil circuit is opened independent of any control logic. This independence provides a fail-safe circuit.
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The normally open contacts may be used by the PLC to indicate the state of the motor starter. This feedback signal could be read by the PLC to provide an “overload detected” state.
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Tech Tip This UDFB modification provides the technician additional troubleshooting symptoms to better troubleshoot the fault. Instead of guessing, they will quickly be able to identify an overload event. This is especially useful when the operator has access to the motor starter’s reset button or if an automatic overload reset is incorporated.
Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey