Author: PLC Play Ground
HMI Number Looks Normal but Never Changes? Check Data Age and Quality
A believable number can be stale.
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OPC UA Worked Yesterday but Fails Today? Check Certificates and the Clock
An OPC UA connection can stop after a certificate renewal, hostname change, restore, or clock error even when the network is reachable.
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VFD Receives a Run Command but Won\’t Start? Trace the Command Source
A true run bit in the PLC does not prove a drive has accepted a valid start request.
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VFD Runs at the Wrong Speed? Check Reference Units Before the Motor
A speed mismatch can begin with a number that means different things at opposite ends of the connection.
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VFD Trips on Overcurrent During Acceleration? Don\’t Just Raise the Limit
An overcurrent trip during acceleration can involve the load, ramp demand, motor configuration, or an electrical fault.
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VFD Overvoltage on Deceleration? The Motor May Be Returning Energy
A DC-bus overvoltage trip that occurs while slowing down can be related to energy returning from the moving load.
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Replacement Motor Runs Badly on a VFD? Read Both Nameplates First
A replacement motor with the same power rating is not automatically an equivalent drive load.
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Encoder Counts Go Missing at High Speed? Do the Frequency Math
An encoder that works slowly but loses counts at production speed may exceed the input path\’s capabilities.
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PLC Misses a Sensor Pulse? A Faster Screen Won\’t Fix the Scan
A short pulse can occur entirely between ordinary input samples.
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PLC Watchdog Fault Appears Randomly? Average Scan Time Can Hide It
A task can look fast on average and still exceed its allowed execution time during a rare path.
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