Fault 11

Pro

Fault 11 — Thermal Overload Trip

What you'll learn

**Reported symptom:** The control supply is healthy and Start works mechanically, but KM1 cannot energise.

Lab time: ~12 minutes.

Lesson briefing

Thermal Overload Trip

Reported symptom: The control supply is healthy and Start works mechanically, but KM1 cannot energise.

The circuit was commissioned from a known-good drawing. Treat every terminal as suspect, but test from source to load instead of guessing. Use voltage mode on an energised control circuit and continuity only after the simulated circuit is isolated.

Your task

Trace the series control chain and find which normally closed protective contact is open.

Choose the fault category only when the measurements support it.

Hints

Hint 1

Measure voltage to each side of Stop and then OL terminals 95 and 96.

Hint 2

Hint 3

This lesson uses 11 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.

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Wiring 1A — Power the PLC

Wiring 1 — E-Stop Control Circuit

Wiring 2 — NPN / PNP Sensors (Sinking & Sourcing)

Competency and practice field guide

Motor overload trip fault lesson: practice plan

Direct answer

The learner can preserve trip evidence, separate control and power paths, inspect likely electrical and mechanical causes and prove protection and load behavior after correction.

Written for electrical maintenance and PLC learners diagnosing a motor stopped by overload protection without assuming the protective device itself is defective.

a supervised industrial control-panel bench used to trace power, protection, terminals, contactors, PLC I/O and field wiring with a correctly selected meter while studying overload-trip diagnosis, reset and verified recovery

System map / 02

NODE 01observable

Scope

Operator symptom, trip indicator and history, motor data, overload type and setting, current per phase, starts, duty, cooling, mechanical load, supply condition, auxiliary contact and safe work state.

NODE 02observable

Signal path

Run demand through contactor and three-phase path to motor and load while current and thermal state operate the overload and auxiliary feedback stops and alarms the control circuit.

NODE 03observable

Baseline practice

The corrected machine starts, carries declared load, remains within approved current and thermal limits and trips and stops through the intended protective path when tested as allowed.

NODE 04observable

Edge cases

Phase loss, low voltage, overload, jam, excessive starts, blocked cooling, wrong setting, imbalance, damaged motor, reset and recurring trip.

NODE 05observable

Fault practice

A demand, contactor, phase, protection-setting, current, motor, cooling, mechanical-load, feedback, alarm or reset mismatch.

NODE 06observable

Transfer to the job

The cause and equipment condition assessed by qualified personnel and the target protection settings and functional tests documented before handover.

Answer surface / 07

Should I simply reset a tripped motor overload?

No. Preserve the trip state and investigate load, current, phases, voltage, starts, cooling, setting and equipment condition before an authorized reset.

Why does a motor overload trip on phase loss?

The motor can draw abnormal current and heat when one phase is missing, causing correctly selected protection to respond even though the root fault is elsewhere.

Continue the signal path / 08

Thermal overload relay guide Complete DOL starter Motor-control simulator Electrical troubleshooting training Explore dry-contact plc input wiring lesson PLC variable table and cross-reference guide field guide

Motor, wiring and VFD path

Connect the control command to physical motor behavior

Move from safe control wiring and contactors into VFD parameters, measurements, faults and PLC command paths.