Motor protection · beginner
Thermal Overload Relay
A motor-protection device that watches sustained current. Too much current heats internal strips, trips the mechanism, and opens the contactor control circuit before the motor overheats.
Why it exists
Protect a motor from overload and phase-loss heating. It does not replace a fuse or circuit breaker for short-circuit protection.
Quick answer
What is a thermal overload relay?
A thermal overload relay protects a motor from sustained overcurrent and phase-loss heating. Motor current warms bimetal elements; when accumulated heat exceeds the adjusted threshold, the trip mechanism opens auxiliary contact 95–96 in the contactor control circuit. It is deliberately slower than short-circuit protection so normal starting current can pass, and it does not replace fuses or a circuit breaker.
Inside the control system
How the PLC relates to it
The overload should interrupt the contactor circuit in hardware so the motor drops even if the PLC program or output fails to respond. A separate trip indication—often 97–98 or another auxiliary—is read by a PLC input for alarm text, sequence inhibition and maintenance history. The program should latch a meaningful fault, prevent automatic restart, and require the underlying overload or phase-loss condition to be corrected before reset. Do not use software to bypass the physical overload contact.
Cause and effect
How it works, step by step
- 01
Motor current heats one bimetal strip per phase.
- 02
A hotter strip bends because its two metals expand differently.
- 03
The trip bar releases when bending exceeds the adjusted threshold.
- 04
The 95–96 contact opens, dropping the contactor and reporting a fault.
01 / Open the case
Watch the mechanism do the work
Follow one highlighted causal link at a time, then operate the component and deliberately create the fault.
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Know the terminals
- L1 L2 L3 — from contactor
- T1 T2 T3 — to motor
- 95–96 — NC trip contact
- 97–98 — NO indication
Recognise the faults
- Dial set above motor nameplate current
- Single phasing
- Frequent manual reset without finding cause
- Wrong trip class for the load
Read the field guide
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Selection checks
- Motor full-load current range and service-factor guidance.
- Trip class appropriate to motor starting time and load inertia.
- Manual or automatic reset mode—manual is normally safer for machinery.
- Coordination with the selected contactor and short-circuit protective device.
- Phase-loss sensitivity and ambient-temperature compensation.
Commissioning sequence
- 1Read the motor nameplate current and set the overload according to the manufacturer instructions and installation arrangement.
- 2Confirm all three motor phases pass through the correct overload current paths.
- 3Verify 95–96 is in series with the contactor coil permissive and that the PLC receives an unambiguous trip indication.
- 4Use the test function or an approved controlled test to prove trip, contactor dropout, alarm and restart inhibition.
- 5After a real trip, inspect current balance, load condition, cooling, supply voltage and mechanical binding before resetting.
Troubleshooting answers
Frequently asked questions
What do overload terminals 95 and 96 do?
They are the normally-closed trip contact commonly placed in series with the contactor coil. A trip opens 95–96 and removes the motor run command in hardware.
Why does an overload not trip instantly on motor start?
Motors draw high current while accelerating. The thermal model and trip class permit that short-duration inrush while still responding to sustained heating.
Can I simply reset a tripped overload?
Reset only after identifying why it tripped. Mechanical overload, phase loss, low voltage, blocked cooling or an incorrect setting will usually trip it again and may damage the motor.
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