If the AC contactor is the “commander” of the motor, then the thermal overload relay is the “thermometer” strapped to the motor itself—it doesn’t issue commands, but it constantly monitors whether the motor is running a “fever.” Motors are among the most expensive pieces of power equipment in any factory; a medium‑voltage motor rated at several hundred kilowatts can cost hundreds of thousands of yuan, while a thermal relay may cost only a few hundred yuan. Yet this small, inexpensive device silently protects those expensive assets every single day. The working principle of a thermal relay is essentially high‑school physics: it uses the heating effect of current on two metal strips with different coefficients of thermal expansion bonded together (the bimetallic strip). Under normal current, the heat generated is just enough to keep the strip straight; when current becomes excessive, the strip bends from heat and pushes the trip mechanism to切断 the circuit. This purely mechanical principle actually makes it more immune to interference and more reliable than many electronic protectors. The most critical step in selecting a thermal relay is setting the correct current setting value. Many entry‑level electricians simply set it to the rated current on the motor nameplate—this is a common mistake. The correct approach must consider two factors: the actual load factor of the motor under operating conditions, and the duration of starting current. If the motor operates under light load for extended periods, the setting can be lowered appropriately (about 0.8 to 0.9 times the rated current) for more sensitive protection; if the motor undergoes frequent heavy‑load starts, the setting should be raised slightly (about 1.05 to 1.15 times the rated current) to avoid nuisance tripping during start‑up due to inrush current. However, there is an upper limit—never exceed the maximum current allowed by the motor nameplate’s service factor. Another easily overlooked point is that the setting dial on a thermal relay becomes inaccurate at different ambient temperatures—most thermal relays are calibrated at 20°C or 30°C; if the actual temperature inside the cabinet is higher, the bimetallic strip already has some initial deflection even without overload, causing the actual trip value to be lower than the dial indicates, meaning the relay becomes more sensitive and prone to nuisance tripping in hot environments, so you should adjust the setting upward based on the manufacturer’s temperature compensation curve when the relay is installed in a high‑temperature cabinet. What do three‑phase motors fear most? Not overload, but phase loss—when running with one phase missing, the motor windings heat up rapidly, but the total current may not exceed the rated value, and ordinary (non‑differential) thermal relays cannot detect this at all. The “differential” type thermal relay internally compares the displacements of the three bimetallic strips; whenever one phase has abnormal current while the other two are normal, an additional offset is generated inside the mechanism to trigger rapid tripping. This is why for three‑phase motors, you must choose a thermal relay with differential protection. When you select a model, if the suffix doesn’t include a “D” (or a similar designation), it’s likely only a two‑phase protection type, which is practically useless for three‑phase motors. After a thermal relay trips, the bimetallic strip needs time to cool before it can reset. There are two reset modes: manual reset and automatic reset. Manual reset requires an operator to press the “reset” button, while automatic reset restores the circuit as soon as cooling is complete. Which one should you choose? For unattended or remotely controlled installations, manual reset is strongly recommended—why? Because with automatic reset, the motor could restart without anyone being aware, and if a maintenance technician is working on the fault at that moment, the unexpected startup can have disastrous consequences. Only manual reset ensures that someone physically visits the site to confirm the fault has been cleared before the motor is allowed to run again. Thermal relays are typically mounted directly below the contactor, and the cross‑sectional area and length of the connecting wires between them affect the relay’s tripping accuracy—because the connecting wires themselves generate heat, and that extra heat transfers to the bimetallic strip, causing premature tripping. Manufacturers usually provide recommended wire sizes and length limits, which must be strictly followed during installation. Additionally, if the copper bars or wires between the thermal relay and the contactor are loose, the increased contact resistance produces localized high temperatures that also “fool” the bimetallic strip into tripping unnecessarily. That’s why, during routine inspections, using an infrared thermometer to check the temperature of the relay terminals is an extremely practical habit. For fault diagnosis: if the motor trips just seconds after starting but the clamp meter shows no excessive current, the setting is likely too low, or the ambient temperature is too high, or the connecting wires are undersized causing extra heat  conduction; if the motor is clearly overloaded (current exceeding the rated value) but the thermal relay refuses to trip, the setting may be too high, or the bimetallic strip may have aged and lost its deformation capability, or the trip mechanism is stuck—this situation requires immediate replacement; if the relay has tripped but feels cool to the touch and won’t reset, it’s not broken—the cooling period simply hasn’t elapsed, and the bimetallic strip needs to cool naturally to a certain point before resetting, so forcing the reset mechanism with tools will damage the internal structure, so patiently wait 1‑3 minutes before attempting a manual reset. Although the thermal relay may not be sophisticated technology, it is the first line of defense for motor protection—more direct and more physical than any electronic monitoring. Set the value correctly, and it silently guards your motor; neglect it, and when the motor burns out, it may not even utter a “protest.” Treat this small bimetallic device well, and you are treating your factory’s expensive rotating machinery with the care it deserves.