Motor Winding Temperature Monitor with PT100 and Relay Trip for Industrial Motors

Motor winding temperature is one of the earliest signs that an industrial motor is in trouble, yet on many small machines it is never measured. Overloading, blocked ventilation, a failing bearing, or a long run under a heavy load all push winding temperature upward, and the heat attacks the winding insulation before the motor stops turning. Once insulation degrades, a shorted turn or a burnt winding can follow, taking the motor out of service. The Motor Winding Temperature Monitor with PT100 and Relay Trip for Industrial Motors addresses this gap by watching winding temperature continuously with a PT100 resistance sensor mounted on the motor body and tripping a relay when a user-set limit is crossed. A buzzer and display warn the operator at the same time. The build is a bench demonstrator on a small motor model, not a certified industrial protection device, and its thresholds are design targets set by the user for demonstration.

Builder note from the build: the prototype uses 3-wire PT100 wiring so the MAX31865 converter can cancel lead resistance. With 2-wire wiring, the same leads would add their own resistance to the reading and shift every temperature upward, which is why the build keeps the lead pair matched in length and routed together. The mechanism behind the reading is first-principles simple: platinum resistance rises with temperature, by approximately 0.385 Ω per °C near 0 °C for a PT100, so temperature is recovered from resistance rather than from a voltage that drifts with supply noise.

The Motor Winding Temperature Monitor with PT100 and Relay Trip for Industrial Motors works as a continuous sense, compare, and act loop. The PT100 probe, clamped to the bench motor housing, changes resistance as the winding temperature rises, and the MAX31865 converter turns that resistance into a digital temperature value that the Arduino reads over SPI. The controller compares each new reading against two user-set limits: a lower warning level that sounds the buzzer and a higher trip level that drives the relay. Below both limits the motor runs normally and the display shows the live temperature. At the warning level the buzzer sounds while the motor keeps running, giving the operator a chance to act. At the trip level the relay opens, the motor stops, the display shows the tripped state, and the trip latches until the reset button is pressed after the motor has cooled.

1. The PT100 probe, clamped against the bench motor housing, changes resistance as winding temperature rises, following the platinum resistance-temperature relationship.
2. The MAX31865 converter measures that resistance through the 3-wire connection, cancels lead resistance, and converts it to a digital temperature value.
3. The Arduino reads the temperature over SPI, applies the converter's linearisation, and updates the LCD with the live temperature in °C and the current motor state.
4. Each reading is compared with the user-set warning limit; crossing it sounds the buzzer while the motor keeps running.
5. Each reading is also compared with the higher user-set trip limit; crossing it drives the single-channel relay module to open the motor supply circuit.
6. The trip latches in firmware: the relay stays open and the display shows the tripped state even as the motor cools down.
7. After the motor has cooled, pressing the reset button clears the latch, silences the buzzer, and allows the motor to be restarted for the next demonstration run.

Tech stack:

• Arduino Uno (ATmega328P controller)
• PT100 RTD sensor · 3-wire probe
• MAX31865 RTD-to-digital converter
• Single-channel relay module
• 16 × 2 character LCD display
• Buzzer · reset push-button
• Arduino IDE (C++ firmware)
• Bench motor model · low-voltage supply

The Motor Winding Temperature Monitor with PT100 and Relay Trip for Industrial Motors combines four capabilities that matter most for motor protection work: accurate RTD temperature sensing, a live local display, a two-stage warning and trip response, and a latching trip that stays off until a person resets it. The PT100 sensor and MAX31865 converter give a stable digital temperature reading instead of a rough analogue estimate, so threshold decisions rest on a repeatable measurement. The LCD shows the live temperature and the motor state at a glance, while the buzzer draws attention the moment a warning level is reached. When the trip level is crossed, the relay opens the motor supply circuit and latches, so a cooling motor cannot restart itself unattended. Together these capabilities turn a simple temperature reading into a complete protection demonstration that students can explain, test, and present.

• [PT100 RTD temperature sensing] A stainless steel PT100 probe clamped to the bench motor housing tracks winding temperature through the resistance of platinum, a measurement principle trusted in industrial instrumentation for its stability and repeatability.
• [MAX31865 digital conversion] The MAX31865 RTD-to-digital converter reads the PT100 over a 3-wire connection and delivers a digital temperature value to the controller, with lead-resistance cancellation and built-in open and short fault detection.
• [Live LCD temperature display] A 16 × 2 character LCD shows the live winding temperature in °C together with the motor state, so the reading and the protection state are visible without a computer or phone.
• [Two-stage warning and trip] A lower warning level sounds the buzzer while the motor keeps running, and a higher trip level opens the relay, giving the operator a chance to act before the motor is stopped.
• [Latching relay trip with manual reset] Once tripped, the relay stays open even as the motor cools, and the motor restarts only after the reset button is pressed, preventing an unattended automatic restart.
• [User-set thresholds] Warning and trip limits are set in the firmware as clearly labelled constants, so the demonstration can be tuned to different motor models and classroom test conditions.
• [Sensor fault indication] If the PT100 lead breaks or shorts, the converter flags the fault and the display reports a sensor fault instead of a plausible-looking false temperature.

• Working bench prototype: bench motor model, PT100 probe, MAX31865 board, Arduino Uno, relay module, LCD, buzzer and reset button
• Complete firmware source code with labelled warning and trip constants
• Circuit diagram and wiring documentation, including the 3-wire PT100 connection
• Component list with part values used in the build
• Setup and demonstration instructions, including how to trigger a warning and a trip safely
• Project report PDF (background, RTD theory, methodology, results discussion)
• PPT presentation for final review
• Viva Q&A preparation document (RTD principle, 3-wire wiring, trip logic, limitations)