Monitoring Relays for Motor Protection: Phase and Thermistor
What monitoring relays protect a motor beyond its thermal overload relay? A phase-sequence/phase-loss relay checks the incoming three-phase supply before and during running, while a PTC thermistor relay reads actual winding temperature through a sensor embedded in the stator, both switching per IEC 60947-5-1 / IEC 60947-8 respectively. Skip the thermistor input and a motor can overheat from a blocked cooling fan or restricted airflow while the overload relay, sized on current alone, never trips because the current stays inside its curve. This article covers phase monitoring for motors, PTC and PT100 thermistor protection, how the two functions combine in one control circuit, output logic, and where each stops short of the other.
Why a Thermal Overload Relay Isn't Enough
A thermal overload relay (or the overload function inside an motor protection circuit breaker) reacts to current. It trips when line current exceeds a curve derived from the set full-load amps, with a time delay that mimics how fast the winding heats. That protects against a locked rotor, a jammed load, or a general overcurrent condition. It does not see the incoming phase rotation, and it does not measure the winding directly.
Two failure modes slip past it. Wrong phase sequence spins the motor backward on start — current looks normal, the load just runs the wrong way. And degraded cooling raises winding temperature gradually, without pushing current outside the overload curve, especially on a TEFC motor where the cooling fan is shaft-mounted and slows down with the same fault that's heating the motor.
What we see in the field: a motor with a partly blocked cooling duct sits in a hot electrical room for years. The overload relay, calibrated on nameplate current, stays satisfied the whole time while the winding insulation ages faster than it should. A thermal overload relay's phase-loss response covers current imbalance from a lost phase, but not a fan that has simply worn out.
Phase Monitoring for Motors: Sequence, Loss, and Asymmetry
A three-phase supply relay checks phase presence, sequence, loss, and usually asymmetry before the contactor is allowed to close and continuously while the motor runs. Wrong rotation means a pump runs backward or a conveyor drives the wrong direction; the relay blocks the start rather than letting the operator find out the hard way. See the full mechanics in our phase sequence and phase failure relay guide.
Lose one phase mid-run and the motor tries to keep turning on two. Current on the remaining phases rises, and the winding heats unevenly — one coil group carries more than its share. An overload relay eventually catches the current rise, but a dedicated phase relay reacts faster and reports which fault occurred, which matters for diagnosis. Schneider Zelio Control RM17TE covers phase sequence, phase loss, and over/undervoltage in one 22.5-45 mm module; ABB CM-MPS does the same and, notably, needs no auxiliary supply — it draws power straight from the measured three-phase lines, so a wiring mistake on a separate control supply can't disable it.
PTC Thermistor Protection: How It Detects Winding Heat
A PTC (positive temperature coefficient) thermistor sits embedded directly against the stator winding, one sensor per phase, with up to six wired in series into a single relay input. Resistance stays low and roughly flat with temperature until the sensor nears its reference point, then rises sharply over a narrow band. The relay reads loop resistance, not current, so it responds to actual winding heat regardless of what caused it — blocked ventilation, high ambient, frequent starts, or a developing winding fault.
Formula: PTC Trip Threshold — Source: IEC 60947-8
Rtrip ≈ 3.3 kΩ
| Symbol | Description | Unit |
|---|---|---|
| Rtrip | Loop resistance at which the relay switches (nominal reference value per IEC 60947-8) | kΩ |
| R25 | Sensor resistance at 25°C, low and stable across the normal operating range | Ω |
| Rreset | Loop resistance below which the relay allows reset, lower than Rtrip | kΩ |
The sensor loop is a distinct low-voltage circuit, separate from the main power wiring, so shielded or twisted-pair cable to the relay input matters — stray induced voltage on a long run can cause nuisance trips or a short-circuit fault indication. Most PTC relays also detect a sensor short (very low resistance) and a broken sensor loop (open circuit) as separate fault states, not just the temperature trip.
PT100/PT1000 vs PTC: Which Sensor for Which Motor
PTC gives a switch point, not a reading. PT100 (or PT1000) is different: it's a resistance temperature detector with a near-linear resistance-to-temperature curve, roughly 100 Ω at 0°C, so the relay can display an actual temperature value and trip at a chosen setpoint rather than a fixed reference. Some ranges use two setpoints — an alarm point for early warning and a trip point that actually drops the motor.
Which one to specify depends on the motor. Smaller general-purpose motors typically get PTC: cheap, simple, binary, adequate. Larger or critical motors — a compressor driving a process line, a large pump with no standby — often get PT100 so the panel or SCADA system shows a trend, not just a trip after the fact. A rising temperature curve over weeks tells a maintenance team more than a single alarm contact ever will. Schneider's RM35ATR-class relays and ABB's CM-TCS / CM-MSS / CM-MSN range both cover PTC and PT100/PT1000 inputs, so the sensor choice, not the relay brand, usually drives the decision. See our dedicated guide on PTC and PT100 temperature monitoring relays for the setpoint math.
Combining Phase and Thermistor Protection in One Motor Circuit
A typical control circuit puts the phase-sequence/loss relay's output contact in series with the start button and the contactor coil, upstream of the run condition. The PTC (or PT100) relay's output sits in the same series chain, or feeds a PLC input that then drops the coil. Either fault opens the loop and the contactor drops out — the motor stops regardless of which relay tripped.
This depends on how the panel builder wants fault indication split out. Some prefer one relay per function so a technician standing at the panel sees exactly which LED lit — phase fault, or thermistor fault — without opening a PLC program to find out. Others consolidate onto a multifunction module like an RM35TF-class device to save DIN-rail space, accepting that fault detail then lives in wiring labels or a PLC diagnostic screen instead of a dedicated LED. Neither approach is wrong; it's a trade between panel footprint and troubleshooting speed.
None of this replaces the motor protection circuit breaker or the thermal overload relay upstream — those still cover short-circuit and general overcurrent. Monitoring relays add the phase-quality and direct-temperature layer neither one measures. For the fuller mechanics of how phase-loss propagates into motor damage, see our note on MPCB phase-loss and single-phasing protection.
Wiring and Output Logic for Motor-Protection Relays
Fail-safe wiring is the default for motor protection: the relay's output contact stays closed (energized) during normal operation, holding the contactor coil circuit live. A fault — or loss of the relay's own auxiliary supply — opens the contact and drops the motor out. That second part matters: a dead relay behaves the same as a detected fault, not a silent pass-through. Our guide to wiring a monitoring relay covers the output and auxiliary-supply options in more detail.
Reset behavior is worth setting deliberately. Automatic reset lets the relay re-close as soon as the fault clears and the hysteresis band is satisfied — fine for a nuisance voltage dip. Manual (latched) reset is common for PTC thermistor faults on motors, because a winding that just tripped on temperature is still hot; forcing someone to check before restart avoids an immediate re-trip or, worse, restarting into a real winding fault. Getting the hysteresis and delay settings right — covered in our piece on hysteresis, trip delay, and latching — is what keeps a motor-protection circuit from nuisance-tripping on transients while still catching a real fault fast.
Frequently Asked Questions
Can a phase monitoring relay replace a thermal overload relay on a motor?
No. A phase relay checks the incoming supply — sequence, loss, asymmetry, voltage — while an overload relay or PTC relay protects the winding from excess current or temperature. The two cover different fault types and are normally wired in series so either one can drop the motor.
Why would a correctly phased motor still overheat?
Heat can come from restricted airflow, high ambient temperature, or frequent starts, none of which register as a phase fault and none of which necessarily push current outside the overload relay's curve. A PTC or PT100 sensor embedded in the winding is what catches this directly.
Where is the PTC sensor installed on the motor?
Embedded against the stator winding at the point most likely to run hottest, installed during motor manufacture rather than retrofitted in the field. Up to six sensors, one per phase winding section, are typically wired in series into a single relay input, so the relay reads whichever winding is hottest.
What happens if a fail-safe monitoring relay loses its auxiliary supply?
Its output contact drops just as it would on a detected fault. Wired in series with the contactor coil circuit, that also drops the motor out, so a power-supply failure is treated the same as a real measured fault rather than being ignored.
Can one relay combine phase monitoring and PTC thermistor input?
Some multifunction ranges offer both, but most panels still use two separate devices. The PTC sensor loop is a distinct low-voltage circuit, and keeping it on its own module simplifies isolation, cabling, and fault indication.
Does a motor protection circuit breaker replace the need for a monitoring relay?
No. An MPCB gives short-circuit and current-based overload protection on the supply side. It does not check phase sequence or asymmetry beyond a basic phase-loss trip, and it has no direct winding-temperature input.
Conclusion
Motor protection is layered, not single-device. A phase-sequence/loss relay guards the supply side before and during the run. A PTC or PT100 thermistor relay reads the winding directly, catching heat from causes a current-based device can't see. A thermal overload relay or MPCB still handles general overcurrent. Combine the three, wire fail-safe into the contactor coil circuit, and each layer catches what the others miss. For the broader picture across all monitoring relay types, see our monitoring relay engineering guide, and browse the full range of monitoring and control relays stocked for immediate dispatch.
