PTC and Pt100 Temperature Sensors in Motors

Short answer

A PTC is a thermistor whose resistance jumps when it reaches a fixed response temperature, so it acts as a thermal switch that a thermistor relay watches. A Pt100 is a platinum sensor with 100 ohms at 0 °C whose resistance varies almost linearly with temperature, so it gives a continuous measurement. The PTC protects with direct tripping at low cost; the Pt100 allows pre-alarm, trending and monitoring from a relay or a PLC. Both complement current protection and do not replace it.

What measuring motor temperature is for

Current protection estimates heating from what the motor draws. But a motor can overheat without the current showing it: a blocked fan, high ambient temperature, dirty filters, fins full of dust, repeated starts that leave no time to cool or an altitude that reduces cooling. Measuring temperature directly in the windings and bearings covers those cases. It is a complementary protection, not a substitute, for those explained in induction motor and its protections and motor protection relay.

The two sensors most often found embedded in motors are the PTC and the Pt100.

PTC: the thermal switch

The PTC (positive temperature coefficient) is a thermistor. While temperature is below its nominal response temperature, its resistance is low and almost constant. On reaching it, the resistance rises very sharply, by an order of magnitude or more. That characteristic makes it a threshold sensor: it does not deliver a temperature, but the information that a preset temperature was exceeded.

How it is used:

One per phase is installed in the winding, normally three connected in series to two motor terminals.
A thermistor relay (or a control module of the starter or drive) passes a small current through the chain and watches for the resistance jump. When it detects it, it opens its trip contact, which is integrated into the contactor control.
The response temperature is chosen when the motor is manufactured and is related to the winding insulation class. With that value, the trip occurs before the insulation suffers.

Advantages: low cost, fast response, no transmitter needed and good integration with soft starters and drives that have a dedicated input. Limitations: no value is read, the threshold cannot be adjusted, there is no pre-trip alarm and no trend information.

Pt100: the measuring sensor

The Pt100 is a platinum resistance temperature detector (RTD). Its nominal value is 100 ohms at 0 °C, and its resistance grows almost linearly with temperature following a curve standardized in IEC 60751. The relationship between resistance and temperature is known and stable, so it delivers a continuous measurement.

How it is used:

They are installed in the winding, one per phase or several per phase in large machines, and also in the bearings and, sometimes, in the inlet and outlet air.
They are connected to a motor protection relay, a PLC temperature input module or a transmitter.
The receiving device can set alarm and trip with different thresholds, record the trend and compare between phases.

Advantages: real measurement, pre-alarm, adjustable thresholds, trend analysis and diagnosis (a phase that rises more than the others warns before the fault). Limitations: higher cost, needs a device that interprets the signal and more careful wiring.

Pt100 wiring

Cable resistance adds to the sensor resistance and produces error. So:

2 wires: the cable resistance enters the measurement in full; only suitable for very short distances.
3 wires: compensates cable resistance if the three conductors have the same length and gauge; it is the industrial standard.
4 wires: separates the excitation current from the measurement and eliminates the cable effect; used where accuracy matters.

Sensor signal wiring goes in shielded cable, separated from the motor power circuits. The shield is connected at one end only, per the receiving device guide, to avoid induced noise that corrupts the reading. On a motor with a drive this point is especially sensitive.

Comparison

Aspect	PTC	Pt100
Signal type	Threshold (sharp resistance change)	Continuous measurement
Information	Threshold exceeded or not	Temperature value
Pre-alarm	No	Yes, with a threshold different from the trip
Threshold adjustment	Fixed at the factory	Adjustable in the receiver
Associated device	Thermistor relay or dedicated input	Motor relay, PLC module or transmitter
Wiring	2 wires	3 or 4 wires
Cost	Low	Higher
Typical use	Small and medium power motors, windings	Large or critical motors, windings and bearings

Illustrative example

Invented values.

A 200 kW motor carries one Pt100 per phase in the winding and one in each bearing (illustrative). The protection relay is configured with alarm and trip for the winding, and a different alarm and trip for the bearings. In operation, the trend shows that one phase runs a few degrees hotter than the other two; current is balanced. The sustained difference points to a local problem, for example a coil with degraded insulation or a blocked ventilation duct. With one PTC per phase nothing would have been seen until the trip. The pre-alarm allows an inspection to be scheduled without stopping production.

Sensor location and what they really measure

Where the sensor sits determines what it reports.

Stator winding. This is the main protection point, because winding insulation is the element that limits motor life. The sensors are embedded between the coils during manufacture, so they reproduce well the temperature of the copper and insulation. Even so, they measure one point: a hot spot away from the sensor can be hotter than it indicates. That is why trip thresholds are set with margin below the insulation class limit.

Bearings. A Pt100 in the end shield or in the bearing housing reveals lubrication failures, misalignment or wear before there is serious mechanical damage. It is a measurement that does not depend on current and that, together with vibration analysis, forms the basis of condition monitoring. On large motors with sleeve bearings, oil temperatures are also measured.

Inlet and outlet air. The temperature difference between the two indicates the state of ventilation and the thermal load. It is a complementary measurement, not direct protection.

Space heaters. Some large motors carry heaters to prevent condensation at standstill; their supply is cut when the motor starts. They are not sensors, but they share the terminal box and are confused with them if each terminal is not properly identified.

Response in time. The embedded sensor responds with a delay relative to what happens in the copper, and the rotor, which heats differently, has no direct sensor. In a locked rotor, rotor temperature rises faster than stator temperature. That is why locked-rotor protection is based on current and not on temperature. Temperature sensors complement current protection in the slow-heating range and do not replace it.

Maintenance. In periodic inspections, the continuity and resistance of the sensors are checked with instruments suited to avoid damaging the PTC. It is also verified that the trip circuit really cuts the contactor control.

Common mistakes

Connecting a Pt100 to a PTC input, or the other way around.
Running the sensor cable together with the motor power cable without shielding.
Measuring PTC resistance with an instrument that applies high voltage and damages the thermistor.
Using a 2-wire cable over long distances with a Pt100.
Leaving the sensors unconnected to the relay: the motor has them, but nobody watches them.
Setting the Pt100 trip above the motor insulation limit.
Not testing the sensor circuit at commissioning.

Checklist

Type and number of motor sensors per its nameplate or terminal diagram.
Receiver suited to the sensor type.
Shielded wiring, separated from power and with the shield grounded at one end.
Alarm and trip thresholds defined with the motor manufacturer.
Trip contact integrated into the contactor control.
Functional test of the complete circuit at commissioning.
Insulation and sensor continuity measured without damaging it.
Trend recording for critical motors.

When to call us

If a motor has sensors but the protection does not use them, the protection exists only in the catalog. Our team can review the protection design of your motors remotely, worldwide, and test the complete circuit and commission it in Colombia, or elsewhere on request. Write to us through contact with the nameplates and the protection diagram.

References

IEC 60751, Industrial platinum resistance thermometers and platinum temperature sensors.
IEC 60034 (series), Rotating electrical machines.
Motor and thermistor relay manufacturers’ technical documentation on winding sensors.

Frequently asked questions

What is the difference between a PTC and a Pt100 in a motor?

The PTC is a threshold sensor: its resistance changes sharply at a fixed response temperature and it only reports that the threshold was exceeded. The Pt100 is a measuring sensor: its resistance follows temperature, so you can read the value, set alarms and trips and record trends.

How many sensors does a motor have?

It depends on size and manufacturer. Motors commonly include one sensor per phase in the winding, for example one PTC per phase connected in series or one Pt100 per phase, and large motors add sensors in the bearings. The quantity and location are on the nameplate or the motor terminal diagram.

Why is a Pt100 connected with three or four wires?

Because cable resistance adds to the sensor value and produces a reading error that grows with distance. With three wires the cable resistance is compensated and with four its influence is eliminated entirely.

Do these sensors replace the overload relay?

No. Current overload protects against heating caused by load; temperature sensors cover cases that current does not see, such as poor ventilation, high ambient temperature, failing bearings or repeated starts. They are used together.