Motor Thermal Protection Compared: Bimetal Klixon vs PTC vs PT100

The most important line of defense protecting an electric motor from burning out is the thermal protection sensors that monitor the winding temperature. When the winding temperature exceeds the limit of the insulation class due to overload, phase loss, insufficient cooling or frequent starting, the motor is quickly and irreversibly damaged. External thermal relays measure current but cannot see the real temperature of the winding; whereas temperature sensors embedded inside the motor detect the heat right at its source. There are three basic types of these sensors: bimetal (klixon) thermal contact, PTC thermistor (compliant with DIN 44082) and PT100 RTD. In this article we compare in detail the operating principle of each, which is for alarm and which for trip (cut-off), their sensitivity differences, the monitoring relay requirement, and in which application each sensor provides the right protection.

When a motor winding burns out the result is usually expensive: the cost of rewinding, lost production while the motor is out of service, and sometimes replacement of the entire motor. Yet a correctly selected and correctly wired thermal protection sensor prevents this disaster within seconds. The cost of the sensor is very small next to the motor and the production loss; therefore thermal protection is an investment that should not be neglected. Understanding the differences between the three sensor types both helps avoid unnecessary cost and ensures choosing the protection level most suited to the application. Below we examine these three technologies step by step, from operating principles to field wiring.

Why Is In-Winding Temperature Protection Needed?

The insulation of a motor winding is designed for a specific temperature class (usually F or H). Every 10-degree rise above this limit roughly halves the insulation life. An external thermal relay provides indirect protection by looking at the current the motor draws; however even if the current looks normal, the motor can overheat due to insufficient cooling, high ambient temperature or frequent start-stop. In these situations sensors placed inside the winding come into play and measure the real temperature directly.

• Phase loss: When one phase is cut, the remaining windings overheat; the current balance can be misleading.
• Insufficient cooling: Blocked fan, dirty fins or fan inadequacy at low speed.
• Frequent starting: Starting current heats the winding at every start.
• High ambient temperature: Even if the current is normal, the winding approaches the limit temperature.

Therefore in critical and continuously running motors, in-winding temperature protection is a strong second line of defense in addition to the external relay. For the general logic of external protection elements, our article on thermal, relay and fuse selection is a fundamental guide.

Operating Principle of the Three Sensor Types

All three sensor types monitor the winding temperature but work on completely different principles. This difference determines the purpose of use and system integration.

Bimetal (Klixon) Thermal Contact

A bimetal contact is a switch made of the bond of two different metals that bends with temperature. When a certain threshold temperature is reached the contact opens (or closes) and can directly cut a contactor coil. It does not require a separate monitoring relay; this is a simple and economical solution. When the temperature drops the contact resets automatically. However its threshold is fixed and it does not measure the real temperature; it only tells whether the threshold has been exceeded.

PTC Thermistor (DIN 44082)

A PTC (Positive Temperature Coefficient) thermistor is a semiconductor whose resistance increases with temperature. As the rated response temperature (for example 130, 150 degrees) is approached, its resistance rises very sharply. This sudden resistance increase is detected by a PTC monitoring relay and the relay stops the motor. PTC responds very sensitively and quickly around the threshold; it is produced according to the DIN 44082 standard and usually three sensors are connected in series and placed in the three phases.

PT100 RTD

PT100 is a platinum resistance whose resistance changes linearly with temperature (100 ohms at 0 degrees). It does not switch on/off at a certain threshold; it continuously measures the temperature with the real value. This way, with a monitoring device, you can read the winding temperature instantly, set both alarm and trip levels and track the trend. PT100 is the most sensitive and most informative solution but requires a separate reading/monitoring device. For the PT100 and PTC wiring application, our article on PTC/PT100 wiring offers practical information.

Comparison Table: Klixon, PTC and PT100

Which Is for Alarm and Which for Trip?

The distinction between alarm and trip is critical in the protection strategy. An alarm warns the operator as the temperature approaches a dangerous level and gives a chance to intervene; a trip stops the motor immediately when the limit is exceeded. Ideal protection uses both together:

• Bimetal (klixon): Usually single-level trip; for simple and direct cut-off.
• PTC: Typically for trip; alarm+trip can also be set up with two PTC sets of different thresholds.
• PT100: Since it measures the real value, both alarm and trip levels can be defined in one device.

In large and critical motors the common approach is to monitor continuously with PT100, warn at the alarm level and cut off at the trip level; additionally an independent backup trip layer can be added with PTC. This multi-layer approach ensures the preservation of the insulation and thermal class; you can explore the topic further in our article on insulation and thermal class.

Which One in Which Application?

The right sensor selection depends on the motor's size, criticality and monitoring need. The following guidance offers a practical decision path:

• Small/medium, simple application: Bimetal klixon; economical, no monitoring relay needed.
• Standard winding protection: PTC thermistor; DIN 44082, reliable trip.
• Large/critical motor, continuous process: PT100; real temperature monitoring, alarm+trip.
• Drive-fed, variable-speed motor: PT100 (on bearing and winding) with thermal trend monitoring.
• High burn risk (dusty/gassy) environment: Redundant protection with PTC + bimetal combination.

In sudden and excessive heating scenarios such as locked rotor, correct setting of the thermal limit becomes critical; our article on locked rotor withstand time (tE) and thermal limit is useful on this. For the basic logic of monitoring winding temperature with PT100 and thermistor, our article on monitoring with PT100 and PTC thermistor is a good start. On the current-based protection side, our article on the motor protection circuit breaker (MPCB) is complementary.

Number and Placement of Sensors

The effectiveness of thermal protection depends not only on the sensor type but also on how many sensors are used and where they are placed. The winding temperature is not the same at every turn; the hottest point is usually inside the winding overhangs. Therefore the sensors must be placed as close as possible to the point where the temperature is highest.

• Three sensors (one per phase): The typical placement for PTC and PT100; ensures protection of all three phases and catches heating from phase imbalance.
• Embedded in the winding overhang: The sensor is embedded between the winding overhangs during winding; this gives the fastest and most accurate measurement.
• Separate PT100 for bearing temperature: In large motors not only the winding but also the bearing temperature can be monitored with PT100; this enables early detection of mechanical faults.
• Redundant set: In critical motors two separate PTC sets, one for alarm and one for trip, are common.

Embedding the sensors in the winding is done at the factory during winding; therefore the thermal protection need must be specified at the ordering stage. Adding a sensor later in field conditions is both difficult and, since full contact with the winding overhang cannot be achieved, the measurement is delayed. The right approach is to order the motor with the desired protection sensors from the start; this way the protection is at factory quality and close to the hottest point of the winding.

Common Mistakes and How to Avoid Them

Thermal protection systems are very reliable when set up correctly; however a few mistakes frequently made in the field cause the protection to fail or lead to unnecessary downtime due to false trips. Being aware of these mistakes improves both safety and operational continuity.

• Wrong threshold selection: If the response temperature is not chosen to suit the insulation class, the sensor either protects too late or trips unnecessarily during normal operation. The typical PTC threshold for class F differs from class H.
• Running the signal cable in the same conduit as the power cable: PT100 and PTC are low-voltage signals; if they run beside power cables, electromagnetic noise corrupts the measurement and false trips occur.
• Leaving the monitoring relay in the wrong function: Choosing manual where automatic reset is needed, or automatic where manual is needed; automatic reset can be dangerous in a critical application.
• Not connecting the sensor at all: Having PTC or PT100 in the motor but not connecting it in the panel is the most frequent and most dangerous oversight; the sensor exists but there is no protection.
• Overloading the bimetal contact: If the current limit of the klixon contact is exceeded the contact sticks or burns; an auxiliary relay should be inserted if necessary.

Most of these mistakes can be prevented with a simple checklist during commissioning. When the motor is received, the sensor ends in the terminal box should be documented, connected in the panel to the correct relay with the correct threshold and function, and tested before commissioning. The most reliable way to verify that the sensor actually provides protection is to deliberately open the sensor circuit during the commissioning test and see that the relay stops the motor.

Correct Connection and Wiring Notes

No matter how good the sensors are, they provide no protection if wired incorrectly. A few basic rules:

• Sensor ends are connected to separate terminals (usually small terminals) in the motor terminal box; they must not be mixed with the power ends.
• PTC and PT100 are low-voltage signal circuits; they should be run with shielded cable, separate from power cables.
• The monitoring relay must be set to the correct threshold and function (automatic/manual reset) in the panel.
• Attention must be paid to the load current limit of the bimetal contact; an auxiliary relay should be inserted if necessary.

Frequently Asked Questions

What is the basic difference between PTC and PT100?

PTC is a threshold sensor: when the rated temperature is reached its resistance suddenly rises and the relay stops the motor, but it does not tell what the temperature is. PT100, on the other hand, continuously measures the real temperature; with a reading device you can see the instantaneous value and define both alarm and trip levels. PTC is suitable for a simple and reliable trip, while PT100 is suitable for places requiring monitoring and precise control.

Is bimetal klixon sufficient on its own?

In small and medium power, simple applications a bimetal klixon is often sufficient; since it does not require a monitoring relay it is economical and practical. However, because it does not measure the real temperature and works with a fixed threshold, it is right to set up stronger protection together with PTC or PT100 in critical and continuously running large motors.

Which sensor should be preferred in a drive-fed motor?

Since drive-fed motors can run long at low speed and cool insufficiently, PT100, which monitors the real temperature, is preferred; this way the heating trend at low speed is seen and measures such as an external fan are activated in time. For extra safety, an independent trip layer can be provided by also placing a PTC in the winding.

Secure Your Motor with the Right Thermal Protection

Bimetal klixon, PTC thermistor and PT100 serve the same purpose, namely protecting the winding from overheating, but offer different levels of sensitivity and integration. Klixon provides simple and direct, PTC reliable and standard, and PT100 sensitive and traceable protection. The right choice should be made according to the motor's size, criticality and monitoring need; layered protection should be preferred in critical applications. As HEM Motor we supply electric motors equipped with different thermal protection options (klixon, PTC, PT100) from stock with fast delivery; contact us for the most suitable protection configuration for your application to request a quote.