Motor Winding Temperature Monitoring: Protection with PT100 and PTC Thermistor

When an electric motor's winding burns out, the business's loss is not just the cost of the motor: the production halted during replacement, the equipment removed and refitted, and the unplanned maintenance make up the real cost. Most winding burnouts share a single root cause: the winding temperature exceeding the limit the insulation can withstand. The critical part is that measuring this temperature from the outside is not reliable, because the internal temperature of the winding rises far faster than the value read from the body surface. For this reason, on critical motors the most reliable protection is provided by embedded sensors that measure temperature directly from inside the winding. As electric motor manufacturer and supplier HEM Motor, manufacturing since 1979, we offer our motors orderable with a winding-embedded PT100 (RTD) or PTC thermistor sensor option.

In this article we compare the two most common solutions in motor winding temperature monitoring — the PT100 temperature detector and the PTC thermistor — and address which is essential in which application, and how to request the sensor option correctly when ordering a motor. Our aim is not to give a technical definition; it is to help the person making the purchasing decision find a clear answer to "does my motor need this sensor, and if so, which one?" The panel-side protection equipment such as breakers, relays and contactors we gathered earlier in a separate article; the in-winding sensors are a different protection layer that complements them.

Why Is It Necessary to Measure Winding Temperature Directly?

In asynchronous motors, heat arises from the losses the current produces as it passes through the winding. While the motor runs at normal load, this heat is dissipated in balance through the fan and the body surface. However, under conditions such as overload, phase loss, frequent starting, insufficient cooling or high ambient temperature, the winding temperature rises far faster than the body surface can respond. The insulation material has a withstand limit; once this limit is exceeded, insulation ageing accelerates and eventually winding burnout follows.

Classic protection methods measure current: a thermal relay indirectly estimates heating from the current the motor draws. This method works in many cases, but it is indirect. For example, when the motor's cooling fan is blocked or the ambient is very hot, the current may look normal while the winding has heated dangerously; the thermal relay cannot detect this situation. A winding-embedded temperature sensor, on the other hand, measures temperature directly at its source and catches these dangers invisible to current. That is exactly why on critical motors an embedded sensor is an indispensable layer complementing current-based protection. You can also look at our article on preventing overheating where we cover why a motor heats up and how to prevent overheating more broadly.

What Is PT100 (RTD) and How Does It Work?

PT100 is a resistance temperature detector (RTD). The "PT" in its name means platinum; the "100" indicates that the sensor shows 100 ohms of resistance at 0 °C. The resistance of the platinum wire changes with temperature in a proportional and fairly linear way: as temperature rises, resistance rises. A measuring device reads this resistance to show the winding temperature in degrees, continuously and as a real value.

The greatest advantage of PT100 is that it gives temperature as a continuous number, not a threshold. This way not only "danger / no danger" information but also how the temperature changes over time can be monitored. The business learns the motor's normal operating temperature; when the temperature slowly begins to climb, it can intervene early as a herald of a fault. This feature makes PT100 ideal for plants that want predictive maintenance and continuous monitoring. Typically more than one PT100 is embedded in the motor (for example one per phase winding) and these are connected to a temperature display, PLC or monitoring system.

What Is a PTC Thermistor, and How Does It Differ from PT100?

A PTC thermistor is a "positive temperature coefficient" semiconductor sensor. Its fundamental difference from PT100 lies in its operating logic: a PTC thermistor does not show temperature as a number; instead it shows low resistance up to a certain critical temperature (the rated response temperature), and once this temperature is reached its resistance rises suddenly and very sharply. The tripping device it is connected to detects this sudden resistance jump and takes the motor out of service.

So a PTC thermistor is a threshold protector: it says "the temperature has reached this degree, stop the motor." It does not say exactly how many degrees the temperature is, but it reports that the danger threshold has been reached without leaving room for doubt. These sensors are usually used by series-connecting three thermistors embedded in the motor's three phases; when any phase's winding reaches the threshold, protection engages. PTC thermistors are defined in industry according to the DIN 44081 and DIN 44082 standards; these standards set the thermistor's resistance-temperature characteristic, so that it works compatibly with the tripping devices of different manufacturers.

In short, the two approaches complement each other: PT100 is used for continuous measurement and monitoring, and the PTC thermistor for precise and economical threshold protection. Which one to choose depends on the criticality of the motor and the monitoring needs of the business.

PT100 or PTC Thermistor? Which One in Which Application?

The right choice is determined by the criticality of the motor and its operating conditions. Roughly, the following framework guides:

• A PTC thermistor is preferred: when the motor needs protection but continuous temperature monitoring is not mandatory. Threshold-based protection is economical, simple to install, and provides sufficient safety on many standard industrial motors. On motors that start and stop frequently, can be loaded suddenly, or have strained cooling, it is an effective protection layer.
• PT100 is preferred: when temperature needs to be monitored continuously and numerically. PT100 is the right choice in critical processes, on motors whose stoppage causes great cost, in plants applying predictive maintenance, and in businesses that want to track the temperature trend via PLC/SCADA.
• Both together: on the most critical motors, continuous monitoring with PT100 and independent threshold protection with a PTC thermistor can be used together; one measures, the other stops the motor as a backup safety layer.

The typical cases where temperature monitoring becomes especially important are: motors that start frequently (each start brings high current and extra heating), motors running at low speed with a variable frequency drive (the self-cooling fan slows down, cooling weakens), motors working in high ambient temperature or dusty/covered environments, and critical drives whose stoppage completely locks production. Because cooling weakens at low speed, a winding temperature sensor is often essential in variable frequency drive applications.

Which Failures Does a Temperature Sensor Protect the Motor From?

A winding-embedded temperature sensor engages in many scenarios that current-based protection cannot see:

• Insufficient cooling: if the fan cover is blocked, the ambient is very hot or the motor is covered, the current looks normal but the winding heats; the sensor catches this.
• Frequent starting and overloading: starts repeated at short intervals heat the winding cumulatively; the sensor measures the cumulative heating directly.
• Long running at low speed: on a motor running at low speed with a variable frequency drive, self-cooling decreases; the sensor reports the dangerous temperature.
• Phase imbalance and voltage problems: grid-sourced imbalances heat the winding asymmetrically; the embedded sensor sees the hottest spot.

This protection directly extends the motor's life: insulation wears according to the temperature it is exposed to over its lifetime. Keeping the winding temperature within limits means protecting the insulation life. Our article on the winding and insulation class, where we cover in detail the effect of the winding and insulation class (F/H) on motor life, complements this subject. The panel-side protection equipment to be considered together with the sensor you can find in our article on protection equipment.

The Cost Return of Sensor Protection for the Business

A temperature sensor looks at first glance like a small cost item added to the motor; yet the cost it prevents in return is far greater. When a winding burnout occurs, the business faces three separate costs: the replacement or rewind cost of the motor itself, the production halted until the motor is replaced, and the labour and logistics burden brought by unplanned maintenance. The largest of these three is usually the production loss, because when a critical drive stops, not only that machine but also the line connected to it stops.

A winding-embedded sensor prevents this chain loss from the start by stopping the motor before it reaches the danger threshold. When the motor temperature exceeds the limit it goes out of service and runs again after cooling; the winding is not damaged. So the sensor saves you from buying "a large and unplanned fault" in exchange for "a small and controlled stoppage." Especially on motors that have no spare and whose stoppage completely locks production, this protection is one of the fastest-paying items of investment. We covered the symptoms, causes of motor failures and when to intervene in detail in our article on motor failures.

How Temperature Monitoring and Periodic Maintenance Work Together

A temperature sensor does not replace a maintenance programme on its own; it is a tool that strengthens it. When a continuously measuring sensor such as a PT100 is installed, the maintenance team learns the motor's normal operating temperature and records this value as a reference. Over time, the temperature beginning to rise slowly is often an early herald of a problem: bearing wear, cooling channels clogged with dust, or an increase in load drive the temperature up. When this trend is noticed, intervention can be made in a planned maintenance window before the fault has even occurred.

For this reason, temperature monitoring is one of the most practical entries to predictive maintenance. Although a PTC thermistor does not give such trend information, it provides the maintenance team a safe buffer by stopping the motor at the most critical moment. Both solutions support the periodic check schedule. We gathered the motor's regular maintenance and check steps in our article on the periodic maintenance schedule; temperature monitoring feeds this schedule with real data from the field.

Ordering a Motor with the Sensor Option

Although it is possible to add a temperature sensor to a motor later, the soundest method is to order the motor with the sensor option from the start. If the sensor is embedded correctly into the hottest spot of the winding during manufacturing, it gives the most reliable measurement; solutions added externally afterward do not provide the same accuracy. As HEM Motor, we can manufacture our motors with the PT100 or PTC thermistor option. There are a few points you should state at the ordering stage: which sensor type you want (PT100, PTC thermistor, or both), how many sensors (usually one per phase) and the type of monitoring/tripping system it will connect to.

Our general purpose industrial motors across a wide power range and our high-efficiency IE4 high-efficiency electric motors can be ordered with the temperature sensor option. To see the whole motor family you can review our electric motors category, and for our technical supply articles our electric motors blog category. It is especially important to plan the sensor option in variable frequency drive installations; we covered starting and drive selection in our article on motor starting.

Frequently Asked Questions

What is the fundamental difference between PT100 and a PTC thermistor?

PT100 is an RTD sensor that measures temperature continuously and numerically; it shows the motor's instantaneous temperature in degrees and is suitable for trend monitoring. A PTC thermistor works on a threshold basis: when a certain critical temperature is reached its resistance rises suddenly and gives the signal to take the motor out of service, but it does not show the exact temperature value. In short, PT100 measures and monitors, the PTC thermistor protects at the threshold. PT100 is preferred in critical processes that want continuous monitoring, and the PTC thermistor in applications that want economical and precise threshold protection.

On which motors is a temperature sensor really essential?

A temperature sensor is strongly recommended on motors that start and stop frequently, run at low speed with a variable frequency drive, sit in high ambient temperature or dusty/covered environments, and on critical motors whose stoppage completely locks production. In these applications, current-based protection alone may fall short, because cooling problems can heat the winding without showing in the current. On motors running at standard load, in a well-ventilated environment and at continuous constant speed, thermal protection is often sufficient.

Can a PT100 or thermistor be added to my existing motor afterward?

Although it is possible in some cases, the most correct method is to order the motor with the sensor option from the start. This is because, for the sensor to give a reliable measurement, it must be embedded correctly into the hottest spot of the winding during manufacturing. Solutions added externally afterward cannot provide the same accuracy. If you are buying a new motor, stating the sensor type and quantity at the moment of order is the soundest path for both cost and reliability.

According to which standard is a PTC thermistor selected?

In industrial applications, PTC thermistors are defined according to the DIN 44081 and DIN 44082 standards. These standards set the thermistor's resistance-temperature characteristic and the rated response temperature; this way the sensor works compatibly with the tripping devices of different manufacturers. The correct rated response temperature is chosen according to the motor's insulation class: because the insulation class determines the maximum winding temperature the motor can withstand, the thermistor threshold is set at a suitable value below this limit. If you tell us the insulation class and the application at the order stage, we select the thermistor with the appropriate threshold value together.

Why is a temperature sensor more important on a motor running with a variable frequency drive?

A standard asynchronous motor's cooling fan is connected to the shaft; that is, as the motor slows the fan also slows and cooling weakens. On a motor running for a long time at low speed with a variable frequency drive, this can lead to the winding not being cooled sufficiently. Moreover, because this heating may not always show in the current, thermal protection may not notice it. For this reason, a winding-embedded temperature sensor is often essential in variable frequency drive installations; by measuring temperature directly at its source, it catches the cooling shortfall that arises at low speed.

Get a Quote for a Motor with a Temperature Sensor

If you want to protect your critical motors against winding burnout in the most reliable way, order the motor with the PT100 or PTC thermistor option. If you tell us your application (frequent starting, variable frequency drive, critical process, ambient temperature), we determine together which sensor type and how many are right, and manufacture your motor accordingly. The right sensor option extends motor life and reduces unplanned downtime.

To get a quote right away you can call us on +90 (532) 345 49 86 or reach us through our contact us page. In the supply of motors with temperature sensors, we are by your side with the assurance of HEM Motor.