What Is a Thermistor and How Is It Used in Electric Motors?
Short Answer
A thermistor is a temperature-sensitive resistor whose electrical resistance changes in response to variations in temperature. In electric motors, it monitors winding temperature and provides protection against overheating. This creates a critical temperature monitoring mechanism for motor safety.
Summary
- The resistance of a thermistor changes as temperature rises, enabling precise temperature monitoring.
- Sensors integrated into motor windings provide early detection of overheating.
- Working in conjunction with a protection relay, the system initiates an automatic shutdown in hazardous conditions.
- By interrupting the circuit at high temperature thresholds, it provides thermal protection for equipment.
- Widely used in industrial applications, this solution enhances the long-term operational reliability of motors.
- Proper configuration and connection deliver stronger protection performance throughout the system.
What Is a Thermistor?
A thermistor is a semiconductor-based temperature sensor whose electrical resistance varies with temperature. In electric motors, it is typically integrated into the stator windings for direct thermal monitoring.
The operating principle of a thermistor is based on the resistance value changing according to a characteristic curve as temperature increases. A PTC temperature sensor produces a sharp increase in resistance when a predefined temperature threshold is reached, triggering the protection relay and disconnecting the motor.
This design provides an active layer of thermal protection for industrial motors with high power ratings, various frame sizes, and different numbers of poles, helping preserve the winding insulation class. In GAMAK electric motors, thermistor protection is provided as a standard feature on models with frame sizes 160 and above.
What Types of Thermistors Are There?
Thermistors are classified according to how their resistance responds to temperature changes. In industrial applications, selecting the correct sensor type plays a decisive role in motor safety and system stability.
The main types of temperature sensors are:
- PTC (Positive Temperature Coefficient): When the temperature reaches a predefined threshold, the resistance increases sharply. This is the most commonly used type of motor temperature thermistor in electric motors. It is typically used together with a temperature monitoring relay to provide winding protection.
- NTC (Negative Temperature Coefficient): As temperature increases, the resistance decreases. NTC thermistors are preferred in applications that require continuous temperature monitoring, and NTC thermistor circuits are commonly used in analog temperature measurement systems.
- Silicon-Based Sensors These sensors are used in electronic control systems that require high measurement accuracy. They are typically applied in systems focused on measurement precision rather than industrial motor protection.
Thermistors are widely used in applications that require direct thermal control, including electric motors, compressor systems, and process equipment.
Motor thermistor selection should take into account factors such as motor power rating, frame size, insulation class, and operating environment.
What Is the Function of a Thermistor in Electric Motors?
The primary function of a thermistor in an electric motor is to directly monitor winding temperature and protect the motor against overheating. A motor thermistor serves as an active thermal protection component, particularly in high-power motors and systems operating under heavy-duty conditions.
The key functions of a temperature sensor in electric motors include:
- Winding Temperature Monitoring The motor temperature thermistor is installed within the stator and detects the actual winding temperature.
- Overheating Protection When the threshold temperature is exceeded, the thermistor triggers the relay and disconnects power to the motor.
- Protection of the Insulation Class By preventing the temperature limits of Class F and Class H insulation systems from being exceeded, the thermistor helps extend motor life.
- Early Warning Function It detects overload conditions or insufficient cooling by monitoring increases in temperature.
- Safety in Explosion-Proof Motors Thermistors help maintain surface temperatures within safe limits in explosive environments.
- Stability in Compressor Motors In the event of a sudden temperature rise, the thermistor safely shuts down the system to prevent damage.
The use of motor thermistors is more than just sensor integration; it is a fundamental component of long-term operational reliability in motors with larger frame sizes, varying numbers of poles, and higher power ratings. A properly selected and correctly connected temperature sensor is considered a critical element of sustainable performance in industrial applications.
How Is a Thermistor Connected to an Electric Motor?
A motor thermistor is connected by installing the sensor directly within the stator windings and wiring its leads to a protection relay. The temperature sensor leads are integrated into the control circuit rather than the power circuit, providing indirect protection through the motor contactor.
In PTC applications, the connection is made through a dedicated temperature monitoring relay. When the winding temperature exceeds the threshold value, the resistance rises sharply, causing the relay to open the contactor and disconnect the motor.
In NTC thermistor circuits, resistance changes are monitored continuously in an analog manner. This method is typically preferred in systems that require continuous temperature monitoring, with temperature values calculated through a measurement module.
When connecting a motor temperature sensor, factors such as cable insulation class, terminal configuration, and motor power rating should be taken into consideration. Proper configuration improves measurement accuracy and ensures the thermal protection of the electric motor.
What Are the Most Common Thermistor Failures?
Thermistor failures directly affect the motor’s thermal protection performance. Sensor-related faults can weaken the overall electric motor protection system.
Common types of thermistor failures include:
- Open-circuit failure
- Short-circuit failure
- Resistance drift
- Calibration drift
- Loss of contact with the winding
- Incorrect motor thermistor connection
- Cable insulation damage
- Thermistor relay incompatibility
Regular testing and measurement of temperature sensor faults are essential for maintaining operational safety in motors operating at high power ratings.
Key Considerations When Using Thermistors
The use of temperature sensors directly affects the thermal protection of a motor. Proper selection and correct integration are critical for effective electric motor protection.
The main considerations include:
- Selecting a sensor suitable for the motor's power rating
- Setting a temperature threshold compatible with the insulation class
- Correct motor temperature sensor connection
- Using cables with appropriate conductor size and insulation properties
- Full compatibility with the protection relay
- Proper positioning within the winding
- ATEX compliance for explosion-proof motors
- Threshold optimization in high-load applications such as compressor motors
- Periodic resistance and circuit checks
When properly configured, a temperature sensor is one of the key components of safe and long-term motor operation.
Discover GAMAK's Thermistor-Protected Products
In GAMAK electric motors, temperature sensor protection is provided as a standard feature on models with frame sizes 160 and above. This design enables direct monitoring of winding temperature in high-power motors and helps preserve the insulation class.
Motor thermistor integration, available across both standard-series and explosion-proof motor ranges, enhances operational safety, particularly in heavy industry, HVAC, and process applications. With manufacturing capabilities ranging from 0.12 kW to 1,000 kW, GAMAK offers solutions in IE2, IE3, and IE4 efficiency classes.
You can explore GAMAK's product range to discover motor solutions that integrate thermal protection through a wide selection of frame sizes, pole configurations, and efficiency class options.
Frequently Asked Questions
How Can a Thermistor Failure Be Identified?
A temperature sensor failure is typically identified when the protection circuit fails to activate despite motor overheating, or conversely, when it causes an unnecessary shutdown under normal operating temperatures. A mismatch between the measured resistance value and the specified technical data is another indication of a fault. Physical cracks, broken connections, or loose wiring may also be signs of thermistor failure.
Will a Motor Run If the Thermistor Fails?
Whether a motor continues to operate after a temperature sensor failure depends on the connection configuration. In systems connected to a protection relay, a fault may prevent the motor from starting or cause it to shut down for safety reasons. In some cases, however, the protection function may fail to activate, allowing the motor to continue running and increasing the risk of overheating.
What Is the Service Life of a Thermistor?
The service life of a temperature sensor depends on operating conditions, temperature exposure, and electrical load levels. A high-quality thermistor used within its specified operating range can function reliably for many years. However, factors such as frequent overheating, moisture, and vibration can reduce the sensor’s sensitivity and shorten its service life.
