Introduction

Temperature monitoring is an important part of protecting low-voltage industrial motors. During operation, motor windings generate heat due to electrical losses, while factors such as overload, insufficient cooling, frequent starting, and high ambient temperatures can cause winding temperatures to rise beyond acceptable limits.

To monitor winding temperature and prevent overheating, two commonly used temperature sensors in low-voltage motors are PTC thermistors and PT100 resistance temperature detectors (RTDs).

Although both are designed to help protect motor windings against excessive temperatures, they work differently and serve different monitoring purposes. So, what are the main differences between PTC and PT100, and which one is more suitable for a particular motor application?

1. What Is a PTC Thermistor?

PTC stands for Positive Temperature Coefficient. A PTC thermistor is a temperature-sensitive resistor whose electrical resistance increases significantly when its temperature reaches a specific threshold.

In motor applications, PTC thermistors are commonly embedded in the stator windings to monitor winding temperature. When the temperature approaches the sensor’s rated response temperature, its resistance rises sharply. A connected PTC thermistor protection relay detects this change and sends a signal to trigger an alarm or disconnect the motor, depending on the protection arrangement.

For many low-voltage motors, three PTC sensors are installed in the windings, typically one per phase, and connected in series. This arrangement allows the protection relay to monitor the winding temperature through a two-wire connection.

The main characteristic of PTC is that it provides threshold-based temperature protection rather than continuous temperature measurement. It can effectively identify overheating conditions, but it does not provide an accurate real-time temperature reading.

2. What Is a PT100 Sensor?

PT100 is a platinum resistance temperature detector (RTD). Unlike a PTC thermistor, which is mainly used for threshold protection, PT100 is designed for continuous temperature measurement.

The number “100″ indicates that the sensor has a nominal resistance of 100 ohms at 0°C. As temperature increases, its electrical resistance changes in a predictable and approximately linear manner over its operating range.

A PT100 sensor is usually embedded in the motor stator winding. Its resistance is measured by a compatible temperature monitoring device, which converts the resistance value into an actual temperature reading.

Depending on the motor design and monitoring requirements, PT100 sensors can be installed in different winding locations. They are commonly supplied with two-wire, three-wire, or four-wire connections, with three-wire and four-wire configurations helping to compensate for lead-wire resistance and improve measurement accuracy.

The major advantage of PT100 is that it provides continuous temperature information. Users can monitor actual winding temperature, set different alarm and trip limits, and integrate the readings into a PLC, DCS, or other industrial control system.

3. PTC vs PT100: Key Differences

Although both sensors are used for motor temperature protection, their functions and characteristics are quite different.

1. Monitoring Method

PTC provides threshold-based protection. When the temperature reaches its rated response point, the resistance increases sharply and activates the protection relay.

PT100 provides continuous temperature measurement, allowing users to see the actual winding temperature during motor operation.

2. Temperature Information

PTC generally indicates whether the temperature has reached a predefined protection threshold. It cannot provide an accurate continuous temperature reading.

PT100 provides measurable temperature values, making it more suitable for applications requiring detailed temperature monitoring and operational analysis.

3. Protection and Control

PTC is commonly used with a dedicated thermistor protection relay to trigger an alarm or trip signal.

PT100 can be connected to a temperature monitoring device, PLC, or DCS, allowing users to configure multiple alarm and shutdown thresholds.

4. Wiring and Monitoring Requirements

A typical three-phase PTC arrangement uses three sensors connected in series, requiring only two external connection terminals.

PT100 wiring depends on the selected measurement configuration. Three-wire and four-wire connections are commonly used where higher measurement accuracy is required. Additional monitoring channels may also be needed.

5. Cost and System Complexity

PTC protection systems are generally simpler and more economical, making them suitable for standard industrial motor applications.

PT100 systems usually require more sophisticated temperature monitoring equipment and additional wiring, but provide more detailed temperature information and greater flexibility.

4. When Should You Choose PTC or PT100?

The choice depends mainly on the motor’s operating conditions, protection requirements, and the level of temperature monitoring required.

PTC is generally suitable for:

• Standard industrial low-voltage motors.
• Applications where basic winding overtemperature protection is sufficient.
• Motors with relatively straightforward control and protection systems.
• Projects where cost and system simplicity are important considerations.

PT100 is generally recommended for:

• Motors operating under demanding or variable load conditions.
• Applications requiring continuous winding temperature monitoring.
• Critical equipment where temperature trends need to be recorded and analyzed.
• Systems requiring integration with PLC or DCS monitoring platforms.

For some critical motor applications, PTC and PT100 can also be installed together. In this configuration, PTC provides an additional independent overtemperature protection function, while PT100 supplies continuous temperature information for monitoring and operational management. The final arrangement should be determined according to the motor design and protection system.

5. Conclusion

PTC and PT100 are both widely used temperature sensors in low-voltage industrial motors, but they are designed for different purposes.

PTC is primarily intended for threshold-based overtemperature protection, offering a simple and economical solution. PT100, on the other hand, provides continuous and more precise temperature measurement, making it suitable for applications requiring detailed monitoring and integration with industrial control systems.

There is no universally better choice between the two. The most appropriate solution depends on the motor’s operating environment, application requirements, protection philosophy, and overall project budget.

When selecting a low-voltage motor, it is important to consider not only the sensor type but also its rated response temperature, installation position, wiring arrangement, and compatibility with the external monitoring system. Proper selection helps improve motor protection, operational reliability, and equipment service life.