Thermistor Relay: Working Principle, Selection, Basics

With the push to reduce component volumes, lower costs, and reduce weight without sacrificing performance or reliability, the challenges associated with thermal management increase for electrical motors. Thermal management for electric motors will become more important as the automation industry continues to grow. That is why you have to know the fundamentals of a thermistor relay. (Sometimes it is referred to as a PTC relay or thermistor motor protection relay.)

What is a Thermistor Relay?

The thermistor relay monitors and measures the winding temperature of electrical motors. When a high temperature occurs in the windings, it protects motors from overheating, overload, and insufficient cooling.

Many motor protection devices protect motors from overheating and overload based on the motor current information. The real temperature is not detected by these devices. The temperature in the thermal hot spots of a motor can be higher than expected due to motor location or insufficient cooling.

To avoid this, motors should be protected against high temperatures. This can be done by measuring the temperature with PTC sensors in the motor windings and/or bearings and monitoring these signals with a thermistor relay.

Advantages of a directly monitored motor temperature

Monitoring of all possible different disturbing influences like:

Insufficient cooling
Heavy duty
Too small dimensioning
Overload
Differences in the ambient temperatures between the panel board and the motor environment
Rolling bearing defects etc.

What Does a Thermistor Relay Do?

Accurate direct temperature monitoring in the motor windings is critical. Thermistor relays evaluate the resistance of PTC sensors integrated into the motor windings. This is a direct and therefore very accurate measurement method.

What does a thermistor protection relay do

A motor has stationary and rotating elements. The solid part is the stator, the rotating part is the rotor. In asynchronous motors, the windings are located in the stator. The stator is a cast-iron housing with radiator fins with a core assembly of stamped, electrically insulated sheets.

Normally the three-phase squirrel cage motor has three-phase windings U, V, W. The windings are coiled around the stator slots with a shift of 120° between the windings.

The windings start and end in the motor’s terminal box. The three phases on the terminal box can be interconnected either in star or in delta connection. The supply voltage is connected to these terminals. A generator works on the same principle. Here, U1 and U2, V1 and V2, and W1 and W2 are the points where the generated voltages can be tapped off.

A certain amount of current flows in each winding. This induces the magnetic field, which makes the rotor move.

The PTC sensors are one of the first parts to be installed in a motor. One thermistor is sufficient for each winding as the current causing the heating is constant in each winding. PTC sensors are connected to the monitoring device at terminals T1 and T2 on the terminal box.

Working Principle of a Thermistor Relay

Thermistor relays have different characteristics and different functioning.

To explain the function we can look at an application with a motor speed controlled by a frequency converter. The motor is usually equipped with three PTC sensors connected in series to terminals T1 and T2. The total resistance of the sensors connected to the terminals should be under 1.5-kilo ohm. Each thermistor usually has 100 ohms in normal operation, so connecting three sensors in series gives 300 ohms. The relay works on the closed-circuit principle, so the output is de-energized as long as the supply power is not connected. As in our example terminal, 12 can be used to connect an alarm lamp. In normal conditions, the output relay is energized. The total resistance is below the temperature threshold of 3050 ohms ± 550 ohms for the relay.

Even if only one of the PTC sensors in the circuit heats up excessively, the output relay de-energizes and switches off the motor.

All thermistor motor protection devices feature an automatic reset, in other words, the relay re-energizes automatically after the resistance falls below the temperature hysteresis, in this case, 1900 ohm ± 400 ohms.

Selection of a Thermistor Relay

You can select the device according to the supply voltage range, the rated frequency, the number of output contacts, the status indication, and the enclosure width required in the application.

The next important parameter is the accuracy of the temperature threshold and hysteresis settings. Another important factor is the number of sensor circuits that the device can monitor. The devices feature different monitoring functions such as wire break monitoring, short circuit detection, and non-volatile fault storage.

Reset options, Atex approvals should also be considered.

Wiring Diagram of a Thermistor Relay

Features and Applications of Thermistor Relays

Thermistor relays can be useful in all applications where motors are used especially where overloads are frequent and may cause motor damage: pumping stations, water treatment, conveyors, material handling, HVAC, and chillers. etc.

The characteristic of a resistor is not influenced by harmonics. Therefore, thermistor protection is used in motor applications such as frequency converters, and soft starters.

Another argument for using a thermistor relay is the reset option. Thermistor relays feature automatic, manual, or remote reset, triggered by a control input, for example from a PLC. The kind of reset should be chosen carefully and depends on the application.

Sophisticated devices feature short circuit detection and broken wire monitoring. Thermistor relays can be tested with a test button. To attain a certain safety level in applications with configured auto reset, the thermistor protection devices feature a non-volatile storage function, to prevent resets caused by a supply fault.

One important feature of thermistor relays is the reset possibilities. The basic automatic reset is implemented in all thermistor motor protection devices. Automatic reset means that the output re-energizes when the resistance level falls beneath the hysteresis for re-tripping. This can cause a dangerous situation since a connected motor can restart automatically.