Nonlinear PTC thermistor probes
What is a non-linear PTC (EN) - PTC (FR) thermistor probe?
Thermistor CTP or PTC (in English) is a small non-linear resistance sensor, which can be integrated into the winding of electric motors. It is made from a metal oxide or semiconductor material.
Practical advice for installing and using the PTC thermistor:
- The relationship between resistance and temperature is non-linear, it varies greatly with small temperature changes around the set point.
- For correct positioning, the thermistors must be located near thermally critical areas, or hot spots of the winding. This allows the temperature of the copper wires to be closely monitored. However, an offset will remain depending on the size of the probe and their installation in the winding.
- Thermistors are to be inserted in non-rotating parts of motors, such as the stator winding in an AC motor or the field windings in a DC motor.
The advantages of the nonlinear PTC - PTC thermistor probe
- Their small size allows them to be installed in direct contact with the stator winding.
- Their low thermal inertia gives a quick and precise response to changes in winding temperature.
- They measure the temperature directly regardless of how these temperatures are initiated.
- They can be used to detect overload conditions in motors driven by frequency converters.
The change in resistance is relatively small and remains almost constant up to the nominal response temperature. As the rated response temperature is approached and exceeded, the resistance gradient increases sharply, giving the PTC thermistor high sensitivity to small changes in temperature.
At the Curie point, an increase in temperature of a few degrees results in a significant increase in resistance. The resistance is monitored by a protection relay and, when the sudden change in resistance is detected by the protection relay, it activates a contact to trigger an alarm or shut down the protected device.
Thermistor protection relays should trip reliably when the resistance of the sensor exceeds approximately 3 kΩ.
They also respond to an open circuit, either in the cable or in the thermistor sensor, thus providing protection against failure. Modern relays are also designed to detect a short circuit of the PTC probe, when the resistance of the sensor drops below approximately 50 Ω.
Frequently used in AC variable speed drives to protect the AC motor supplied by the inverters. Many modern AC drives have a built-in thermistor protection unit thus eliminating the need for a separate thermistor protection relay.
Tripping temperature according to the insulation class of the electric motor
|Motor insulation class||E||F||H|
|Lower nominal temperature||120°C||140°C||165°C|
General technical characteristics
|Max. operating voltage||Vmax||30V|
|Max. measurement voltage||Vmeas, max||2,5 V|
|Nominal resistance (2,5 V max)||R25||≤100 ohm|
|Insulation voltage||V||2,5 kV|
|Ts response time||t||≤5 seconds|
|Detection temperature tolerance Ts||ΔT||± 5K|
|Max. Operating temperature range (V≤Vmeas, max)||° C||200C|
Due to the relatively slow transfer of heat to the sensors through the insulation, PTC thermistors do not provide sufficiently fast protection against short circuits in motors or transformers. Also, as they are typically located in stator windings, they do not provide adequate protection for critical motors or high inertia starting or rotor stall conditions.
In these cases, to achieve full protection, it is recommended to use PTC thermistors in combination with electronic motor protection relays, which monitor the primary current drawn by the motor. The application of PTC thermistors as temperature sensors is only effective when:
1. The rated response temperature of the thermistor is correctly selected for the insulation class used on the winding.
2. Thermistors are correctly located near thermally critical areas.
3. There is low thermal resistance between the winding and the PTC thermistor. It depends on multiple thermistor sensors can be connected in series in a relay to one input, provided that the total resistance at ambient temperatures does not exceed 1,5 kΩ. In practice, up to six thermistor sensors can be connected in series. The electrical insulation between the winding and the thermistor. Since thermistors must be isolated from high voltages, it is more difficult to achieve low resistance to heat transfer in HV motors, which have a greater thickness of insulation.
For a three-phase AC motor, two thermistor sensors are usually provided in each of the three windings and connected in two groups of three series. One group can be used for alarm and the other group for motor triggering. The alarm group is usually selected with a lower rated response temperature, typically 5 ° C or 10 ° C lower than the trip group.
If the operator takes no action, the trip group is used to stop the motor to prevent damage to the winding insulation. The physical location of thermistor sensors in an AC motor depends on the construction of the motor, whether it is a cylindrical rotor or a salient pole rotor, and several other design and design variables. manufacturing. In some cases, the optimal location may need to be determined from real-world testing.
Relays for PTC thermistors
The thermistor protection relay is designed to be mounted inside a control cabinet or motor control center, usually on a standard Din rail. The figure below shows a typical connection of two thermistor protection relays, and their associated groups of thermistor sensors.
For alarm and trip control of a three-phase AC induction motor. The performance of thermistor protection relays can be affected by external electrical interference, where voltages can be induced in the sensor cable.
Therefore, the cables between the thermistor protection relay and the PTC thermistor sensors should be selected and installed to minimize the effects of induced noise.
Cables should be as short as possible and should avoid running near noisy or high voltage cables for long distances!
During the tests, care must be taken not to blow the thermistors as this can damage them !! The correct procedure is to connect all thermistor leads together and apply the test voltage between them and the earth or phases.
Some practical recommendations for the type of cables to be used are as follows:
- Distances ≤ 20 m - Standard parallel cable is acceptable
- Distances ≥ 20 m, ≤ 100 m - Twisted pair cable is required
- Distances ≥ 100 m - A shielded twisted pair (STP) cable is required
- High interference level - Shielded twisted pair (STP) cable is required