A thermistor is a non-linear resistance temperature detector, made from semi-conductor material. Each specific thermistor has its own unique resistance vs.temperature characteristic.

There are two general types of Thermistors. The positive temperature coefficient (PTC) type has a resistance that increases with increasing temperature.   The PTC thermistor has a sharp rise in resistance when the temperature reaches the 'knee', or switching point.

The negative temperature coefficient (NTC) type thermistor has a resistance that decreases with increasing temperature. PTC type thermistors are supplied on U.S. MOTORS^® brand product lines.

A control module must be used with the thermistors. The control module is generally an externally excited solid state device that senses the thermistor resistance and switches contacts when the thermistor input resistance rises above a set resistance value.

The thermistor trip temperature determines the temperature at which the thermistor resistance rises to a very high value, thus tripping the controller. Thermistors are available with different trip temperatures. Each thermistor has one pre-set, non-adjustable switch point.

We will bring all six (6) leads out unless specified otherwise.

Thermistors have several advantages over RTD's, thermocouples and thermostats. Like thermostats, thermistors have a tamper proof pre-set switch point, with a reset only after the motor cools down. Due to their small size and heat sink construction, they have fast response times. In mush wound motors, non-rotor limited designs, thermistors can provide locked rotor protection. Also, since the controller trips at a high resistance value, resistance variations due to long lead runs can be tolerated. This feature allows for one controller to be used for several temperature sensing locations.

Selection

Unless specified otherwise on the face of the order, quantity three (3) thermistors will be supplied.

Types of Thermistors:

Siemens B59100M, PTC thermistors must be wired in series. Up to six (6) Siemens thermistors may be wired in series without false tripping the controller. Our standard procedure is to install quantity three (3) thermistors in series and bring all six (6) leads out, making the series connection in the outlet box. For Canadian orders, the three thermistors are wired in series internally in the motor, and quantity two (2) leads are brought out. We ask that this configuration be specified when ordering. The Siemens standard controller is a 3UN tripping unit control module. The control module has one normally open and one normally closed contact.

Table 1: THERMISTOR SELECTION CHART

Table 2: WINDING THERMISTORS