Winding Thermistors
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. U.S. Electrical Motors supplies the PTC type thermistors.
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.
Thermistors are normally installed in the end turns of the motor. Depending on the controller, they are either wired in series, or in a special configuration that can be referred to as a 'common lead circuit' (4 leads out including a common ground). The thermistors wired in series can be either wired in series internal to the motor with only two leads brought out (usually for Canadian orders), or wired in series in the outlet box so that all six (6) leads are brought out of the motor. U.S. Electrical Motors 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
Except for Therma Sentry, the customer must advise the exact type of thermistor required so it will match the controller. All orders that do not specify the exact type of thermistor required will be delayed. Unless specified otherwise on the face of the order, quantity three (3) thermistors will be supplied.
Types of Thermistors:
1. Power Control Corporation (PCC) 8000 series are usually installed in the 'common lead circuit' configuration. They cannot be installed in series as false tripping will occur. PCC makes numerous controllers, including a special controller for a Therma Sentry system. They also make a controller with a normally open contact and one normally closed contact. The PCC controller (non- Therma Sentry) brand name is "MOTOGARD".
2. Texas Instruments (TI) 4BA series thermistors are usually wired in series. Up to quantity three (3) thermistors may be installed in series without false tripping the controller. Our procedure is to bring out all six (6) leads and make the series connection in the outlet box. The standard TI controller is a 50AA control module.
3. Siemens Q63100-P, PTC thermistors must be wired in series. Up to six (6) Siemens thermistors may be wired in series without false tripping the controller. U.S. Electrical Motors 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.
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