Frequently Asked Questions & Answers

Winding Temperature Detectors

Question:What types of winding temperature detectors are utilized by U.S. Electrical Motors?

Answer: Specific types of winding temperature detectors include thermostats, RTD’s, thermistors, and thermocouples. Following is a brief description of each.

Winding Thermostats

Winding thermostats are snap action, bi-metallic, temperature actuated switches. Their purpose is to activate a warning device, or simply shut down the motor upon excessive winding temperatures, when wired into the motor control circuit.

Thermostats are made either with contacts that are normally closed (open at high temperatures) or contacts that are normally open (closed at high temperatures). The thermostat temperature switch point is pre-calibrated by the manufacturer and is not adjustable. Reset is automatic after a decrease in temperature. Thermostats are normally installed in the connection end turns of the motor winding. Standard procedure is to wire three thermostats together in a set, with one thermostat embedded in each phase of the winding. Open thermostats are normally wired in parallel, while closed thermostats are wired in series. Refer to the figure below for further explanation.

As seen in the figure above, only two leads come out to the motor outlet box. The leads of a normally closed (N.C. thermostat) are marked P1 and P2. Those of a normally open thermostat are marked P3 and P4.

Refer to the table below (Table 6) for thermostat alarm and shutdown temperatures.

Table 6: Thermostat Temperature Chart

Temperatures shown in ° C

Winding RTD’s

RTD’s (Resistance Temperature Detectors) are precision, wire-wound resistors, with a known temperature resistance characteristic. U.S. Electrical Motors uses flat, molded strip type RTD’s that are only .030 inch thick. RTD’s are installed in the slot portion of form wound motors, and in either the slot or end turns of mush wound motors.

RTD’s used in motor windings are either 10 ohm, 100 ohm, or 120 ohm. Each type of RTD has its own specific resistance characteristic. The basic detectors are listed below in Table 7.

Table 7: Winding RTD’s

* Also available with 3 leads.

All the RTD leads are brought out to a motor outlet box. RTD’s leads are identified in sets, using C1, T1, T1, and C11, T11, T11 for the same phase. Since leads are always brought to terminal strips, the leads are terminated with fork-tongue terminals.

See alarm and trip temperatures based on the motor service factor, HP rating, and class of temperature rise.

Winding Thermistors

A thermistor is a non-linear resistance temperature detector, made from semi-conducting material. USEM utilizes positive temperature coefficient (PTC) type thermistors, which have a resistance that increases with increasing temperature. Each individual thermistor has its own unique resistance vs. temperature characteristic. Thermistors are normally installed in the end turns of the motor. Depending upon the controller, they are wired either in series or in a ‘common lead circuit’. Both circuits are shown below.

The following is a brief description of the controllers and thermistors supplied by various companies:

Power Control Corporation (PCC): In the past, USEM supplied PCC 600, 900, 8000, and 9000 series thermistors. USEM now uses only the 8000 series thermistors. A maximum of three PCC 8000 series thermistors are installed in the common lead circuit configuration. Do not install them in series, or false tripping will result. PCC makes numerous controllers, including a special controller for the therma-sentry system. The PCC controller brand name is ‘MOTOGUARD’. For non-therma-sentry PCC thermistors, the thermistors are internally wired in the common lead configurations with the leads marked TM5, TM6, TM7, and TM8. Lead TM5 is the common lead.

Texas Instrument (TI): TI currently uses 4BA and 7BA series, PTC thermistors. The 4BA series thermistors are normally used on new and rewound motors and contain a copper heat collector for a fast response time. The 7BA series is normally used on existing motors, and contains only a small thermistor bead to ease installation. TI thermistors are wired in series. Three thermistors may be installed in series without false tripping the controller. Our procedure is to bring out all six leads and make the series connection in the outlet box. The thermistor lead pairs are marked TM1, TM2, and TM3. The standard TI controller is a 50AA control module.

Siemens: Siemens presently uses a Siemens Q63100-P, PTC thermistor. Siemens thermistors must be wired in series. Six thermistors may be wired in series without false tripping the controller. Our standard procedure is to install three thermistors in series and bring all six leads out, making the series connection in the outlet box. The thermistor lead pairs are marked TM1, TM2, and TM3. The Siemens standard controller is a 3UN tripping unit control module, which has an N.O. and an N.C. contact.

The following table (Table 8) shows alarm and shutdown temperatures (in ° C) for 1.0 and 1.15 SF thermistors, based on the required class of temperature rise.

Table 8: Thermistor Temperature Setting Chart

Temperatures shown in ° C

A thermocouple is a pair of dissimilar conductors joined at one point, in a way that causes an electromotive force (EMF) to develop due to the thermoelectric effects. Any given set of thermocouple wires has a known EMF vs. temperature characteristic. Thermocouples are only able to generate a low-voltage, low-power signal in the millivolt range. There are many types of thermocouples. USEM’s standard types include copper-constantan, chromel-constantan, and iron-constantan. Thermocouples are normally installed in the slot between coil sides, on both mush wound and form wound motors. However, if necessary, they can also be installed in the end turns. The standard quantity of thermocouples is six, installed two per phase. If quantity-3 thermocouples are specified, leads are marked TC1, TC2, and TC3. If quantity-6 are specified, leads are marked TC1, TC2, TC3, and TC11, TC22, TC33, such that TC1 and TC11, etc. are in the same phase.

See alarm settings for alarm and trip temperatures based on the motor service factor, HP rating, and class of temperature rise.

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