Standard Motor Catalog

Section TR Technical Reference Guide

A. Line-interrupting Devices - Current-Sensing Thermal Protectors

2. Thermistors (Not Allowed on DIV 1 Motors) Thermistors are small non-linear resistance devices placed in the stator windings. As the critical temperature is reached, the resistance of the thermistors changes radically, causing operation of a control relay. Normally, 3 thermistors are furnished, one per phase. Thermistors may be furnished with or without a separate control relay (greater than 600 nameplate volts). 3. Resistance Temperature Detectors RTD’s are precision wire-wound resistors with calibrated temperature-resistance characteristics. These devices are used in conjunction with customer supplied instruments and are available in 10, 100 and 120 ohm designs. Normally six RTD’s (2 per phase) are furnished per motor, suitably distributed around the circumference of the stator winding, located between coil sides and positioned to detect the highest slot temperature. RTD’s are constructed using a fine, pure, metallic, spring-like wire surrounded by an insulator and enclosed in a metal sheath. • 120 Ohm Nickel • 100 Ohm Platinum • 10 Ohm Copper

These protectors sense both motor temperature and current and offer inherent protection against all abnormal stalled and running conditions. They are built into the motor conduit box for automatic* reset operation on both automatic* and manual reset operation on single-phase motors 182-215 frame.

These protectors are not available on explosion-proof motors or on two-winding two-speed motors.

* WARNING: A MOTOR WITH AUTOMATIC-RESET THERMAL PROTECTION SHOULD NEVER BE USED WHERE AN UNEXPECTED RESTART MAY INJURE PERSONNEL OR DAMAGE PROPERTY. B. Pilot Devices 1. Thermostats These protectors are not available on two-winding two speed motors. Thermostats are mounted on the stator winding and are temperature-sensing only. They are available with normally closed contacts and only for automatic reset. Thermostats will protect the motor from high ambient temperature, exceedingly long accelerating cycles (where the design is not rotor limited), repeated or excessive overloads and loss of ventilation. However, they do not give protection under stalled or locked-rotor conditions and must be used with an external current sensitive motor control for complete motor protection. Thermostats can be installed in all motor enclosures They are not recommended for machines with name plate voltages exceeding 600 volts.

Thermostats are suitable for the following pilot circuits:

Table 19

Power Supply


Current 6.0 amp 3.0 amp 1.5 amp 1.2 amp 2.2 amp 1.1 amp 0.4 amp


110-115 volts 220-230 volts 440-480 volts

550 volts

115-125 volts 230-250 volts

550 volts


Data subject to change without notice. 02/23 •

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