Abstract: A thermal model of an electric apparatus such as a transformer or motor, from which signals can be obtained representing temperatures of the conductors and of the adjacent core material, includes storing first and second values, respectively, representing conductor temperature and core material temperature. The magnitude of the current(s) in the apparatus is sensed and a digital signal representative thereof is added to the first value to represent heat generated in the conductors. Values are determined for both heat transferred from the conductors and heat transferred into the apparatus core based on the difference in the first and second values. The value representing heat transferred from the conductor is subtracted from the first value to give a new first value and the value representing heat transferred to the core is added to the second value.

Abstract: A method of and apparatus for protecting three phase electrical equipment where a signal is derived for each phase representing current in each phase, and the three signals combined to form an analog composite signal. The analog composite signal is converted to digital form. One of the analog signals is used to relate the values of the digital signals to respective phases. The largest value is used to determine heating in the equipent and from the heating the temperature of the equipment is derived. The temperature is compared to critical temperature levels and when a critical level is exceeded, the supply of power to the equipment is interrupted. The digital signals representing the current peaks in each phase are used to determine phase loss and phase unbalance, and the power supply is interrupted when there is a phase loss or when the unbalance exceeds a predetermined critical level.

Abstract: Apparatus and method for protecting a squirrel cage induction motor against excessive temperatures which can occur in the rotor, provides a thermal model of the rotor. The thermal model has three regions A, B and C representing the starting portion of the rotor conductor, and the rotor core. The rotor is frequency sensitive and currents of higher frequency induced in the conductors tend to be in region A, and currents of lower frequency tend to be in region B. Equipment connected to the stator winding determines values for the positive and negative sequence components of the supply for the fundamental frequency and preferably also for the second and third harmonics present, and provides signals representing these values. Heat calculation circuitry receives the values for all the positive and negative sequence components and a signal representing speed, determines the currents that would be induced in regions A, B and C, and derives equivalent currents which would produce the same heat.

Abstract: A circuit for monitoring the temperature of the contacts of an electrical switching device (such as a circuit breaker), comprises an electrical analogue circuit of the thermal characteristics of the switching device. The analogue circuit comprises electrical resistance and capacitance, interconnected so as to form an electrical model of the heat flow paths and capacitances respectively, in the switching device. The preferred embodiment also provides a temperature sensing device to introduce to the analogue circuit a voltage signal the magnitude of which is proportional to the exterior temperature of the switching device. The analogue circuit is connected in a series circuit relationship with a current controller. The current controller controls the amount of current flowing through the analogue circuit. The current controller is in turn controlled by a voltage control signal proportional to the amount of current being carried by the switching device.

Abstract: A circuit for monitoring semiconductor junction temperature includes means for obtaining a signal representing the current flowing through the semiconductor, a current controller, an electrical analog of the thermal system of the semiconductor, a direct current supply, and a voltage level detector. The current controller is connected in circuit with the analog and the combination to the d.c. supply. The current conducted by the controller via the analog is controlled by the signal such that the voltage across the analog indicates junction temperature and this voltage is monitored by means of the level detector.