The temperature of the stator winding of an induction motor is monitored by measuring
the resistance of the winding while the motor is in operation. By computer simulation
of an induction motor controlled by thyristor phase control, using back to back
connected thyristors in each phase of a 3 phase system. it is shown that the motor stator
winding resistance can be deduced from the DC components of the phase voltage and
current resulting from intentionally unbalancing the non-conducting periods (notches)
in the voltage waveform. The DC voltage and current components are measured by
integrating the unbalanced phase voltage and current over an exact number of electrical
cycles. The stator winding resistance is determined by dividing the DC voltage
component with the DC current component. A generalised d-q axes mathematical
model of the induction motor system has been developed for the computer simulation.
The practical implementation of the method using a phase controlled microprocessor
motor controller and support circuitry is presented.
A motor protection algorithm calculates the stator winding temperature from the
measured stator resistance. displays the winding temperature and provides a motor
protection function by comparing the calculated winding temperature with the
temperature limit of the motor and acting accordingly. A calibration procedure before
installation measures the motor stator winding resistance at cold and reads the motor's
cold temperature. full load current and insulation class. Experimental results are
presented and the features and the limitations of the method are discussed.