The goerges phenomenon-induction motors with unbalanced rotor impedances

The Goerges phenomenon is the operation of a 3-phase wound-rotor induction motor at half of synchronous speed, and it is obtained by unbalancing the rotor circuit. If the motor is started with one rotor phase opened or with rotor impedances greatly unbalanced, it will accelerate to half speed and operate stably as a motor or generator. Several investigators have made tests and analyses to find the characteristics of a motor operated in this way. In the present work a physical explanation and an equivalent circuit are given. Results of tests and calculations for a 5-horsepower motor are compared and found to be in good agreement. The unbalanced rotor impedances cause unbalanced rotor currents of slip frequency, and balanced stator currents of two frequencies, the fundamental and a subharmonic, the frequency of which depends on the speed. The motor may be operated stably at half or full speed. The torque at half speed is greater than at full speed, and may be almost as great as the rated torque for a balanced rotor. It may be desirable to operate an induction motor at half speed intermittently or as a temporary measure. Disadvantages of half speed operation are the low power factor, low pull-out torque, the subfrequency line currents, and motor vibrations.