The Influence of Magnetic Retaining Rings on AC Motors for Boosting Torque and Reducing Starting Current

There are applications where low-current inrush and/or high starting torque are specified for reducing disturbance on the electrical distribution network. There are also situations where it is required to successfully start large ac motors at reduced voltage conditions when only one substation transformer of a double-ended transformer is in service at petrochemical plants. The latter condition requires the motors to draw lower inrush current while producing high starting torque. These requirements can create unfavorable design characteristics where one or both requirements cannot be accomplished using standard techniques at the final design stage or motor test. The authors have accomplished the design and installation of magnetic material retaining rings on induction motors with significant improvement in inrush currents and starting torques. Inrush currents have been reduced from 600% to 508%, and starting torques have been increased up to 14%. Several case studies of using magnetic retaining rings on large induction motors are described in this paper for machines ranging from 6000 up to 14 000 hp and testing as low as 380% of inrush for machines with magnetic rings. The application of this concept is not limited to only ac induction motors. The same process can also be applied to synchronous motors. Very limited information is available on the benefits of applying magnetic retaining rings and what the consequences are for large ac motors. The application of magnetic retaining rings is a practical approach to solving inrush and torque problems at the manufacturing or design stage. The theory, test, concepts, and consequences along with electrical and mechanical considerations are described in this paper.