Spotting drives for large motors

High power line-fed, constant speed motors sometimes must be operated at very low speed for controlled positioning of the attached load equipment. This process is referred to as ´spotting´ or ´inching´. Examples include cement grinding mills (both raw and finish) and metal processing rolls. These motors are usually medium voltage (>2300 V). Two methods of spotting motors have been used in the past: (1) bumping the motor with the medium voltage motor starter, and (2) an electromechanical inverter. This paper describes the application of a low voltage (460 V) PWM adjustable speed drive to medium voltage motors for spotting purposes. The drive is capable of rotating a synchronous or induction motor at low speed under manual control to position the machine attached to the motor to a desired position