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.