Spot Don't Bump: The Use of Pulsewidth Modulated Adjustable Speed Spotting Drives is More Effective than Bumping Large Motors

High-power, line-fed, constant-speed motors must sometimes 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 (more than 2,300 V). Two methods of spotting motors have been used in the past: bumping the motor with the medium-voltage motor starter and spotting with an electromechanical inverter. This article describes the application of a low voltage (460-V output) pulsewidth modulated (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.