Excerpt from-- "AC Motor Drive Selection for Stage Effects"
originally printed in USITT's TD&T (Spring 2000)
The need to control the output of a motor is clear to the technical designer: an effect will need to run backward as well as forward, a braking force may need to be applied to slow or halt movement, the director may want to vary the timing of an effect, acceleration and deceleration rates must be chosen so as not to jar performers or scenery, and in the case of stock machinery and equipment, the load and required speed may vary from use to use. To satisfy these needs, the theater has looked to the technology available to industrial applications at large, and motor drives are the devices that allow this control. Specifically, a motor drive controls an electric motor's speed and torque output and also provides the interface between either an operator or some other mechanical and/or electronic control device. The following presents much of the necessary background for technical designers who need to choose motor drives suitable for three-phase AC induction motors to be used in theatrical effects on the stage. While this information is certainly applicable to many of the types of machines found as permanently installed stage equipment, it is primarily intended in aiding in the design of production specific scenic solutions or in the selection of stock machinery.
DC permanent magnet motors have been the historical standard in stage machinery where variable speed control is required. This is because a reliable technology to vary their speed, the Silicon Controlled Rectifier (SCR) drive, has been available for several decades. Although multi-phase AC induction motors are generally less expensive, less complicated, and more reliable machines than DC motors of equivalent capacities, until recently their speed could not be controlled as simply and economically. As a result AC motors have been found primarily in constant speed applications, and thus rarely used to produce scenic effects. Fortunately for the theater (and other industries), the last decade has shown a rapid growth and proliferation of the technology necessary to achieve control of AC motors that is comparable to their DC counterparts. Variable frequency drives (VFDs) for AC motors offer a flexible and affordable alternative to SCR based DC drives.