Special Multi-Speed Induction Motors for Energy Savings using Pole-Modulation

For the past several years substantial use has been made of a class of induction motors developed for application in driving loads which operate at multiple speed points to gain efficiency. While adjustable frequency power supplies may be more attractive for certain loads which operate over a very wide range of speeds, the multiple-speed machines are economically more viable in many cases. In addition, the technology base for the design and control of multiple speed machines is opening additional application areas in the machine tool industry. This paper will describe the most recent technology of multiple-speed motors, including the essential motor and control features. These include both the physical aspects of the system incorporating the motor, as well as the economic tradeoffs which must be made to establish whether multiple-speed motors are appropriate for the heavy manufacturing industries. The technology of multiple-speed machines is divided into two principal types, including pole-changing and pole-amplitude-modulation (PAM) motors. In those cases where the efficient load speeds are multiples of two, a pole changing motor may be appropriate, while PAM machines are most favorable for the efficient operating speeds which are closer together. The newest three-speed, single-winding induction motor being developed will be examined to illustrate the principles of decision making between the various candidate steel industry drives.