Application and advantages of PAM multi speed motors in petroleum and chemical industries

For years, engineers have been looking for economical methods to reduce the initial and operating cost of equipment including motors and controls. Not many options exist that will save money and operating cost using a reliable service of a squirrel cage induction motor. Engineers have developed adjustable speed drives (ASD) used with large induction motors in applications such as pumps, fans, mixers, compressors etc. where the power consumption is reduced at lower operating speeds. However, the initial cost of ASD may have to be evaluated carefully. Meanwhile motor designers have been attempting to design induction motors to run at two different speeds. Many large two speed single winding motors have been installed in the field with two operating speeds without the need of an ASD. However, the speed ratio obtained in such motors is limited to 2:1; for example, 3600/1800 RPM, 1800/900 RPM, 1200/600 RPM, etc. These motors have only one winding and are called consequent pole machines. Other two speed machines exist where two independent windings, one for each desired speed are wound into the stator to accomplish different speeds other than 2:1 ratio of operation. The latter type of motors requires more space in stator to accommodate the dual windings making the machine larger, less efficient and more expensive. Pole Amplitude Modulated (PAM) motor is a two speed single winding machine that can accomplish two speeds at different speed ratio such as 1800/1200 RPM, 1200/900 RPM, 900/720 RPM, 900/600 RPM etc. The PAM motors can also be designed for 3 or 4 speeds. This paper will describe the advantages of using multi-speed PAM motors in applications where power required by the load drops significantly when operated at lower speeds, thus saving operating cost. It will also compare and discuss the total losses of the system using an induction motor driven by ASD versus a single PAM motor for typical petrochemical applications. An understanding of the theory and operating aspects of Pole Amplitude Modulation are explained.