Operation and starting of PAM motors using vacuum contactors

PAM (Pole Amplitude Modulation) motor is a single-winding two speed motor, used for ID fans. Its cost is much less than that of a ASD motor. This paper is prompted by a rigorous study that was undertaken to replace an existing, oil filled, leaky, transfer switch type two speed motor starter, with an adjunct of tens of hard wired timers in a utility installation. The paper describes the fundamental operation of a PAM motor, different torque speed and thermal withstand characteristics at two speeds and design of an alternate starter using conventional vacuum contactors with modern multi-function microprocessor relays (MMPR) to provide effective protection during normal two-speed operation and starting.