Optimum pole configuration of AC induction motors used on adjustable frequency power supplies

For a "fixed frequency" (line powered) AC induction motor, a simple relationship exists between the number of magnetic poles and the synchronous speed. Specific choices of pole number and the common line frequencies of 50 and 60 Hz result in discrete available speeds for these motors. When matching AC induction motors to variable speed applications using adjustable frequency power supplies, there is a temptation to assume that 50 or 60 Hz will be the "base frequency", and therefore the "base speeds" achievable are those provided by varying the number of motor poles. That assumption not only limits the choices of base speeds, but also results in suboptimal performance from the motor. This paper explains the physics of why the optimal choice of the number of motor poles is more a function of the motor torque (size), rather than the motor speed, when considering adjustable frequency applications. In fact, the exclusive use of a four pole configuration results in optimal performance for a significant range of ratings. The parameters which can be optimized via the correct choice of pole configuration include-torque density, speed range, efficiency, power factor, overload capability, and acoustic noise.