Torque capability of high phase induction machines with sinusoidal and trapezoidal airgap field under steady state

The paper assesses the torque capability of high phase induction machines taking a conventional three phase machine as the basis for comparison; analytical expressions for torque as a function of phase number for machines with sinusoidal and trapezoidal airgap field are derived and discussed. A trapezoidal field is assumed to be produced through the imposition of a third harmonic current in the stator current besides the fundamental. The torque capability comparison is performed keeping the airgap peak induction and the linear stator current constant, thus resulting about the same amount of magnetic and winding losses for variable phase number. It is shown that the main factors influencing the torque capability of machines with sinusoidal airgap field are the winding characteristics - including number of slots per phase and pole - and the winding pitch. Higher torques are associated with windings without pitch shortening and with low values of slots per phase and pole. It is also shown that machines with trapezoidal field can have higher torque capability than sinusoidal field machines, being the greatest torque improvement associated with low values of slots per pole and phase. Under the assumptions made, torque improvements from 10% to 15% can be obtained. The paper also shows that for sinusoidal field machines, the maximal torque practically obtained with seven phases, while for trapezoidal field the maximal torque is obtained with eleven phases.