Specific design considerations for AC induction motors connected to adjustable frequency drives

There has been an increased usage of adjustable frequency drives (AFD) coupled with induction motors due to the demand for higher operating efficiency levels. Due to the starting requirements, motor manufacturers are forced to employ high resistivity rotor bars which lead to higher losses and lower motor efficiency. This paper will show that AFD offer the advantage of decreasing the energy consumption by eliminating the constraints of starting requirements. Short rise time voltage pulses produced by AFD may adversely affect the stator winding insulation and accelerate its aging process. The paper will discuss how motor manufacturers have addressed this problem by introducing surge resistant insulation materials, turn insulation and reinforced ground insulation. The voltage waveform harmonics of an AFD could also lead to higher motor temperature rise. Therefore, additional thermal consideration should be given. The paper will review the new IEC standards which define the stator winding insulation requirement when the induction motor operates with AFD. Another aspect discussed in the paper, related to the design of an induction motor coupled with an AFD, is how to minimize the magnetic noise that may be generated if higher frequency voltage harmonics are present in the AFD.