Analytical expression of pulsating torque harmonics due to PWM drives

This paper proposes an analytical approach to reconstruct the air gap torque waveform of an electrical motor from its stator voltages and currents. This approach is consistent with practical implementation requirements of large variable frequency drives, where a direct measurement of the air gap torque is generally not possible. The method is based on an analytical reconstruction of the air gap torque in stationary and orthogonal coordinates. The proposed approach can be used as a design tool to accurately predict the frequencies of torque harmonics along with their magnitudes and phases. The results can be used to predict the shaft's torsional behavior and they can help assessing shear stresses of the shaft, thus providing a better understanding of the shaft's design life time. Moreover, the proposed approach is a tool for root cause analysis of drive failures. By reconstructing the air gap torque one can accurately prove whether or not one of the shaft's eigenmodes was excited. The proposed technique is applied to a 35 MW drive system based on the parallel connection of four back-to-back neutral point clamped converters. Through simulations, the pulse-width modulated voltage source inverters are analyzed for different switching frequencies and over a wide operating range. The results confirm the accuracy of the proposed method.