Getting the Most from a Variable Frequency Drive: The Optimal Way to Improve Lifetime and Reliability

When using a three-phase variable frequency drive (VFD) powered from a single-phase ac source, the drive's output power has to be limited to prevent higher component stresses in it and in the input power system. To address this issue, many VFD manufacturers derate the drive. Typically, based on the dc-bus-voltage ripple, the output frequency of the drive is limited so that the capacitors are not overstressed. This method does not take into account the stresses in other VFD components, especially the input diodes and terminal blocks. In this article, a new method of protecting the drive using the motor q-axis current is proposed. The motor q-axis current has information that is relevant to the stresses in various parts of the VFD, including the input diodes, input terminal block, and the dc bus capacitors. The output power can be limited by reducing the output frequency based on the average and ripple amplitude of the q-axis current instead of the dc-bus-voltage ripple. The traditional dc-bus-voltage-ripple-based fold-back method is experimentally compared with the proposed method using a VFD fed from a single-phase ac source to prove the concept.