An optimal solution for operating a three-phase variable frequency drive from a single-phase AC source

When using a three-phase Variable Frequency Drive (VFD) powered from a single-phase AC source, output power of the drive has to be limited to prevent higher component stresses in the drive and in the input power system. To overcome this issue, many VFD manufacturers de-rate the drive. Typically, the output frequency of the drive is limited, based on the DC bus voltage ripple so that the DC bus capacitors are not overstressed. Though the traditional method reduces the stress in the DC capacitors, it does not take into account the stresses in other components of the VFD, especially the input diodes and terminal blocks. In this paper, a new method of protecting the drive using the motor q-axis current is proposed. The motor q-axis current, is shown to have information pertaining to the stresses in various parts of the VFD including the input diodes, input terminal block, and the DC bus capacitors. The output power is proposed to 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. To prove the concept, the traditional DC bus voltage ripple based fold-back method is experimentally compared with the proposed method using a low power VFD fed from a single-phase AC source.