Open-Delta Systems Affect Variable Frequency Drives

To avoid premature drive failure, proper precautions must be taken when installing VFDs on open-delta supplies.

Written by:
Dan Peters, Yaskawa America, Inc.

In Figure 3, the voltage waveform for each phase has been added and the time per division increased. This allows users to see the current pulse relationship to the voltage sine wave. Even though current draw is not sinusoidal, the current pulses phase to phase are much closer to equal. Phase A and Phase B have eliminated the zero conduction time, but it is still present during Phase C. Overall, the voltage waveform in Figure 3 looks clean.

 

Figure 3. Figure 2's voltage waveforms after a DC link choke was placed in the circuit

 

The main reason for installing an open-delta supply is to reduce the cost of power installation. It is always preferable to have a supply with balanced impedance phase to phase whether the intended load is a VFD or an across-the-line motor.
To ensure a long VFD service life, understanding the type of supply power being fed to the VFD and taking appropriate countermeasures are important.

While this case study is too small to draw broad conclusions about all open-delta installations, in this particular topology, the effects of the unbalanced impedance phase to phase on the VFD are seen. The DC link choke still has a positive effect in mitigating harmonics on this open-delta system and additionally benefit the VFD by bringing the load phase to phase closer to balance.

End users, installers and salespeople should be aware of the power supply topology before selecting any equipment to be supplied power on an open-delta supply. The imbalance may require that a VFD or across-the-line motor be derated to handle the phase-to-phase imbalance. Consult the manufacturer for recommendations.



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