An open delta power system is a power configuration using only two transformers instead of three to generate 3 phase power. In essence, one side of the secondary delta is physically missing but a 3-phase measurement is still possible phase to phase. This power configuration is not as efficient as a 3-transformer bank system but saves the consumer and utility company the cost of using one less transformer and is generally found at locations where the majority of the load uses single phase. As a result, an open-delta power system is not a true 3-phase balanced system and thus provides an unbalanced power supply. There are two key areas that are affected by the installation of a VFD on an open-delta power system:

• The load imbalance impact on the components of the VFD can lead to input diode and/or DC bus capacitor failures.
• Mitigation of harmonics produced by the VFD.

Refer to document: PR.PS.01 for more information. It is important for a drive to be properly sized when used on an open-delta power system. The electrical diagrams below show a basic representation of the different types of an open delta power system.

Ungrounded Open Delta	Corner-Grounded Open Delta	High-Leg Open Delta

Most* Yaskawa's VFDs are compatible with an open delta power system with an appropriate derate. It is best to speak with an outside sales rep for additional information. Alternatively, one of the methods below can be used when sizing a drive on an open-delta power system.

• Choose the correct drive model as if single phase input were present.
  • Refer to the appropriate drive Technical Manual for sizing.
• Choose the next larger frame size and add a 5% line reactor.

* Z1000, U1000, Z1000U, and U1000 iQpump drives are not compatible with an open delta power system as these require a balanced, 3 phase supply.

Note: If an EMC filter is being used, it must be removed from the main circuitry or internally disabled (GA500 drives).