Occasionally, a new water district pumping station will be installed in a location that is not served by 3-phase power.
The options to run the pumps on 3-phase include (but are not limited to):
- Pay the electric utility a fee (often prohibitive) to bring 3-phase power to the site.
- Generate the 3-phase power on-site.
To generate 3-phase power on-site for a pump motor, a VFD is often a very economical option. However, a VFD that is powered by single-phase must generally be derated to account for the increased current load on certain components within the front end of the VFD. A common derating rule-of-thumb is to derate a single-phase driven VFD to 57.8% of its 3-Phase rating, or in other words, up-size the VFD to at least 173% of motor rating.
When powered by single-phase, Rockwell recommends that the PowerFlex 400 VFDs be conservatively derated to 35%; and the PowerFlex 523 VFDs conservatively derated to 50%.
For example, if a pump and motor are rated for 7.5 HP 3-Phase, then a PowerFlex 400 rated for at least 7.5/0.35=21.4 HP is required, or a PowerFlex 523 rated for at least 7.5/0.50=15 HP is required.
Keep in mind that it is always possible to over-size a VFD for its application, regardless of whether the VFD is utilized within a 3-phase generation scenario. However, an engineering rule-of-thumb is to never exceed the VFD rating by more than a factor of 10; and of course economics would also dictate avoiding such gross oversizing.
The use of an AC Line Reactor is also beneficial — especially in single-phase-powered situations. Because the power-up inrush current is more concentrated due to the missing phases, the inrush current-dampening action due to the reactor’s impedance will help to protect the input diodes and DC capacitors.
In such single-phase-powered situations, the AC Line Reactor should be sized to 173% of the motor 3-phase rating. For details, a good reference is available HERE at AutomationDirect.com.
Are you interested in using a VFD to create 3-phase power in a new pump station application? Or perhaps you already have a station with an add-a-phase device that you would like to eliminate? If so, give us a call. We’ll be glad to discuss the VFD solution with you in detail.
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