Jim Mimlitz

Jim Mimlitz, NRI

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.

 

Booster Pump Application: Station Fed by 480VAC Single-Phase Power. Note the Single Transformer on the Power Pole. Phase Generation Performed by Station’s VFD’s Shown in Next Photo. (Hamilton, IL)

 

Booster Pump Application: 3-Phase 480VAC Pump Motors Driven by Allen Bradley PowerFlex Variable Frequency Drives, Which Generate 480VAC Three-Phase from 480VAC Single-Phase Inputs. (Hamilton, IL)

 

Single-Phase VFD Pump Application. Note The Power Input Has Two Wires, And The Motor Output Has Three Wires.

A Single-Phase VFD HVAC Compressor Pump Application: Note that the 240VAC Power Input (Left) Has Two Wires L1,L3 (Single-Phase), and the 240VAC Motor Output (Right) Has Three Wires L1, L2, L3 (3-Phase).

 

At Navionics Research Inc., our most commonly-used Rockwell/Allen-Bradley VFDs include the PowerFlex 400 (3-350 HP) and PowerFlex 523 (0.5-30 HP) families.

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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Jim Mimlitz

About Jim Mimlitz

Licensed Professional Electrical Engineer @ Navionics Research. Specialties: Radio-Telemetry / Controls / SCADA System Integration for Water Utilities, Flow Metering, Variable Frequency Drives, Motorized Valves, Electronic Circuit Design, Software Development.

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