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)
A Single-Phase VFD HVAC Scroll 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 from AutomationDirect.com, or HERE from MTE Corporation.
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,
I have an older AB 1336 impact (1336-B075-AE-EN-HA2-L6) that I would like to use to convert 240V single phase to 240V three phase.
Is this drive capable of this conversion?
Thank you,
Jeff Rutherford
Yes, Jeff I think it should. Remember to de-rate the drive for your application (see my notes above regarding de-rating).
What is the HP of your desired 3-phase load? What is the rated HP of your 1336 VFD?
Hi Jim, do you have any tips for doing this for a submersible well pump motor? Like if I plan to install a 2 HP motor, would you recommend that the motor be single phase or three phase? And if you recommend three phase, then what size should the VFD be, if it’s a single phase input?
Hello Rebecca,
When it comes to motors, I’m almost always in favor of 3-phase over single-phase.
For many VFD’s 3 HP and below, there will not be a need to de-rate when powering with single-phase. So you may be able to just use a 3 HP VFD for your 2 HP pump. Of course, for the VFD that you select, I recommend that you check for de-rating requirements with the VFD manufacturer.
Dear Jim
very interesting
thanks a lot for this article
i want to run 5 HP submersible pump (which is either star connected 3 phase 440 volt or delta connected 3 phase 240 Volt )with single phase power available 240 Volt 50 Hz in my farm
which VFD i can use
Requesting to please suggest
Regards
Mridul Sharma
Hello Mridul,
Since you have 240V Single-Phase available at your farm, I’d probably go with a 15HP 240VAC VFD, which would be sufficiently de-rated to run your 5 HP pump at 240VAC/3-Phase.
A complete Allen Bradley VFD package, with Rotary Disconnect / Circuit Breaker / and Line Reactor is as follows: Part No. 23C-X022A103NNMANN-LR
Although of course other makes/models are also available.