Phase Converters: Running a Three-phase Pump on Single-phase
November 29, 2012
Consider this scenario: You go to a flea market, and spot an electric motor that looks similar to the one on the wash-down pump in your barn that is starting to make noise. You know that a new motor would cost way more than what this guy is asking for his so-called slightly-used-but-not-abused motor, so you buy it. You get it home, and in the process of installing it, you find out that it is a three-phase motor, and you only have single-phase power on the farm. You call the utility company, and are told that it will be glad to bring in three-phase power from the road to your barn for about $15,000, but you will have to wait six months because it is a little backed up. What to do?
This is the perfect place to use a phase converter, a device that converts single-phase power to three-phase power. Phase converters come in various shapes and sizes, costing from a few hundred dollars to many thousands, depending on the HP rating and type. The phase converter will artificially generate the third leg of a three-phase system from the two legs of a single-phase system.
There are three basic types – static phase converters, rotary phase converters and solid-state phase converters. Plus, there is a forth option that can be used for motor applications – a variable-frequency drive (VFD).
Static Phase ConverterThe static units consist of a several capacitors, and often a transformer, sized to the amp draw of your pump motor, to generate the third leg of three-phase power. They have been around for a long time, and are the least expensive of the three types. Their advantage is price, and their disadvantage is that they do not provide as closely balanced power as do the more expensive units.
Rotary Phase ConverterThe next least expensive units look like an electric motor with a large electrical box attached to the side that houses the capacitors. The capacitors generate the third-phase, and the motor, called an idler motor, cleans up and balances the power (see Figure 2, above). With a rotary phase converter, you can run more than one pump at a time, as long as you do not exceed the maximum capacity of the unit. The power is balanced better than with a static phase converter.
Solid-state Phase ConverterFor applications that require stricter control on the power, solid-state phase converters are the best (see Figure 3, above). They have the advantage of providing very clean power with no harmonics or distortion. Solid-state phase converters are very efficient, and operate in conjunction with the load motor at nearly unity power factor. They can power multiple loads, and can be located remotely from the load. Some pump installers use them in situations where there already is three-phase power available, but it is so far out of balance, it will not safely run a pump. In this case, a solid-state phase converter is hooked to two legs of the three-phase power, and the pump gets perfect three-phase power from the converter.
VFDs as Phase ConvertersAlthough variable-frequency drives (VFDs) are designed primarily to control the speed of AC motors, they can be adapted to function as phase converters (see Figure 4, right). They have the added advantage of allowing you to control the speed of the pump to match the load.
Problems can arise when using VFDs as phase converters if:
1. They are used to power loads other than motors – not an issue here.
2. There are multiple loads on the VFD – one pump per VFD.
3. The motor needs to provide braking action – not a problem for pump applications.
4. The distance between the motor and the VFD is appreciable – can be an issue with submersible applications, but with proper filtering, usually not a deal killer.
5. The current drawn by the VFD is large compared to the rating of the utility step-down transformer – usually not an issue.
When VFDs are used as phase converters, the drive must be de-rated. Typically, it is necessary to double the size of the drive to the load.
A precaution: Regardless of which type of phase converter you choose – static, rotary, solid-state or VFD – make sure the unit has enough capacity to start your motor. Remember that the starting current drawn by an electric induction motor can be six or more times full load amps (FLA). Some 10-HP phase converters only will power a 10-HP motor if it is started without a load, and it may take a 15-HP or even a 20-HP unit to start your motor under load. Consult the pump, motor and/or phase converter manufacturer for information on starting recommendations.
The Internet has a ton of technical and supplier information on phase converters. Also, check with your local pump distributor. It may have a source of phase converters, and offers the advantage of being there for you if there is a problem.
Next month, we will continue our series on the electrical side of pumped water systems with a look at reduced voltage starters. ’Til then …. ND
Road Trip! Participating in the 2012 RodFather Goodguys RoadIn mid-September, intrepid National Driller columnist Bob Pelikan left behind the concerns of the industry for a time to participate in the 2012 RodFather Goodguys Road Tour. The more-than-two-week road trip took him cross country in a pilgrimage to the Indianapolis Speedway for the Goodguys 2nd WIX Filters Speedway Nationals. It’s an event, which, according to its producers, “fills the infield of Indianapolis Motor Speedway with thousands of customized, candy-colored classics and hot rods.” A sponsored excursion to the event, the RodFather Goodguys Road Tour was organized by the Goodguys Rod & Custom Association, the world’s largest hot-rodding association with more than 70,000 active members worldwide.
Pelikan and his 1931 Buick Victoria set out from the Bay area of California, joining other hot-rodders from around the nation at the Bonneville Salt Flats, Utah, for the official start of the journey, where en masse, the 100-plus vehicles hit the road. They set out, traversing the midsection of the country, and touring places like Ft. Collins, Colo.; Des Moines, Iowa; and the Chicagoland area before arriving at their destination. After clocking mileage on the Speedway, they returned to the Golden State via a southerly route through St. Louis, Tulsa, Okla.; and Albuquerque, N.M. Overall, the tour encompassed 13 states on both back roads and interstates.
Their trip is reminiscent of songs on hitting the highway – in fact, the group did stop in Winslow, Ariz. – and it combined history, popular culture and, of course, cars and the storied Route 66. On the way to and from Indianapolis Motor Speedway, the caravan made stops at various points of interest – hot rod shops, historical museums, private car collections, and much, much more. Pelikan chronicled his journey on his blog, and a sense of camaraderie, love of classic cars, and general fun is conveyed in the online journal.
If you’re a car enthusiast, enjoy traveling, or just would like to see more photos and read further about his adventures on the road, visit Bob Pelikan’s blog at http://www.bobpelikan.com/category/indy-2012/.