How to properly operate a three-phase motor using single-phase power

There are several methods to operating a three-phase motor using single-phase power to make what would be an otherwise expensive and arduous process a little easier.

10/10/2016


Figure 1: Comparison single-phase (top) and three-phase (bottom) power. Courtesy: EASASo, you told a neighbor you work with electrical equipment and now he thinks you can solve his problem because he or she bought a three-phase motor that can't run on single-phase power. Being asked to convert this motor already sounds like more trouble than it's worth. That's not quite true though. There are some methods to make the process easier. 

The phantom leg method

Three-phase power involves three symmetrical sine waves that are 120 electrical degrees out of phase with one another (see Figure 1). One method of converting single-phase power that worked well for decades was connecting two phases to the incoming 220 V single-phase power and create a "phantom leg" for the third phase by using capacitors to force an offset between the main and auxiliary windings. In this case, the offset is 90 electrical degrees.

For this method, the capacitors must be appropriately sized for the load. The current will be unbalanced if it isn't. Rather than the 120-deg phase shift depicted in the lower half of Figure 1, the incorrect pairing of the capacitor and load may result in a large deviation. The bigger the discrepancy, the lower the torque. 

Rotary phase converter method

Figure 2: Example of rotary phase converter. Courtesy: EASAAnother viable method is with a rotary phase converter (see Figure 2). A wood shop, for instance, might use a rotary phase converter to run several three-phase machines on a single-phase power input. One drawback is that the process can be very expensive during the entire time the rotary phase is converting, regardless of whether any machinery is being used. The current might be balanced when specific machinery is running, but if few machines are operating, or all of them are heavily loaded, the three-phase power—current and voltage—is drastically unbalanced.

"NEMA Stds. MG 1: Motors and Generators" calls for motors to operate from voltage that is balanced within 1%. If the 10x rule is applied (percent current unbalance can be as high as 10 times the percent voltage unbalance) to a motor operating with 1% voltage imbalance, the current imbalance could be 10%. This is beneficial because a majority of three-phase motors running on a system described above are operating between 15% and 50% current imbalance. Even with the NEMA MG 1 derating graph (see Figure 3), no motor should operate with such a large current imbalance. 

Figure 3: Medium motor derating factor due to unbalanced voltage. Courtesy: EASAVariable frequency drive method

A variable frequency drive (VFD) rectifies each pair of phases to dc and inverts the dc to the power for the three-phase output, which means a VFD can be used with single-phase input to operate a three-phase motor. Manufacturer support varies, and it is cautiously recommended to derate the drive by 1 divided by the square root of 3 (about 58%). Also note that the hp/kW VFD rating is there for convenience in sizing drives since they're rated by current. For example, a 10 hp (7.5 kW) motor would use a VFD rated for 15 hp (11 kW). It is strongly recommended for the user to work with the drive manufacturer in selecting and sizing the VFD for this use.

Compressors, machine shop and woodworking equipment and decorative fountains are good candidates for this method. Rather than buying an expensive single-phase motor, changing the controls and dealing with speed control and starting torque issues, it is a better idea to use a VFD to operate the existing motor from single-phase power. For many applications up to 5 hp (4 kW), a suitable VFD can be purchased for far less than the cost of rewinding a three-phase motor and providing the necessary controls to operate it.

Chuck Yung is a senior technical support specialist at the Electrical Apparatus Service Association (EASA). Courtesy: EASAThe additional benefits are that a three-phase motor is usually less expensive to buy, the controls do not require replacement or modification, and the VFD has the value-added bonus of providing speed control. Best of all, you don't have to ruin a weekend helping someone who does not fully understand just what it is you do.

Chuck Yung is a senior technical support specialist at the Electrical Apparatus Service Association (EASA). EASA is a CFE Media content partner. Edited by Chris Vavra, production editor, CFE Media, cvavra@cfemedia.com.

ONLINE extra

See additional articles from EASA linked below.



Virgilio , , 12/04/16 12:27 AM:

I've been using capacitors in running a three phase motors and I've found out that the efficiency of the motor is somewhat low. This article is very informative for me and really appreciate it. Thanks.
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
Safety standards and electrical test instruments; Product of the Year winners; Easy and safe electrical design
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Diagnostic functions for system safety; Specifying industrial enclosures; Effective decision support for a crisis
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Natural gas for tomorrow's fleets; Colleges and universities moving to CHP; Power and steam and frozen foods

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me