Chuck Yung
EASA Senior Technical Support Specialist
We all have that occasional customer who got a “deal” at an auction: a compressor, or lathe, or wood-working equipment, only to discover when he started to install it that this equipment has a three-phase motor and only single-phase power is available. Maybe it’s your neighbor or a friend from church. In any case, you know that you are about to be called upon to “convert” that piece of equipment, and you probably realize that it’s going to cost you more than you can charge.
For decades, there was a procedure that worked OK, connecting two phases to the incoming 220V single-phase power and using capacitors from one energized lead to create a “phantom leg” for the third lead. As Figure 1 illustrates, the three-phase power involves three sine waves that are 120 electrical degrees out of phase with each other, which is symmetrical. The single-phase power, however, relies on capacitors to force an offset between the main and auxiliary windings. The offset of 90 electrical degrees is considerably different than the 120-degree difference between phases for three-phase power. If you’ve ever tried this, you know that the run capacitors have to be sized appropriately for the load, or the current will be horrendously unbalanced. Rather than the 120-degree phase shift depicted, incorrect pairing of capacitor and load may result in large deviation from the graphic. The further we are from three 120-degree phases, the lower the torque. The worse the discrepancy, the lower the torque.
Use of rotary phase converter
Likewise, the rotary phase converter (see Figure 2) could be used for similar purposes. A wood shop, for instance, might use a rotary phase converter to convert single-phase power input to run several three-phase machines. The first drawback for this is that the capacitors cannot be sized for all the possible loads. So the customer might have balanced current when specific machinery is running, but if he is operating fewer machines, or loading all of them heavily, the three-phase power is drastically unbalanced. A second drawback is that someone is paying the power company the entire time the rotary phase converter is running, regardless of whether any machinery is being used. I once designed and built a test panel using this approach, with a rotary phase converter feeding through a three-phase transformer for a service center that had no three-phase power available. (I don’t recommend it.) The current unbalance varied widely, depending on the power rating of the motor being run.
NEMA MG1 calls for motors to operate from voltage that is balanced within 1%. If we generously apply the 10x rule (percent current unbalance can be as high as 10 times the percent voltage unbalance), the current unbalance might exceed 10%. It is not an exaggeration to state that the majority of three-phase motors operating from one of the systems described above are operating between 15% and 50% current unbalance. Even if we applied the often misunderstood NEMA MG1 derating graph, a motor operating with such a large current unbalance should not be used.
A solution to dilemma
Fortunately, there is a better way to solve this dilemma. Because a Variable Frequency Drive (VFD) rectifies each pair of phases to DC, then inverts the DC power for the three-phase output, a VFD can be used with single-phase input power to operate a three-phase motor. Manufacturer support varies, with a cautious recommendation to derate the drive by (58%, basically). We should recognize, too, that the hp/KW rating of a VFD is there for convenience, since the drive is actually rated by current. For example, a 10 hp (7.5 kW) motor would use a VFD rated for 15 hp (11 kW).
At least two manufacturers state that their drives can be used up to 5 hp (4 kW) without derating the drive. Anecdotally, there are people using a VFD to run up to 30 hp (22 kW) motors through a VFD from single-phase power. While I have heard of at least one case where this was done with a 40 hp (30 kW) motor, I strongly caution you to work with the drive manufacturer in selecting and sizing the VFD for this use.
Good candidates for this include compressors, machine shop or woodworking equipment and decorative fountains. Rather than buying an expensive single-phase motor, changing the controls, etc. and dealing with speed control and starting torque issues, use a VFD to operate the existing motor (which, hopefully, came with the equipment and is therefore correctly sized) 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.
The additional benefits to the customer are that a three-phase motor is usually less expensive to purchase and more efficient than a comparably sized single-phase motor; the controls do not require replacement / modification; and the VFD has the value-added bonus of providing speed control.
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