Convert 3 phase motor to single phase with capacitors
Answers
So, 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
Motion Industries
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.
