Smith Electric Motorworks ©  Legal

Appendix A:

"Wild-Leg Voltage"

Most rotary phase converters produce 3-wire Delta Power. The "wild leg" on a phase converter is usually defined by measuring the three voltages to ground (neutral) rather than line-to-line.


 In Figure 9, the L1 and L2 points on the triangle represent the single-phase line and T3 is the Generated Leg. The ground symbol shows the single-phase neutral (N).



The dotted lines show the distance through the rotary windings from each leg to Neutral (N).


When you look at the path the T3 voltage takes to get to neutral, it is obvious that the "wild leg" is not really wild at all. It is simply electrically farther from T3 to the neutral point, and the distance through the winding simply increases the voltage reading by a factor of 1.73.


Remember that the system neutral is never connected to the rotary windings, and the 3-phase load does not recognize line-to-Neutral voltages.


The T3-to-Neutral voltage is useful for identifying T3 among a bundle of wires, but do not use it to determine whether voltages are balanced. Imbalance is only gauged by line-to-line voltage measurements.

Figure 9--Delta rotary

phase relationships, Line to Neutral