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Open Delta Transformers
- Thread starter Will96
- Start date
Are you sure that you have an open delta?
Typically, a wild leg is used to refer to a 3-phase delta system(typically 230-240V phase-to-phase) with one winding center-tapped and grounded. The potential between ground and the two phases of the tapped coil are 1/2 the phase-to-phase voltage, while the potential between ground and the third phase(the wild leg) is higher by a factor of 1.73.
Typically, a wild leg is used to refer to a 3-phase delta system(typically 230-240V phase-to-phase) with one winding center-tapped and grounded. The potential between ground and the two phases of the tapped coil are 1/2 the phase-to-phase voltage, while the potential between ground and the third phase(the wild leg) is higher by a factor of 1.73.
Hello all,
DanDel is right, in normal operating conditions, the higher phase-to-ground voltage is 1.73 time the lower phase-to-ground voltage for a transformer configuration with mid-tap grounded open delta. See also NEC fig 250-3, art 215-8, 230-56 and 384-3.
In a not normal operating condition, continuous overvoltage on the red-leg delta (wild leg) occasional could occur particularly on unstable open delta system. Some of those possible overvoltages are ungrounded connection, ferroresonance, harmonics, unbalance voltage, etc.
Unfortunately, there is not a cookbook to make a specific recommendation to determine the abnormal conditions. Consider checking first the obvious, transformer connection, balance voltage, regulation, neutral/ground connections, harmonic content, etc.
Good luck
DanDel is right, in normal operating conditions, the higher phase-to-ground voltage is 1.73 time the lower phase-to-ground voltage for a transformer configuration with mid-tap grounded open delta. See also NEC fig 250-3, art 215-8, 230-56 and 384-3.
In a not normal operating condition, continuous overvoltage on the red-leg delta (wild leg) occasional could occur particularly on unstable open delta system. Some of those possible overvoltages are ungrounded connection, ferroresonance, harmonics, unbalance voltage, etc.
Unfortunately, there is not a cookbook to make a specific recommendation to determine the abnormal conditions. Consider checking first the obvious, transformer connection, balance voltage, regulation, neutral/ground connections, harmonic content, etc.
Good luck
Will—Look over the transformer-secondary configurations at It sounds like you are describing a 3ø 3-wire system with no intentional ground connection to the serving transformer-secondary terminals. As Dan and cuky have suggested for a grounded four-wire delta system, you should not read any more than about 220 volts to ground—or not exceeding 255V for a “corner-grounded” system. If indeed no terminals on the secondary windings are intentionally connected to ground, it may be possible to read higher-than-normal ø-ø voltage to ground. This may be a case where there is primary-to-secondary leakage from the transformer-set primary side, which is almost always fed from a higher-voltage source. Also, a resonant or arcing condition in the 240V system could cause readings greater than nominal ø-ø voltage. As you might imagine, this can be stressful to electrical insulation on anything connected—with motors and transformers being the most easily damaged.
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