V-phase power and 3-phase motors
V-phase power and 3-phase motors
(OP)
Installation is a 50 Hp 3-phase motor driving a pump. Power source is 480V V-phase from two single phase transformers connected in an open-wye primary and open-delta secondary. Motor is delta connected. V-phase driving 3-phase motors is not uncommon in rural U.S. areas (i.e. farms) where true 3-phase isn't readily available. However, the literature I can find indicates that this configuration is OK for "small" three phase motors, but doesn't specify what is considered "small." Is 50 Hp on 480V still "small"? I've also seen comments that the open-delta transformer configuration is prone to unbalance issues. Can a balanced 3-phase motor load collapse the voltage phase relationships from the open-delta source in an unbalanced manner?





RE: V-phase power and 3-phase motors
RE: V-phase power and 3-phase motors
Also, the rise in the open delta leg tends to get worse as the difference between the 2 "real" legs grows, and in areas where there is a lot of 1 phase loads coming on and off, such as rural or residential areas, that unbalance can get somewhat severe. I have seen as much as a 30% unbalance in the open delta 3 phase circuit during the summer when a lot of 1 phase AC units are running in residential areas.
"Venditori de oleum-vipera non vigere excordis populi"
RE: V-phase power and 3-phase motors
To add to stevenal’s remarks, one reference is ANSI C84.1 Annex D, partially described at www.jolietequipment.com/voltage_unbalance.htm
Many submersible-pump and hermetic refrigeration-compressor motors are particularly sensitive/intolerant of voltage imbalance compared to NEMA T-frame motors—given their inherent [purposely engineered] high-current and flux densities.
Some utilities limit open-delta-secondary distribution transformers to 20hp or less.
Particularly for single-bank—single-motor installations, sometimes a phase-rolling exercise may improve imbalance. Example see §10 of ww