V-phase power and 3-phase motors 1

[MNEE]

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?

 

[stevenal]

I've seen 50HP given as the limit. I also understand that the voltage across the open delta tends to rise above the other phase to phase voltages. The result is negative sequence current and motor heating. Electrical World had a short article on this, 5/77.

 

[jraef]

I agree with stevenal, but must add that the 50HP limit is cumulative, so if you already have 10HP connected to that xfmr bank, you should not add any more than another 40HP.

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"

 

[busbar]

 
To add to stevenal’s remarks, one reference is ANSI C84.1 Annex D, partially described at

http://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

http://www.pge.com/docs/pdfs/custom...eenbook/service_requirements/054619noLink.pdf