3 Phase Delta Service 1

[rballance]

Hi all,
I have a question regarding a service change from a 3 phase delta connected system to a 3 phase wye connected system. I am designing a service upgrade from the existing delta connected system to a wye connected system. The local electric utility will not upgrade delta systems anymore, so the wye connection is necessary. Is there anything I need to look out for regarding the existing load side equipment when changing over to the wye connection? My understanding of the purpose of the delta connection was to create 3 phase power when only two transformers were available from the electrical utility, and the high leg should not be used for single pole loads due to the 208V to ground. Thanks for the help.

 

[dpc]

With only two transformers, it's an "open delta". You can also get delta service with three transformers, and often a wild leg for 120V.

If the present secondary system is 120/240 V, then you will lose your 240V when you switch to a three-phase wye. You will be getting 208/120V. If you have motors running at 240V, it could be an issue.

 

[waross]

Where you previously had 240 volts, (or 230 if the system is old enough) you will now receive 208 volts.
Any equipment on three phase and any equipment on 240 volts should be checked. Some equipment will have 208 volt taps and should be changed to the 208 volt taps.
Older 230 volt motors usually run well on 208 volts. Newer motors should not be run on 208 volts unless they are rated for 200 volts.
That said, lightly loaded motors will generally survive the switch. It is mainly motors that are putting out full horsepower and motors with hard to start loads that may have a problem. If in doubt, check the motor current under load. If the current is not over the rated current the motor should work well. If you have one or two motors with a problem you can boost the voltage back up to 240 or close to it with a small "Buck-Boost" transformer. You need two transformers for three phase motors. You can boost the voltage for a group of motors with one set of transformers.
RECAP
Inspect all equipment.
Change tap settings as required.
Monitor your motors for over current.
Don't buy transformers unless you find that you need them.
There are so many millions of 230 volt motors running happily on 208 volts that you shouldn't have to worry.
yours

 

[rballance]

Thanks Waross, that's what I was looking for. My motor loads are some HVAC air handlers and some small kitchen exhaust hood fans. I will definitely check the motor taps as you mentioned.

 

[waross]

The motors won't have taps, check the current.
Some discharge lighting ballasts have taps, battery chargers for forklift trucks, and often any equipment with an internal transformer. Some air conditioning control transformers have a 208 volt tap.
It's well to check but I don't anticipate any problems. The areas where I have spent most of my time used 120/208 volts almost universally. Office buildings, shopping malls, small industry, etc. Virtually all the utilization equipment is 230 volt rated, except equipment with 208 volt taps.
yours

 

[rballance]

Thanks. This particular building is approx. 50 years old, and has been changed many times to suit the new tenants. Currently it is a medium sized restaurant and is located at the entrance to an old residential neighborhood. Most of my experience has also been with 120/208V 3 phase services.