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Xformer delta-delta with high leg situation, (irregular voltage)

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B

Burdaneta

Member
Location
MIAMI, FL
Occupation
Electrician
hi everyone , I need some advice on a situation we got a small electrical work shop with a trainking station and we just purchase a new xformer step-up to convert our 120/208v service to 480v, we got everything connected high leg to phase C like the diagram shows, but when I read the voltages with the multimeter on the secondary side (480v) I got , Phase A(280V), B (230V), C (355V) and I don’t know could be happening , any thoughts? , I’m pretty sure you guys will ask for more details on this please let met know what information I can provide to get a better understanding on this, thanks in advance.
 
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B

Burdaneta

Member
Location
MIAMI, FL
Occupation
Electrician
infinity said:
Can you explain how you wired it? 3Ø, 208 volt, 3-wire to X1, X2, X3?
Click to expand...
Of course it feds from a electrical panel 120/208 high leg,

X1 phase A (BLACK), X2 phase B (RED), X3 phase C (BLUE HIGH LEG FROM PANEL 208V) and finally X4 neutral from panel.
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Burdaneta said:
Of course it feds from a electrical panel 120/208 high leg,

X1 phase A (BLACK), X2 phase B (RED), X3 phase C (BLUE HIGH LEG FROM PANEL 208V) and finally X4 neutral from panel.
Click to expand...
If this is Delta the voltage should be 240/120 with the high leg being 208. The neutral shouldn't be connected to the X4.
 
C

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
In the OP you state the supply voltage is 120/208. This would be a Wye system not Delta. The transformer is designed for 240 volts on the LV side.

As Infinity stated you should not have a neutral connected to the supply side, only the 3 phase conductors.

The HV side does not have a center tap so measurements to ground are meaningless.
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Burdaneta said:
should I remove the neutral them? What is the use of X4? It looks like for me is intended to be used as neutral
Click to expand...
That transformer is really a step down that you are reverse wiring. In a step down scenario you would use the X4 to create a neutral in the secondary side. Since it's reverse wired you do not connect anything to X4. This creates other problems because your 480 system is not grounded.
 
B

Burdaneta

Member
Location
MIAMI, FL
Occupation
Electrician
infinity said:
That transformer is really a step down that you are reverse wiring. In a step down scenario you would use the X4 to create a neutral in the secondary side. Since it's reverse wired you do not connect anything to X4.
Click to expand...
Yes you 100% correct is a step down that I’m using on reverse, ok the point on this is , if I remove the neutral should I get normal readings on the voltage? this is the main issue I have as I stated on my first post, thanks for you help on this
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
@Burdaneta We have to go one step at a time

1) You describe your supply as '120/208'. You also say 'high leg'. I am guessing that you have a high leg delta service, 240V L-L, 120V L-N on two legs, 208V L-N on the high leg. Please confirm this. The other possibility is that you have 208V L-L, 120V L-N in a balanced wye system, but I don't expect that.

2) As others have noted: when you have a delta:delta transformer wired in reverse, you do _not_ connect the neutral. Connecting the neutral when it is on the side used as the _primary_ may cause circulating currents and excess heating, but shouldn't change the output voltage.

3) You've not described how you grounded the 480V delta secondary, nor how you measured the unbalanced voltages. You should measure both L-L and L-G voltage. The voltage measurements you gave look like L-G voltages, but if the transformer secondary isn't grounded than the voltage measurements will be variable, likely not balanced, and essentially meaningless. Depending on the requirements of your system and hardware available, you might need to corner ground your 480V or get a different transformer.

-Jonathan
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Burdaneta said:
Phase to ground and phase to phase, I got low voltages values on phase B and to high on Phase C
Click to expand...
Forget measuring phase to ground the transformer is not grounded. All you need to measure is phase to phase. On the primary it should be 240 volts phase to phase and 480 phase to phase on the secondary.
 
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