TRANSFORMER ISSUES

[jaggedben]

So there is a high leg there. They are feeding the transformer with the 120/208Y. The in house guy is telling me that the voltage across the Delta used to be 120 per phase and 240 phase to phase. I measured 240 phase to phase and A /C phase measured 240 to ground. 0 volts on B to ground.

This is contradictory. Perhaps there used to be a high-leg (different transformer) and at some point it was converted to corner grounded. But a high-leg and a phase 0V to ground should be a dead short, not possible even if the transformer had a center tap for a high-leg (which the label doesn't show).

 

[jeffreylee7469]

Was this at the transformer terminals?
Can you post a better picture of the wiring in the transformer.
Based on the OP nameplate that transformer is not capable of providing a high-leg center tapped output with 120V line to ground.

I have run into more than one person who thought all delta secondaries were center tapped. Industry slang really gets in the way

I was troubleshooting a transformer and found this issue-



It is a step up transformer, 120/208Y to 240v delta - 3-phase. A/C phase used to measure 120 per phase and B was a high leg (130-150).


I measured the voltage Friday and measured 240 across all phases and 240 to ground on A / C phase, but B-phase measured 0 volts to ground. Can you think of any reason why this might be happening?

 

[jeffreylee7469]

These are the best pics I have. They are all tapped at the #3 tap which says it’s 100% voltage equaling 240v and 180a. I never measured 120v per phase, which would seam impossible if the total output is 240v.
This transformer feeds a 240v panel. When I was there previously I was told only the one oven had an issue. I received a call today and another oven on a different panel fed by the first panel mentioned had a conductor that had burned up. I had a debate with their handyman saying this transformer and panel were set up to measure 120v per leg and then the high leg was (130-150v).
I’m not a transformer expert, hence the questions.

 

[jeffreylee7469]

Did anyone consider using a buck-boost setup?

No

 

[jeffreylee7469]

Here is the best picture I have

 

[jeffreylee7469]

I was told a previous transformer was wired incorrectly and the old panel burned up. This was a while ago.
Should the transformer read 240 total across all phases and 90amps per phase? Or is the transformer label stating those voltages from HV side to the LV side not vice versa

 

[jim dungar]

When I was there previously I was told only the one oven had an issue. I received a call today and another oven on a different panel fed by the first panel mentioned had a conductor that had burned up.

Open all of the disconnects on the output of the transformer, or disconnect the load wires. Now measure the voltages.

If the voltages are 240 L-L and some equal L-G voltages (roughly 138V) your transformer does not have a problem. It may not be the proper transformer but it is not behaving badly.

 

[jeffreylee7469]

This is contradictory. Perhaps there used to be a high-leg (different transformer) and at some point it was converted to corner grounded. But a high-leg and a phase 0V to ground should be a dead short, not possible even if the transformer had a center tap for a high-leg (which the label doesn't show).

I think the handyman is referring to the phase that was reading 130-150. When it was installed.

 

[jeffreylee7469]

Open all of the disconnects on the output of the transformer, or disconnect the load wires. Now measure the voltages.

If the voltages are 240 L-L and some equal L-G voltages (roughly 138V) your transformer does not have a problem. It may not be the proper transformer but it is not behaving badly.

Could there be an issue with the line side that is causing an open delta situation? Which is why I’m reading 0v to ground across one phase

 

[jim dungar]

Could there be an issue with the line side that is causing an open delta situation? Which is why I’m reading 0v to ground across one phase

Where are you making your measurements.

An open delta will not provide 0V to ground. 0V to ground can only occur if that phase is shortest ground somewhere. On open phase on the wye source windings will not show up as an 'open delta'.

Has the ovens ever worked on this transformer?
Maybe the ovens really do need a center tapped output, and without they have failed.

 

[jeffreylee7469]

Where are you making your measurements.

An open delta will not provide 0V to ground. 0V to ground can only occur if that phase is shortest ground somewhere. On open phase on the wye source windings will not show up as an 'open delta'.

Has the ovens ever worked on this transformer?
Maybe the ovens really do need a center tapped output, and without they have failed.

I took my readings at the taps on the coil side and at the lugs on the 240 panel. I do not know where the 120v readings were taken from.
Could this delta have been unintentional grounded or a phase bonded to neutral somewhere? As stated earlier a second oven was affected since I was in this shop. This oven is fed by a sub panel from the original 240v panel.

 

[jim dungar]

I took my readings at the taps on the coil side and at the lugs on the 240 pane

Were all of the 240V breakers open?

Open or disconnect everything on the 240V side. Start measuring the voltage at the transformer. Reconnect stuff, one item at a time until you read 0V to ground.

 

[Jraef]

My $ 0.02

The original post said ( in part) "The specs are labeled as 3-phase, corner grounded delta (primary)"

Since the original primary is now the secondary, wouldn't this mean that there is an intentional ground on one corner of the delta ?

Just my take on it, I'm probably wrong, but thought it makes sense.

I think this is a very valid point...
In a corner grounded delta system, you ONLY ground one corner of the transformer, you do NOT intentionally ground it in the LOAD EQUIPMENT. So I find it a bit odd that someone made the equipment spec say "Corner grounded delta", because the equipment should not care and should not be grounded THERE. It should just say “240V delta”. If someone intentionally grounded one of the heater elements, and you connected your supply to where that was NOT the same grounded phase, you will trip the breaker on short circuit.

 

[jaggedben]

I think this is a very valid point...
In a corner grounded delta system, you ONLY ground one corner of the transformer, you do NOT intentionally ground it in the LOAD EQUIPMENT. So I find it a bit odd that someone made the equipment spec say "Corner grounded delta", because the equipment should not care and should not be grounded THERE. It should just say “240V delta”. If someone intentionally grounded one of the heater elements, and you connected your supply to where that was NOT the same grounded phase, you will trip the breaker on short circuit.

The specs do not say corner grounded. The OP misunderstood the label.