1st time installing Scott-T transformer, bonding question

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apjames

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Came across a 50kva 2phase 240v primary to 3phase 240v delta secondary that needs to be replaced.

Incoming utility service to building is in fact 2phase 240v.

My question is what exactly should be bonded before trans, inside trans, and inside secondary protection breaker enclosure. The secondary protection breaker then feeds a 3phase 240v delta panel.

Primary is 5 wire 2phase (4 hots, 1 ground) secondary out is 4 wire 3phase (3 hots, 1 ground) and none of the taps are bonded. Not sure if it was even installed correctly. I'd love to attach a picture but unfortunately it's too big.
 
Couple of things-
Was this in Center City Philly or elsewhere? I'm sort of keeping track of where 2-phase is still delivered.

The bonding won't be any different from any other transformer (both at the service and as an SDS). If the transformer does need to be replaced (why?) does the replacement need to be wired delta? (probably, just asking). It so, you'd need to either corner-ground it, leave it ungrounded (with detectors), or ground the center of one secondary winding.

Also, the EGC doesn't usually count as a wire- a 2-phase 5-wire feed would have 4 hots and one neutral, same as 3-phase 4-wire has three hots and a neutral.
 
If you want a 240V 4-wire high leg configuration on the output, you would bond to ground the center tap of the "main" secondary winding of the T-T transformer, which would be the neutral for single phase loads. A tap up on the "teaser" winding would be needed to provide a neutral and be bonded if you wanted symmetrical L-N voltages on all three phases.
 
zbang said:
Couple of things-
Was this in Center City Philly or elsewhere? I'm sort of keeping track of where 2-phase is still delivered.

The bonding won't be any different from any other transformer (both at the service and as an SDS). If the transformer does need to be replaced (why?) does the replacement need to be wired delta? (probably, just asking). It so, you'd need to either corner-ground it, leave it ungrounded (with detectors), or ground the center of one secondary winding.

Also, the EGC doesn't usually count as a wire- a 2-phase 5-wire feed would have 4 hots and one neutral, same as 3-phase 4-wire has three hots and a neutral.
Click to expand...
Yes Philadelphia. University City to be exact.

Existing transformer burned up at lugs. Definitely had undersized secondary wiring, which we're replacing when we do the trans.

Equipment that this feeds is rated for 3phase 240v so unfortunately yes needs to be delta.

What are the detectors you mentioned?
 
apjames said:
Equipment that this feeds is rated for 3phase 240v so unfortunately yes needs to be delta.
Click to expand...

Does it feed motors in the equipment directly, or are there VFD's involved? Symmetrical L-G voltages are preferred for VFD's, because excess L-G voltage has been known to damage surge protection or other electronics within VFD's. Motors themselves tend to be much more tolerant to a somewhat elevated L-G voltage, such as in a high-leg delta configuration.
 
apjames said:
Came across a 50kva 2phase 240v primary to 3phase 240v delta secondary that needs to be replaced.
Click to expand...
Academic question: If its only a few motors that need 3 phase, and you have a massive 2 phase system like that, I wonder if you could eliminate the scott transformer ?
and feed a 3 phase VFD with 3 240V hots from the two phase? All it does is rectify the 240V to DC right?
:unsure:

apjames said:
Incoming utility service to building is in fact 2phase 240v.

My question is what exactly should be bonded before trans, inside trans, and inside secondary protection breaker enclosure. The secondary protection breaker then feeds a 3phase 240v delta panel.

Primary is 5 wire 2phase (4 hots, 1 ground) secondary out is 4 wire 3phase (3 hots, 1 ground) and none of the taps are bonded. Not sure if it was even installed correctly.
Click to expand...
I have a old scott-T connection diagram somewhere i'll try to dig it up



apjames said:
I'd love to attach a picture but unfortunately it's too big.
Click to expand...
We love pics especially of Philly two phase! Just resize the picture or post it on another site a link it.


Go Phillys!
 
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