Transformer Expert Needed

[kwired]

He is just saying the same thing I said in a couple posts..... make the Xo reference with ANOTHER xfmr. Just hook up any old Wye 460v xfmr winding to the 460V 3ph of concern here. A 250va xfmr, a 250kva xfmr, whatever. Ground the Xo. Now all 3 phases will show the electrician 277v to ground to satisfy the OP question.

I think I see what you are saying, It would give you a voltage reading to ground on all three phases. What happens - especially if you only add 250VA transformer- if you should happen to have a ground fault? The smaller transformer probably becomes a slow blow fuse I would think, but was not listed or tested to be used as a fuse

I would think if they had most any old wye xmfr around they would likely use it instead of the delta in the first place.

 

[steve66]

He had me confused too. But I can't say why my mind triggered on this when reading his post. Maybe the mention of creating an X0 on the secondary?

Or it's the drugs I take...

Sorry I didn't explain it very well. mike_kilroy did a much better job:

He is just saying the same thing I said in a couple posts..... make the Xo reference with ANOTHER xfmr. Just hook up any old Wye 460v xfmr winding to the 460V 3ph of concern here. A 250va xfmr, a 250kva xfmr, whatever. Ground the Xo. Now all 3 phases will show the electrician 277v to ground to satisfy the OP question.

 

[kwired]

Stupid thing won't let me read next page of posts, hasn't happend to me in quite a while. Adding another post usually get it out of it so we are trying that.


Add: looks like it worked.

 

[jim dungar]

What happens - especially if you only add 250VA transformer- if you should happen to have a ground fault? The smaller transformer probably becomes a slow blow fuse I would think, but was not listed or tested to be used as a fuse

Actually taking (3) single phase transformers (even something as small as a 50VA CPT) and connecting them into a primary wye connection is really no different than the traditional wye connected pilot lights used as a ground indicator scheme. The secondary of the transformers are typically connected into a delta and left 'unloaded' however there are many variations.

 

[infinity]

It's in my post; drives have surge protection (usually MOVs) and filters on the front-end, usually set up in a wye to ground. So when you have an delta system feeding the drive or even a resistance grounded wye system, that connection becomes a path of least resistance to ground for the entire distribution system.

So then you're saying that a drive is usually system specific, meaning that if the intended system is WYE then is it's safe to say that it should not be connected to a Delta system even if it's only temporarily.

 

[kwired]

Actually taking (3) single phase transformers (even something as small as a 50VA CPT) and connecting them into a primary wye connection is really no different than the traditional wye connected pilot lights used as a ground indicator scheme. The secondary of the transformers are typically connected into a delta and left 'unloaded' however there are many variations.

But if you do that and the only thing that is intentionally grounded is the common point of the wye - what happens when you have a ground fault from one of the "main conductors" to ground, especially if the wye transformer is significantly smaller than the supplying delta transformer?

Put overcurrent devices in the circuit to protect the smaller transformer and you end up blowing the fuse and creating a high impedance to ground - not what is desired in application of equipment grounding.

 

[mike_kilroy]

keep in mind what the OP was about: how to make a voltage reading an electrician can read to ground from the phases for a temp use. no one ever said anything about making it short to ground proof. a lot of replies were to just answer the OP question that said to ignore code and stuff. get a reading to ground. any old wye xfmr winding of any size will give the electrician's hi impedance meter the required reading. thats all.

as to wye vs delta requirement for vfds, it is NOT an across the board requirement or spec. SOME specify what is required, a lot don't. for instance, a lot of siemens drives require grounded wye input and go on to specify that the input transformer must have 3% or less Z (in their case due to their active front end and any higher impedance lets the horrendous noise jerk the power source around doing things like making clocks in whole buildings run backwards). A lot of Bosch/Rexroth drives require wye also - some stated in really really small print. Hitachi VFDs often have MOVs L-L then ONE MOV to ground - thru a jumper that customer can remove for these kinds of reasons. I have sent many a drive back for warranty repair when the MOV blows and blackens the guts of the drive. Then there are companies like Refu who will work with ANY input - altho as a hi tech vfd drive we have had to remove the input capacitor emi filters on occassion for just the reason Jraef said: it is the lowest Z source to ground for noise on the whole ungrounded system and tries to suck it all down and overheat if not disconnected. So bottom line is: ask the mfgr if you have concerns over whether your power source is acceptable as is.

 

[jim dungar]

But if you do that and the only thing that is intentionally grounded is the common point of the wye - what happens when you have a ground fault from one of the "main conductors" to ground, especially if the wye transformer is significantly smaller than the supplying delta transformer?

Put overcurrent devices in the circuit to protect the smaller transformer and you end up blowing the fuse and creating a high impedance to ground - not what is desired in application of equipment grounding.

The faulted phase will cause the voltage across one transformer to go to 0V, the other two transformers will go to 480V, there will not be 'weird' currents. Remember this is just like ground indicator lights.

 

[Jraef]

So then you're saying that a drive is usually system specific, meaning that if the intended system is WYE then is it's safe to say that it should not be connected to a Delta system even if it's only temporarily.

The real issue is, delta power systems are somewhat unique to North America (and our immediate sphere of influence), whereas in the EU and most of asia, they ONLY have wye systems (in one form or the other) and in commercial installations here, such as HVAC which commands a LOT of attention in the VFD world, all new installations are wye systems now. Since most drives now come from overseas, or American companies now must be able to compete overseas and/or in the HVAC market, the default design is now for wye systems. But as I said, those that place value on the US industrial market allow for simple modifications to accommodate delta systems or HRG wye systems, as long as you RTFM.

Unfortunaterly, that too is often a problem.

 

[mivey]

The faulted phase will cause the voltage across one transformer to go to 0V, the other two transformers will go to 480V, there will not be 'weird' currents. Remember this is just like ground indicator lights.

I can see that if you leave the secondaries open like Steve66 said here:

...The secondary of T2 could just be left open.

But with this configuration:

...The secondary of the transformers are typically connected into a delta and left 'unloaded' however there are many variations.

Unless I've pictured it wrong, a grounded phase on the primary side of T2 would cause two out-of-phase voltages to be paralled on the secondary of T2 because of the delta connection you mentioned.

 

[infinity]

Spoke to the elevator foreman and he said that they ran the drives without incident with the Delta supply. The temporary installation was as described in the OP and everyone seems happy. Hope that ConEd gets done soon.

 

[mivey]

Spoke to the elevator foreman and he said that they ran the drives without incident with the Delta supply. The temporary installation was as described in the OP and everyone seems happy. Hope that ConEd gets done soon.

Sounds good. If you try to make a ground reference, be mindful of limiting the fault current with some schemes. In particular:

Put overcurrent devices in the circuit to protect the smaller transformer and you end up blowing the fuse and creating a high impedance to ground - not what is desired in application of equipment grounding.

In a gounding application, you would insert a grounding impedance into the ground path on the wye side, or break the delta on the secondary side to insert an impedance. Either one will limit the current from a ground fault (or to limit the weird current).