Here's another one

[ggunn]

The last time I connected inverters phase to phase on a three phase service they were the old style Sunny Boys, i.e., transformer interconnected. There was a jumper to change so that they didn't use the neutral, which made sense to me, since on a 208 service line to neutral is 120V but for single phase inverters with 208V phase to phase, line to neutral for each inverter would be 104V, so their neutrals would be at different voltages from one another.

I just was handed a circuit diagram that shows three single phase inverters connected phase to phase (A-B, B-C, and C-A) to 208V with their neutrals all connected together and tied to the service neutral. How can that be? Wouldn't each inverter have to generate two 120V phase to neutral waveforms 120 degrees apart to connect that way? Or am I just having a senior moment?

 

[electrofelon]

since on a 208 service line to neutral is 120V but for single phase inverters with 208V phase to phase, line to neutral for each inverter would be 104V, so their neutrals would be at different voltages from one another.

I dont follow this part. Perhaps I am misunderstanding. Phase to neutral (assuming 208 wye system) is 120 V, regardless of where you measure it....?

 

[GoldDigger]

The last time I connected inverters phase to phase on a three phase service they were the old style Sunny Boys, i.e., transformer interconnected. There was a jumper to change so that they didn't use the neutral, which made sense to me, since on a 208 service line to neutral is 120V but for single phase inverters with 208V phase to phase, line to neutral for each inverter would be 104V, so their neutrals would be at different voltages from one another.

I just was handed a circuit diagram that shows three single phase inverters connected phase to phase (A-B, B-C, and C-A) to 208V with their neutrals all connected together and tied to the service neutral. How can that be? Wouldn't each inverter have to generate two 120V phase to neutral waveforms 120 degrees apart to connect that way? Or am I just having a senior moment?

A very simple confusion here is that the neutral is not always a center tap of anything once you change from a single phase to a three phase system.
The "neutral" connection of an inverter that is connected line to line on a three phase system is *not* carrying any current and is not halfway between the two line voltages.

It is there just to provide a voltage reference point only. And so there is no problem with the three inverters sharing a common neutral.

If the inverter had a transformer winding with a center tap with a neutral terminal, you certainly could not connect the three "neutrals" together.

When you use the same inverter connected to 120/240 single phase three wire, the neutral does in fact correspond roughly to the midpoint of the winding, but it still will not be carrying any current.

In a two phase system the center points of each of the two phase axes are at the same potential and form a true neutral point.


mobile

 

[ggunn]

A very simple confusion here is that the neutral is not always a center tap of anything once you change from a single phase to a three phase system.
The "neutral" connection of an inverter that is connected line to line on a three phase system is *not* carrying any current and is not halfway between the two line voltages.

It is there just to provide a voltage reference point only. And so there is no problem with the three inverters sharing a common neutral.

If the inverter had a transformer winding with a center tap with a neutral terminal, you certainly could not connect the three "neutrals" together.

When you use the same inverter connected to 120/240 single phase three wire, the neutral does in fact correspond roughly to the midpoint of the winding, but it still will not be carrying any current.

In a two phase system the center points of each of the two phase axes are at the same potential and form a true neutral point.


mobile

Thanks; that's what I figured out as the day progressed.