208v to 240v Buck Booster

[LarryFine]

Some time ago, I suggested that, if you select a unit so the secondaries will be connected in series, they could be wired to opposite ends of the primary, maintaining equal voltages to the neutral.

For example, looking at the boost diagram below, separate the secondaries and wire X3-X4 to the H4 end, and X1-X2 to the H1 end.

In other words, wire it like you would an open-delta setup, but with only one transformer, and no neutral connection to the transformer.

It would basically look like this diagram, but with one winding in each place where this diagram shows two (and there would be no "neutral" or B phase connection to the unit):

 

[kwired]

But that design would only work in countries with 3wire systems.
Many manufacturers like to keep as much of their internals as common as possible when they build for the world market.

If they are designing it with 120 volt motor and controls it is intended to operate on a three wire system anyway. What is different is designing a heat element with a center tap vs one without.

 

[kwired]

How would it not not produce max heat at 240?
Were talking about a L1----5Ω----N----5Ω----L2

I believe he was still talking single element there, so if it is rated for 208 volts then it will run hotter than designed if you connect it to 240.

 

[wwhitney]

Some time ago, I suggested that, if you select a unit so the secondaries will be connected in series, they could be wired to opposite ends of the primary, maintaining equal voltages to the neutral.

But if you do that to 120/208 "single" phase, you will get 138/240 "single" phase. Which is fine if your electric dryer is 240V 2-wire, but a problem if it is 120/240V 3-wire.

In theory with a 120/240V 3-wire load, if all the 120V loads are using just one ungrounded leg, you could boost only the ungrounded leg, to get an asymmetric 120/240 "single" phase.

Cheers, Wayne

 

[LarryFine]

But if you do that to 120/208 "single" phase, you will get 138/240 "single" phase. Which is fine if your electric dryer is 240V 2-wire, but a problem if it is 120/240V 3-wire.

What if you used the center tap (H2-H3) as the load's neutral instead of the source neutral?

 

[wwhitney]

What if you used the center tap (H2-H3) as the load's neutral instead of the source neutral?

I looked a bit closer at your diagram and realized that you aren't boosting L1-N and L2-N, but L1 and L2 relative to the midpoint of L1/L2, so my comment about 138/240 "single" phase was not correct.

The L1/L2 midpoint is 60V above ground/neutral, so I don't think you'd want to connect that to the load's neutral. I mean maybe it would work OK, as the load should keep its neutral insulated from the EGC, but I imagine it was designed for about 0V neutral-EGC, not 60V.

With the L1/L2 midpoint 60V above ground, boosting L1 and L2 from 208V to 240V relative to the midpoint of L1/L2 will make the boosted voltage to ground/true neutral sqrt(120²+60²) = 60 * sqrt(5) = 134V.

Cheers, Wayne

 

[tortuga]

Yeah Larry you have a good idea, I know there are some code sections under 210 that deal with auto-transformers, so as long as its legal to have the neutral 60V above EGC that would fly.
I should add for the OP over my career I have done tons of 120/208 units with regular consumer ranges, dryers and heaters and never once got a complaint.

 

[jim dungar]

I looked a bit closer at your diagram and realized that you aren't boosting L1-N and L2-N, but L1 and L2 relative to the midpoint of L1/L2, so my comment about 138/240 "single" phase was not correct.

The L1/L2 midpoint is 60V above ground/neutral, so I don't think you'd want to connect that to the load's neutral. I mean maybe it would work OK, as the load should keep its neutral insulated from the EGC, but I imagine it was designed for about 0V neutral-EGC, not 60V.

With the L1/L2 midpoint 60V above ground, boosting L1 and L2 from 208V to 240V relative to the midpoint of L1/L2 will make the boosted voltage to ground/true neutral sqrt(120²+60²) = 60 * sqrt(5) = 134V.

Cheers, Wayne

Which would result in a short circuit if the system neutral and the boosted neutral were ever connected together, because of the voltage difference.

 

[tortuga]

It appears 210.9 would disallow it?
EDIT however there is exception no.1

 

[ruxton.stanislaw]

134 V might be OK for the components, other than the bulb burning out faster if it's not LED. However, we are assuming the system will be at a nominal 120 V. If it is already running at 125 or even 128 V, things will start to get too hot. It is a bit complicated, but stepping up both sides and stepping down the neutral could make sense. Though, if this set up requires multiple transformers, perhaps a 208:240 or 250 V transformer would be more economic.

 

[wwhitney]

Which would result in a short circuit if the system neutral and the boosted neutral were ever connected together, because of the voltage difference.

The L1/L2 midpoint (which you are calling a boosted neutral) would obviously need to be a color other than white, as it is ungrounded, so hopefully that would help avoid that problem.

But then you have the question of violating the appliance's listing, or the receptacle's listing, by attaching an ungrounded conductor to the grounded conductor terminal of the appliance or receptacle. Along with confusing the heck out of anyone seeing it the first time.

Cheers, Wayne

 

[ptonsparky]

The L1/L2 midpoint (which you are calling a boosted neutral) would obviously need to be a color other than white, as it is ungrounded, so hopefully that would help avoid that problem.

But then you have the question of violating the appliance's listing, or the receptacle's listing, by attaching an ungrounded conductor to the grounded conductor terminal of the appliance or receptacle. Along with confusing the heck out of anyone seeing it the first time.

Cheers, Wayne

I'm already confused and I've yet to get my Volt Tick out.

 

[jim dungar]

The L1/L2 midpoint (which you are calling a boosted neutral) would obviously need to be a color other than white, as it is ungrounded, so hopefully that would help avoid that problem.

But then you have the question of violating the appliance's listing, or the receptacle's listing, by attaching an ungrounded conductor to the grounded conductor terminal of the appliance or receptacle. Along with confusing the heck out of anyone seeing it the first time.

Cheers, Wayne

Because the new neutral is less than 150V to ground, the NEC would require both it and the normal system neutral to both be bonded to ground. this would cause an immediate short circuit.

This is why no manufacturer recommends this as a preferred wiring diagram

 

[tortuga]

for older commercial dryers you can of course get 5kw '208V' element or a 5kw '240V' element.

 

[wwhitney]

Because the new neutral is less than 150V to ground, the NEC would require both it and the normal system neutral to both be bonded to ground. this would cause an immediate short circuit.

I take it you are referring to 250.20(B)(1), which says we need to ground our voltage system "Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts."

A', B' and N' already have a have a well defined voltage to ground of 150V or less, so it already complies with (1). A', B', and N' exist only in the context of A, B, C, and N, where N is already grounded.

Cheers, Wayne

 

[wwhitney]

I'm already confused and I've yet to get my Volt Tick out.

Larry's suggestion in Post #21, along with using H2-H3 as the "neutral" would create the 3 new voltage points A', B', and N' shown in the diagram below. I just eyeballed the length of the new black line (which represents the autotransformer supplied by A and B).

I didn't write in all the voltages (given by the distance between two points in the diagram), but (A,B,C,N) are as usual for 208Y/120V. The new points have voltages to ground (to N) of A' = 134V, N' = 60V, and B' = 134V. But A'-N' and B'-N' are 120V as desired.

Cheers, Wayne

 

[LarryFine]

Bingo! And as was said above, the system is grounded. Nothing says the load must use that conductor.

 

[ptonsparky]

Bingo! And as was said above, the system is grounded. Nothing says the load must use that conductor.

What would we use for sizing the transformer? Same method as a BB?

 

[wwhitney]

What would we use for sizing the transformer? Same method as a BB?

Good question, let's think it through. I'm going to assume a hypothetical transformer that gives us 240V from our 208V. I.e. the turns ratio is 32V : 208V, or 2 : 13. With both the primary and secondary center-tapped, so the post #21 scheme can be implemented.

Then for 240V loads, you would just size it as normal. 13A @ 240V would require 15A @ 208V, and of that 15A, 2A flows on the 208V primary windings, 13A flows in the 32V secondary windings. So for 13A @ 240V = 3120 VA of load, your transformer would need to be rated at 2A * 208V = 13A * 32V = 416 VA.

For a 120V unbalanced load of say 26A on A'-N', that would also require 15A @ 240V. The transformer coil currents would be 26A from A' - A; 11A from A - N' ; and 15A from B - N'. [The 11A from A-N' and the 15A from B-N' are of opposite sense (direction), so the average current over the primary coil is (15 - 11)/2 = 2A, while the average current over the secondary coil is (26 + 0)/2 = 13A, the correct ratio.] So for 26A @ 120V = 3120VA of load, your transformer would need to be rated at max(26A*32V, 15A*208V) = max(832 VA, 3952 VA) = 3952 VA.

For the mixed case, just add up the two results using the unbalanced 120V portion of the load, and the balanced 240V portion of the load. Definitely an inefficient way to provide for the 120V unbalanced load, but plausible if that load is small compared to the 240V balanced load.

Of course, this is all academic, I think, as connecting N' at 60V above ground to either a receptacle's grounded pin or an appliance's grounded conductor is surely a violation of something.

Cheers, Wayne

 

[tortuga]

And its probably cheaper to get a 208V element.
I have done this for 4500W water heaters on 208