How is power used in an autotransformer?

Off_Grid

Member
Location
California
Occupation
Electrician
In an application where the “AT” is being used as a step down with load balancing for a 220v genset to a 120v inverter, How is the genset power used? What I mean by this, I understand it induces load on the L2 leg yo balance the genset, so is this load wasted as a “dummy heat load”, or actually used toward the load? My head keeps getting hung up on the fact that L2 is an opposing phase that would crash if blended into L1.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I'd have to see a circuit diagram to answer you well.

I will add something I posted years ago:

When you energize any winding or winding segment at its design voltage, all other windings or winding segments will become energized at its design voltage.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I get it now. The AT is being used as a "combiner" so the secondary is getting the benefit of using current drawn on both halves of the primary. The current from L2-N is being added to the current from N-L1.

L2 to L1 is energizing both of the 120v windings, and any current drawn from either L2-N or from N-L1 is seen by both halves of the transformer, simply because they all share a common transformer core.

Note that the same setup could be used in reverse, deriving 240v (or 120/240v) from 120v. If you can envision how that would work, you can grasp how it works as a step-down/combiner as shown.

Again (and corrected): When you energize any winding or winding segment at its design voltage, each other winding or winding segment will become energized at its design voltage.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
I agree with Larry in how it works.
Another viewpoint is that the transformer is taking the generator's L2-N output, inverting its polarity so that the transformer L1-N output is in-phase with the generator's L1-N ouput, and then connecting them in parallel so they both contribute current to the load on L1-N.
 

GoldDigger

Moderator
Staff member
The key to the specific question that the OP asked, like any transformer the auto-transformer will draw some magnetizing current from the source connected to its primary, and there will be some resistive and magnetic losses in windings and core. These losses will be small in percentage, but will be present whether the unit is loaded or open circuited. In addition there will be resistive losses proportional to the square of the load current.
But the majority of the power going in will be converted to power out, just at a different current or voltage.
The power is transferred from one winding to another through the magnetic flux in the transformer core.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Another viewpoint is that the transformer is taking the generator's L2-N output, inverting its polarity so that the transformer L1-N output is in-phase with the generator's L1-N ouput, and then connecting them in parallel so they both contribute current to the load on L1-N.
:unsure:
 

Off_Grid

Member
Location
California
Occupation
Electrician
I get it now. The AT is being used as a "combiner" so the secondary is getting the benefit of using current drawn on both halves of the primary. The current from L2-N is being added to the current from N-L1.

L2 to L1 is energizing both of the 120v windings, and any current drawn from either L2-N or from N-L1 is seen by both halves of the transformer, simply because they all share a common transformer core.

Note that the same setup could be used in reverse, deriving 240v (or 120/240v) from 120v. If you can envision how that would work, you can grasp how it works as a step-down/combiner as shown.

Again (and corrected): When you energize any winding or winding segment at its design voltage, each other winding or winding segment will become energized at its design voltage.
That does help..... interesting!
 

Off_Grid

Member
Location
California
Occupation
Electrician
Thank to everyone for the input, that does make sense now.

I have one more question now. I am preparing to upgrade, to two stacked parallel inverters on the same phase for more current.. (The wiring to my house does not provide for series-parallel ability).
Would there be any concerns with using two identical autotransformers, with paralleled primaries to the genset, and each secondary to its own inverter AC input? Picture doubled transformers wired as in my first pic, driving inverters wired as in this pic.
 

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GoldDigger

Moderator
Staff member
Thank to everyone for the input, that does make sense now.

I have one more question now. I am preparing to upgrade, to two stacked parallel inverters on the same phase for more current.. (The wiring to my house does not provide for series-parallel ability).
Would there be any concerns with using two identical autotransformers, with paralleled primaries to the genset, and each secondary to its own inverter AC input? Picture doubled transformers wired as in my first pic, driving inverters wired as in this pic.
There is one thing you need to know here to make sure your paralleled system will work:
Grid tied inverters can be paralleled with no extra equipment because both inverters will independently sync themselves to the grid and hence to each other. Standalone inverters, whether battery powered or in the form of a constantly on UPS without significant battery storage typically contain their own internal frequency reference source and will NOT be able to stay in phase if you simply parallel their outputs. f
Some paralleled inverters use a separate electrical connection between the units to share the reference frequency, with one acting as the leader and the other the follower. (Guess what? There is some well founded opposition to the standard terminology of master and slave. We will see where than ends up.)
Some. like the Honda inverter-based generators, are specifically designed to allow parallel operation by simply paralleling the output leads, with no selection of leader/follower role. I do not know how they do. Maybe someone can enlighten us.
Bottom line: do not assume that you can simply parallel your inverters, But if you can, it should not matter whether you do it by direct connection or by paralleled input transformers.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Assuming the inverter outputs can be paralleled, which certainly needs to be checked as GoldDigger brought up:

Another option is to feed inverter #1 with an autotransformer connected to the generator's L2 and N but not to the generator's L1. So in your first picture, there would be a single-pole breaker from C to D, but no breaker pole or connection from the generator L1 at terminal A to the autotransformer L1 at B. Then the power for inverter #1 is sourced from the generator L2-N output, enters autotransformer terminal L2 and exits L1 with a 180 degree phase shift, and feeds the "hot' input of inverter #1.
Then you can feed the inverter #2 input directly from the generator's L1-N output with no transformer required.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Are these inverter chargers that do not have an AC bypass mode? Or if they do have it can it be permanently disabled? If so, and if the they do not need the AC input at any time for synchronization, then you should not need any autotransfomers. If the AC input is just for charging then feed one inverter with L1-N and the other one with L2-N. That's because L1 and L2 both have the same RMS voltage but are shifted in time by 1/2 cycle, which shouldn't matter just for charging.
 

Off_Grid

Member
Location
California
Occupation
Electrician
The inverters are Outback VFX3648, they can be stacked & synced. I have the HUB10 communication module linking the whole system together so they share a reference.
The two reasons I’m looking at using dual xfmr’s, are 1. Maintaining generator load balancing, & 2. The existing & available xfmr’s are not adequately sized to handle the load of two inverters across a single xfmr. Outback sells a PSX-240 (6kva) xfmr for these 3600w inverters, the one currently installed on my system is a Trace T-240 (3.9kva), which to me seems a bit undersized. (I did not install it), this transformer has been the Achilles heel of my system. Interestingly, both xfmr’s come factory with a 25A input breaker..... I added a 240v fan to the T-240, which made a big difference in reducing nuisance breaker trips, but they do still occur at times.
 

jaggedben

Senior Member
Seems to me that you ought to just replace that transformer with a bigger one, rather than worry about paralleling two. I imagine two paralleled transformers would still work, though.
 

Off_Grid

Member
Location
California
Occupation
Electrician
Seems to me that you ought to just replace that transformer with a bigger one, rather than worry about paralleling two. I imagine two paralleled transformers would still work, though.
I do agree there, that seems to be somewhat of a challenge to find....
 
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