help with buck boost

Bluetick21

Bluetick21

Member
Location
Indiana
Occupation
Electrician
I could use a bit of help here. Acme T111686 transformer. Have 208V. Need to get close to 230V. It seemed like Fig. H would be correct but now I am second guessing that. Thanks for any help.
 

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Here is more info ....T111686 is 120X240 input 12X24 output 3kva.
My trial run (without any load) was..... L1= 120v L2= 143v [ those are to grnd or neutral ] L1 to L2= 238v.
I do see another diagram ( not in the picture) that seems like it makes more sense.....
Line to H4. H2 to H3. Line to H1. Then- X1 to load. X2 to X3. X4 to load.
 
The voltage that you got in your trial is exactly what you would expect when that transformer is connected in the boost configuration supplied with 208V from a 120/208V supply.

In your second post, the connection you suggested is an isolation connection that will provide 24V ( twenty four )
 
winnie said:
The voltage that you got in your trial is exactly what you would expect when that transformer is connected in the boost configuration supplied with 208V from a 120/208V supply.

In your second post, the connection you suggested is an isolation connection that will provide 24V ( twenty four )
Click to expand...
since i did not know that.....maybe says something. Anyway. This is being done for an electric train charger. the cable that plugs into the train has 2 hots a neutral and ground. The Line to N with the higher voltage wont be a problem? I guess that may be a question for the tech from the ride company.
 
Bluetick21 said:
since i did not know that.....maybe says something. Anyway. This is being done for an electric train charger. the cable that plugs into the train has 2 hots a neutral and ground. The Line to N with the higher voltage wont be a problem? I guess that may be a question for the tech from the ride company.
Click to expand...
Yes it will be a problem if the load actually requires a neutral. You will need to know if it actually does. Not the best way, but if only one leg is used for 120, such as controls, it can be paired with that leg. The alternative is to use a step up transformer, which will be much larger. 208 to 120/240.
 
Yes, a basic 'buck/boost' transformer doesn't take 120/208V (two legs from a 3 phase 208/120V system) and convert it into a 120/240V 'split phase' output. It provides 240V but the L-N voltages are uneven.

If you really need 120/240V, then you need a full 'isolation transformer'. (Well, there are some non-standard autotranformer approaches, but let's ignore anything that you can't order out of a catalog :) )

So -> check with the equipment manufacturer. Find out if your system really needs 120/240V split phase.

1) If it doesn't really need split phase, and doesn't really use the neutral, then you can use your existing transformer. See if you can disconnect the neutral so that it doesn't accidentally cause a problem.

2) If it does really need split phase, but you can limit the 120V load to only one leg, and you are willing to take the risk that a mis-connection will put 140V on that leg, then you can carefully insure that the 120V load is on the 120V leg.

3) If it does really need split phase and you want to provide true 120/240V split phase, then you need a different transformer.

-Jonathan
 
Bluetick21 said:
sorry about that. I dont think I can clean it up any.
Click to expand...
I think this is the same pic - and connection "H" is the one in the lower left (circled)...

1774151537825.png

For your case, "H" would show 208 VAC mains connected across the Hi Voltage windings - and the output is taken from the Hi Voltage + the Low voltage, which are all wired in series. So they add up - as Larry said above - 208 + 20.8 or ~228.8 VAC.

Also, as noted, the neutral is NOT in the "middle" of that 228.8.
 
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