Mental fog on transformer windings, single phase.

[fastline]

I'm getting foggy. Need to make 240V from 120V. Have several buck/boost around but I am trying to recall my ampacity and KVA limits when doing this? Assuming say a 120/240p, 16/32s xfmr,, 2KVA. You can only get 2KVA when both windings are driven in parallel. But then I am using one as the primary, and other as boost winding.

Part of me thinks this would have to be 1/2 the rating. Part of me things full rating. Assume 2 primary windings as parallel, each can do 5A or 10A combine. I am driving only one so limited by that 5A. However, that second boost winding can also do 5A and will be opposed sine so basically same amperage but double the voltage so full rated capacity of xfmr.

 

[electrofelon]

Have you tried wiping the mental fog off the transformer windings? Maybe some soap and water will get it off. Acetone?

 

[LarryFine]

However, that second boost winding can also do 5A and will be opposed sine so basically same amperage but double the voltage so full rated capacity of xfmr.

No, double the voltage, but at the same current.

 

[synchro]

Because you are using one of the 120V windings as a primary and the other one as a secondary, the kVA capability of the transformer itself will be 1/2 of its nameplate rating. However, if you use these two windings as an autotransformer to boost up to 240V, it will be capable of supplying a load of 2 x 1/2 = 1 times the nameplate rating. And so a 2 kVA buck boost configured this way could supply a 2 kVA load.

Now this configuration will provide an output of 240V relative to the neutral coming from the 120V input. So this may not be what you would want.

 

[LarryFine]

Now this configuration will provide an output of 240V relative to the neutral coming from the 120V input. So this may not be what you would want.

What if the shared "center tap" is connected to the grounded conductor?

 

[winnie]

What if the shared "center tap" is connected to the grounded conductor?

I agree, that would give 120/240V split phase.

 

[synchro]

What if the shared "center tap" is connected to the grounded conductor?

It seems that the output circuit of the transformer would need to include the grounded conductor (neutral) according to 210.9. Now could you provide a 3-wire output but only connect L-L loads and still be compliant?
That said, I don't see a technical problem with what you are proposing.

From 210.9:
"Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.
Exception No. 1: An autotransformer shall be permitted without the connection to a grounded conductor where transforming from a nominal 208 volts to a nominal 240-volt supply or similarly from 240 volts to 208 volts."

 

[kwired]

I'm getting foggy. Need to make 240V from 120V. Have several buck/boost around but I am trying to recall my ampacity and KVA limits when doing this? Assuming say a 120/240p, 16/32s xfmr,, 2KVA. You can only get 2KVA when both windings are driven in parallel. But then I am using one as the primary, and other as boost winding.

Part of me thinks this would have to be 1/2 the rating. Part of me things full rating. Assume 2 primary windings as parallel, each can do 5A or 10A combine. I am driving only one so limited by that 5A. However, that second boost winding can also do 5A and will be opposed sine so basically same amperage but double the voltage so full rated capacity of xfmr.

I don't see how you are going to go from 120 to 240 with only a 32 volt boost coil?

I can see supplying half a 120/240 primary coil with 120 volts and getting 240 volts out of the entire primary. You are limited to at least half the kVA rating of that unit though and most would just use an isolation transformer in this case as it should be able to be a smaller unit.

 

[LarryFine]

I can see supplying half a 120/240 primary coil with 120 volts and getting 240 volts out of the entire primary.

That's what I took the OP to mean.

 

[kwired]

That's what I took the OP to mean.

So the 16/32 volt secondary was to confuse us? I guess he said he had mental fog...

I guess you could still use it if supplying a rather limited load, otherwise like I said you only getting half the VA rating out of it at the most without overloading it.

 

[ggunn]

I'm getting foggy. Need to make 240V from 120V. Have several buck/boost around but I am trying to recall my ampacity and KVA limits when doing this? Assuming say a 120/240p, 16/32s xfmr,, 2KVA. You can only get 2KVA when both windings are driven in parallel. But then I am using one as the primary, and other as boost winding.

Part of me thinks this would have to be 1/2 the rating. Part of me things full rating. Assume 2 primary windings as parallel, each can do 5A or 10A combine. I am driving only one so limited by that 5A. However, that second boost winding can also do 5A and will be opposed sine so basically same amperage but double the voltage so full rated capacity of xfmr.

Use a 120V to 120/240V autotransformer.

 

[fastline]

Use a 120V to 120/240V autotransformer.

Do you mean isolation?

I only mentioned because I have some of these autos around but I do agree that running a proper xfmr is prob best.

The vodka was fogging me last night but maybe still hung over....lol Yes indeedeee, it appears I could only get 50% of KVA rating. I would be limited by the ampacity of a single primary winding at low voltage. Connection would be 120V applied to H1 and H2, H2/H3 bonded, and 240V would come from H1 to H4. However, the best I can do is the allowed ampacity of a single winding and they would have to be in parallel to get full KVA. So 50%.... GD, thanks for clearing that up guys!

 

[winnie]

I think @synchro is correct in post #4.

Because you are using only one 120V coil for input, and the other for output, you halve the kVA rating of your transformer. But as an autotransformer, some of the load is fed directly, not through the transformer. In your 120V to 120/240V configuration you get double the capacity of the transformer available to feed the load. So the possible load kVA is 2x1/2xtransformer kVA.

Consider a transformer rated 5A per primary coil (thus 5A at 240V in series or 10A at 120V in parallel). The way you are using this transformer it is rated 5A in at 120V and 5A out at 120V. You connect the derived 120V in series with the 'infinite' 120V supply to get 240V. Your output is limited to 5A at 240V.

-Jonathan

 

[ggunn]

Do you mean isolation?

No. If you have 120V two wire you can make 120/240V three wire with an autotransformer.

 

[LarryFine]

No. If you have 120V two wire you can make 120/240V three wire with an autotransformer.

Of course, the line current will be double the load current.

 

[ggunn]

Of course, the line current will be double the load current.

Everything's a tradeoff.