Transformer connections and load sharing

[abrahamJP]

We need a 30W,24v for a toilet device.We have a 240V supply in Qatar.But
our supplier who is in US has send us 2nos of RATING 120V/24V 50VA.

Is there any way by interconnections I can make it 240V/24v.

thanks in advance


Abraham

 

[rhatcher]

You can connect the two transformer primaries in series and take the 24V output from the secondary of one transformer.

 

[waross]

It may work better if you put the primaries in series and the secondaries in parallel. Check the polarity before making the last secondary connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Rhatcher,

Are you sure? What about amp-turn balance in the transformers? The unloaded transformer will behave as a reactor.

Bill's solution would work if the transformers are matched.

 

[rhatcher]

ScottyUK,
Well...After reading your comment, I am not sure. That being the case, I withdraw my solution. Thanks for stepping in.

I do agree that Waross offers a good solution. With matching transformers and proper polarities, this should work well.

 

[mikekilroy]

I love the KISS principle and vote for rthatcher's simpler solution as best here: primaries in series and just one ONE secondary, no paralleling of secondaries since not required for the 30w load. I am certain there will be no issue with the one fmr not being loaded; it will still drop 120v across its primary. If OP wants to be sure he can simply do it and measure voltage across each coil to verify all is well.

 

[mikekilroy]

I wish there were an edit button for too early in the morning...... Now that I think about it more, you may be right SkottyUK.... You have my curiosity up - I will hook a couple up this afternoon and check it if OP doesn't do it first and post result?

 

[waross]

Except for that darn inductive reactance. It may be greater on the unloaded primary with the result that the voltage drop across the loaded primary may be less than 120 Volts. As the load is increased the voltage drop across the unloaded primary will increase until the coil saturates and further voltage increase is limited.
Amp turns balance says it all. Thanks Scotty.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Yep, I'd go with Scotty.

 

[Compositepro]

Having the primaries in series would mitigate the concern about circulating currents currents in the paralleled secondaries, would it not?

 

[davidbeach]

Should. Having the primarys in series but only using one secondary would be like leaving a CT open circuited. Not pretty.

 

[rhatcher]

mikekilroy - You are not the only one wishing for an 'edit' button. In my opinion, ScottyUK is correct and my solution was wrong. However, I am curious about the results of your experiement.

 

[mikekilroy]

I have not been able to find 2 identical small xfmrs in junk box that I thought I had... Having thought more about it, I too have come around to SkottyUK's side To get more current flow into primary due to secondary load, its effective impedance would drop so more current flows.... so it will end up with lower Z than the unloaded one, so the voltage across the loaded one would drop. I still want to find a couple xfmrs and do it tho.

 

[waross]

It may be interesting to try it with two similar transformers. The secondaries tied together will set the voltages across the primaries in the inverse ratio of their turns ratios. The transformers must be close enough that neither goes into saturation. If you use 220 V or 240 V primaries in series across 120 Volts, you will have lots of room for voltage swings before the saturation point.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

The point of my previous post is that the transformers may not need to be identical. Another poster wishing for an edit feature.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikekilroy]

Its nice when theory proves out facts.

75va & 500va xmfrs, 240/460 pri->120v sec. Wire 120v across both H1-H3 (240v)(ignored H2&H4) pri in series, no load produced:

Vpri Xfmr NL sec comments
83 .......75va .... 43v....68 ohm....Rpri (I did not measure I or L or calculate Z)
38 .... 500va .... 19v.... 5 ohm ....Rpri (I did not measure I or L or calculate Z)

Put 60watt bulb across 75va xfmr for 15va load:

Vpri Xfmr NL sec comments
3 ......... 75va .... 0.5v
120 .... 500va .... 61v

Tied sec in parallel (phased properly):

Vpri Xfmr NL sec comments
60 ....... 75va .... 30v
60 .... 500va .... 30v
secondary recirculating current: 0.8ma

It also makes sense that I got the full 60v across the lite in series with the secondaries wired out of phase.

 

[waross]

Is that 0.8 ma circulating current or the contribution of the lessor transformer? At no load it would be circulating current. With a load, it may be circulating current or in phase contribution to the load current.
Thanks for the feed back.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikekilroy]

.8ma no load

 

[rhatcher]

Nice job MikeKilroy.