The current rating(derive from VA rating) will not be vallid then loading the transformer with a cap. Say in a typical voltage regulator or power amp setup. With a bridge and a filter cap.

 

The heating effect (charge pulse) is a mainly a concern for derating when the load is of a virtually continuous maximum nature or a high duty period up to maximum current.
In has been my experience that most applications do not run at continuous VA rating.
The charge pulse current decreases as load current decreases.
Max.

 

What are implying t0..?

Ok. So the last transformer I got.

Need to find it's VA rating.

But first I found this.
It is an Audio amp former. It has 220VAC and 240VAC input.
I checked the secondary voltage.

I found that at 220VAC input I get ±29VAC
and when I switched the input selector to 240VAC and apply 220VAC I get ±26VAC.

I guess this cause another winding is switched in to circuit and the turns ratio changed so the output changes.

I will leave the VA rating for tomorrow and will post the wire gauge and core size.

I want ±35VDC or so, so with ±26VAC I can get it.
Problem is I can get this at 240VAC voltage selector setting. I can only apply 220VAC.
Could this cause an issue with former operation ?

 

What are implying t0..?

I want ±35VDC or so, so with ±26VAC I can get it.
Problem is I can get this at 240VAC voltage selector setting. I can only apply 220VAC.
Could this cause an issue with former operation ?

When a transformer is sized as to VA requirements and a rectified and smoothed DC is required, the size of the smoothing capacitor is selected, based usually on the desired % ripple at maximum current.
When the load is operated at maximum the charge pulse of the smoothing capacitor causes a heating effect that would normally be taken into account when sizing the desired transformer VA.
This is mainly a concern if the maximum DC load is operated on an almost continuous basis.
The larger the Cap, the higher the energy required for the charge pulse.
There is a formula for doing this, I have it somewhere.

Operating the primary on 220 on the 240v winding should not be an issue.
Max.

 

I know I have posted this before. But can not find the thread

 

I have a paper published from the U. of Maryland that was the results of Lab experiments and they came up with the required formula.
The bottom line is, keep the smoothing cap or target % ripple to as small a value as possible.
Max.

 

t0... are those documents in public domain if you know ? I did a quick perousal, and think I would like to print them for future reference...

 

The Yamaha former has two lotta secondaries.
One 5.1VAC for μC regulator.
One 9-0-9 VAC
One 15-0-15 VAC
(these I believe are for various audio preamp's and DAC's)

Main Amp secondary actually is ( at primary 220VAC setting ) 42-29-0-29-42 VAC
What I have in mind is take the secondary and rewind it to 25-0-25 VAC with a bit thicker wires.
I believe I can get better current capacity and regulation, won't I ?

 

Thanks t0. The documents are very informative.
I will go through thoroughly.

 

t0... are those documents in public domain if you know ? I did a quick perousal, and think I would like to print them for future reference...

Yes they can all be regarded as free handouts from those who has published them. They are all meant as a design help for designers

 

What I have in mind is take the secondary and rewind it to 25-0-25 VAC with a bit thicker wires.
I believe I can get better current capacity and regulation, won't I ?

You may want to wind this bifilar, that is start at the common end of both 25v windings and then wind the two together in pairs to keep them evenly balanced.
Max.

 

Crap ! I forgot a bifilar thingy. Thanks for reminding me.

So I figured bifilar winding can be parallel or series connected if desired. Right ?

So how is 42-29-0-29-42 VAC wound ?

 

Right, as long as 4 ends are brought out as the connections will be different.
42-29-0-29-42 would usually be bilfilar and the 29v taps brought out as it is wound.
IOW two 13v overwinds carried on.
Normally there is not so much flexibility as the junctions would be under winding and brought out as a single conductor for each connection.
Max.

 

I can see the junctions they are brought out and back in

 

You may want to wind this bifilar, that is start at the common end of both 25v windings and then wind the two together in pairs to keep them evenly balanced.
Max.

My Error: I noticed I implied that you connect both the starts together, where for a 25-0-25 etc the end of winding A will connect to the start of winding B.
Connecting both starts would be a for a parallel winding.
Max.

 

I figured that much

 

The Yamaha Transformer

Primary wire gauge is AWG #22
Main Secondary wire is AWG #15

Core.
Height is 99mm ( I or E laminate length )
Across is 82mm ( I and E laminate )
Lamination depth is 64mm ( no of lamination's )

Primary 220VAC and 240VAC Winding ( switchable)
Secondary 42-29-0-29-42 VAC at 220V input (220V switch selected)

So what can be the VA rating?

 

This the Tx in the second picture
Mind you tht this the former that gave the BANG! and scared the living daylight out of the guy next to me )

I can only see the secondary. Gauge is AWG #14

As I can see two bobbins looking at the picture I will address as right and left bobbin.

Each bobbin has two secondary connected in parallel. The Secondary was already connected in series. Total of two in parallel ( in each bobbin ) series connected. From the looks of it they are the inner most winding.

Voltages are measured with my fluke

So I connected my Variac to the Secodary ( connected in series so each secondary sees { and measured } ) is 66.6VAC

Total Applied voltage is 133.6VAC.
Total current taken is 1.1 Amps.

The two Orange wire output is 39VAC.

The twisted cables are I believe identical windings from each bobbin.
Color is black - black/red - black/yellow & blue - blue/red - blue/yellow.

Voltage are same for each bobbin.
Voltages are :
black - black/yellow & blue - blue/yellow is 126.6VAC
black - black/red & blue - blue/red is 105.7VAC
black/yellow - black/red & blue/yellow - blue/red is 20.9VAC

The two bobbins are separate only the secondary connected in series.
So I figured the black - black/yellow & blue - blue/yellow is the primary winding and should be in series. Right ?

The core size are
H= 160mm
B= 95mm
Lamination depth = 56mm

So what do you think the VA rating would be ?

{ed}

Found it So I phased the twisted wires.
Left the red strip out. Found the start and end of windings. Series them and gave 220VAC. It takes 200mA.
Secondary is at 60-0-60 VAC ...SWEEEEEEEET !
Orange has 34VAC.

Need the VA rating some how ..

 

Need the VA rating some how ..

Going by past experience with that design I would put in the 1 - 1.5 KVA range.

 

Unfortunately there is virtually nothing in the way of a manufacturing formula to decide the sizing of a transformer, it usually has to be discovered empirically.
IOW apply power with and a load to the secondary and monitor the voltage and heat dissipation.
Increase load until voltage decrease and heat rises to just short of an estimated limit.
Also comparing core size with a known stated VA on an existing transformer.
Also as stated earlier, the nature of the load, DC bridge with large smoothing Cap has an effect on selecting a required VA.
I would est around 1kva also.
Max.