Transformer Winding Calculation

[polashd]

I've got laminated EI iron cores (3cm X2.5cm) from an old transformer (which was burned out).
I re wineded it as follows:
-primary 630 turns of 30AWG for 220v mains (I don't have much knowledge to determine the proper volt to turns ratio & wire size)
-secondary with 20AWG, taped for multiple output eg. 5v, 12v, 24v, 30v (ratio calculated according to primary ie. 600/220v)
when i connected the primary, output from secondary were near the calculated values (with no load).
But it makes a low buzzing sound. After couple of seconds the primary starts to heat up and smokes starts to come out of it.

note: much of the varnish i applied for lamination wear out during re-construction & the cores are not very tightly packed.

Please help me understand the mistake(s)
-Is the winding causing the main problem or it's caused by the core re-construction alone?
-How much output(max Power, Max Amp) can i expect if it is corrected?

 

[alec_t]

Sounds like you're overdriving the primary and probably saturating the core.
Not knowing your core properties I would guess you need a lot more turns on the primary (and hence on the secondaries). IIRC, the last tranny I dismantled (I don't remember its rating) had about 7 turns/Volt.
As for power, look for transformers of similar physical size and construction and note their power ratings.

 

[ronsimpson]

I usually tell people to not mess with the primary. The 220V or 110V primary was designed for power line and a certain power level and you don't know enough to change things.

Modifying the secondaries is relatively common.

 

[JimB]

primary 630 turns of 30AWG for 220v mains

There is your problem!
That is only 3 turns/volt, as a very crude rule of thumb, you probably need 6 turns per volt or may be even 10 turns/volt on a small transormer like that.

I have just found some scribbled information which will give you a guide...
Measure the centre limb of the E section of the transformer core, and calculate the area in Square Centimeters.
Now you can calculate the turns/volt using the formula turns/volt = 45/Area

Try it and see what happens.

JimB

 

[polashd]

There is your problem!
That is only 3 turns/volt, as a very crude rule of thumb, you probably need 6 turns per volt or may be even 10 turns/volt on a small transormer like that.

I have just found some scribbled information which will give you a guide...
Measure the centre limb of the E section of the transformer core, and calculate the area in Square Centimeters.
Now you can calculate the turns/volt using the formula turns/volt = 45/Area

Try it and see what happens.

JimB

Thanks to all
I got the idea. I'll re-wind

 

[JimB]

Earlier I said:

Measure the centre limb of the E section of the transformer core, and calculate the area in Square Centimeters.

I just want to emphasise that the area required is the cross sectional area of the centre limb of the assembled transformer core.

JimB

 

[schmitt trigger]

Please Google "transformer EMF equations".
Depending on your core cross section area, applied voltage and frequency, and the desired flux density, you'll CALCULATE and NOT GUESS the proper primary turns.

For steel lamination transformers, a flux density between 1.3 to 1.6 Tesla is correct.

 

[Ramussons]

A thumb rule from yesterdays...
Wattage = 30 VA per square inch, proportional to Square of the area.
Primary winding = 7 turns / volt.

 

[MrAl]

Hello,

I agree with JimB that you need to tell us the dimensions of the center leg of the transformer. That tells us the cross sectional area of the core from which we can calculate the minimum number of turns required for the primary.

The formula can be broken down for 50 Hz as:
N=(22.5*E)/A

where
A is the center leg cross sectional area in square centimeters,
N is the number of turns on the primary,
E is the maximum RMS voltage assumed to be sinusoidal.

This assumes you are using regular EI laminations used for 50 or 60Hz transformers.

This formula computes the absolute minimum number of turns however, so a good practice would be to use at least 10 percent more turns:
N=1.1*(22.5*E)/A

The wire thickness (wire gauge) must be able to accommodate the required primary excitation current plus the required secondary current reflected back to the primary. For a step down transformer of 10 to 1 for example, the reflected secondary current would be 1/10 (one tenth) of the secondary current, so with a 10 amp secondary current the primary would see 1 amp. The secondary wire must be able to handle the secondary current.

This last requirement also means that there is a constraint on the maximum power that the core can be used for, because of the size of the window area vs the required wire gauges for primary and secondary.

Also, it is customary to increase the number of primary turns to counter the voltage drop in the coils when the transformer secondary is loaded with the full load current. This is based on the resistance of the two windings.

To use the formula at 60 Hz, multiply the calculated number of primary turns by 50/60. At 400Hz this factor would be 50/400.

 

[gary350]

There are formulas to do what you want. I worked with transformers 40 years ago I don't remember the formulas so you need to look them up online.

First thing you need to do is know the dimensions of the EI core. Plug them into the formula. You need to know primary winding voltage and area of the opening in the EI core to determine if wire size and diameter will fit in the space available. You can not change the number of turns but you can change wire size to make it fit in the core holes.

You need to calculate secondary coils to make sure they fit in the space that is available in the EI core with the primary. You might need to make adjustments in wire size so both coils fit and produce the same watt rating.

You should probably do the math on the EI core you have then use what you know about the primary coil to determine some things about the transformer. You should be able to determine the voltage rating of the primary coil, then you know if 220 vac is correct. You need a coil winder if wires are not perfect vibration will rub off the enamel coating and the primary will short out.

There has been several table top hand crank coil winders on ebay priced very reasonable for $40 to $70 and on up to $500.

**broken link removed**