[sdiy] Transformers

[John L Marshall]

There have been several questions about transformers recently.

Transformer primer:

V-out = V-in (N-out/N-in)

The voltage ratio (V) corresponds directly to the turns (N) ratio in a perfect transformer.

I-in = I-out (N-out/N-in)

The current ratio( I) corresponds to the inverse of the turns (N) ratio in a perfect transformer.

P-out = P-in*eff.

The efficiency (eff.) for small transformers may vary from 70% to 95%.

Z-out = Z-in (N-out/N-in)^2

The Impedance (Z) ratio is the square of the turns (N) ratio in a perfect transformer.


Many small transformers are wound approximately 4 (3 to 5) turns per volt. If you can accurately measure the DC resistance of the wire and determine the gauge of the wire in the winding then you can tell how long the wire is by using a wire table. From there the number of turns can be determined  by approximating the circumfrence of each turn for the windings.

A good approximation rule to remember: #10 wire has a diameter of approximate 0.1 inches and has a resistance of approximately 1 milliohm per foot. Each three gauges up will double the resistance and halve the cross sectional area (not the diameter). Each three gauges down will halve the resistance and double the cross sectional area. Approximately.

Use a variable voltage transformer (Variac) for testing unknown transformers. A common light dimmer will generate harmonics that may contribute to additional transformer heating.

If you measure the primary current while increasing the primary voltage the current will approximately linearly increase with the voltage. However, if the current sharply increases then the transformer has reached saturation.

It is a good idea to let an unknown used transformer "cook" with no load. The transformer should get warm but not hot. If the transformer gets hot there may be shorted turns. Time to recycle the transformer.

Measure the size of the secondary winding wire. Allow about 1 Ampere load for each 700 (500 to 1000) circular mils of wire cross sectional area.  

The Volt*Ampere rating of a transformer is related to the transformer core cross sectional area:  0.4sq.in. = 6VA, 1sq.in. = 45VA, 2sq.in. = 120VA. The cross sectional area is the iron area directly inside the winding only. I have not seen a simple formula for iron area versus VA rating.

Transformers are rated in Volt-Amperes not Watts. The transformer must supply full current when the load is reactive.

Avoid transformers with primary and secondary bobbins on separate paths of the core. These transformers are probably, self-regulating, ferroresonant type. If you look carefully you will find an air gap in the magnetic path. There is a special secondary winding that requires an AC capacitor.

I think the question was asked if a transformer designed for 230 VAC will work on 120 VAC. The answer is yes, but. The copper losses will be greater. The primary of seconday windings may not support additional current. So, a transformer designed for 230 VAC to 24 VAC if used on 120 VAC will output 12 VAC but will not supply double the current.

Universal transformers have a split primary that is used in series for 230 VAC or in parallel for 120 VAC.

I have successfully misused transformers. I have used telephone hybrid transformers and repeating transformers as power transformers. This is not efficient but it works.

I arranged for the utility company to supply instrument transformers to power a cellular telephone base station directly from 110kV high tension lines. An unusual approach but it did solve the power problem.

Take care,
John

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20041003/3b89a3f7/attachment.htm>