understanding transformers

[electrondady1]

recently there was a rather heated debate in regards to battery charging voltage.
one solution offered up was to run alternator output into a transformer.

i don't know very much about transformers .
i know there is a primary and secondary winding.
without delving into the complexities of multi phase transformers, my question is,
 what is a good  relationship or ratio  in regards to the resistance of the primary winding to
the resistance of individual phases .

i am curious about how the alternator /windturbine reacts to having only the primary wingdings as a load.

am i correct in thinking that the alternator would see the primary wingdings as a constant load whether there is a load attached to the secondary wingdings or not?

[joestue]

Both primary and secondary coils should weigh the same.
For a 1:1 transformer at 90% efficiency and a turbine at 90% efficiency, assuming no iron loss in the transformer or alternator: the resistance of both the primary and secondary should be 1/2 the resistance of the alternator.

it is cute to model the alternator as a constant volts per Hz
but this is not the case, it is highest at start up and decreases under load due to both resistive and reactive losses in the turbine.

If unloaded your turbine produces 2 volts per hz, it should be able to start up when connected to a standard non saturating 120vac 60 hz transformer.
I say non saturating because if it consumes more than 2 watts per pound of iron(at 120v60hz), it's partly saturated.

the load presented to the turbine by a sufficiently idea transformer is negligible after start up

[kajs]

am i correct in thinking that the alternator would see the primary wingdings as a constant load whether there is a load attached to the secondary wingdings or not?

as far as i understand - no. in transformer the electrical resistance is transformed into resistance to changing magnetic field. without load on secondary winding the current in primary will go high.

[joestue]

am i correct in thinking that the alternator would see the primary wingdings as a constant load whether there is a load attached to the secondary wingdings or not?

as far as i understand - no. in transformer the electrical resistance is transformed into resistance to changing magnetic field. without load on secondary winding the current in primary will go high.

if english is your second language please use symbols instead of defining inductance as "resistance is transformed into resistance to changing magnetic field"
which is difficult to read.
That last part isn't correct though.

[kajs]

if english is your second language...

i apologize for that.

but how am i not correct about high current?
if I=V/R, then where R->0, I->infinity. Unloaded transformer is basically a short circuit.

[DamonHD]

An unloaded transformer secondary would be much more like an *open* circuit on the primary.

Rgds

Damon

[Tight Yorky]

Hi guys,

Damon is correct in that an open circuit on the secondary results in a low primary current. But it is not quite open circuit. As there is no secondary current to affect the magnetic circuit in the transformer the primary acts as simple inductor, or choke. However, the current in the primary is frequency dependant. That is at 50Hz or 60Hz it provides a high impedance with a low magnetising current.
However, if the primary was connected to an alternator spinning slowly (low frequency) the result would be a higher current. A high current can easily saturate the magnetic circuit of the transformer. Try searching for 'ampere-turns' on the Net.
Just as a bit of further info, this is a reason for never stopping a generator 'on load', always disconnect items such as tool transformers.

Hope the above helps.
Clive.

[JW]

Mutual inductance and basic operation
http://www.allaboutcircuits.com/vol_2/chpt_9/1.html

JW

[electrondady1]

any one ever run a capacitor in parallel with a the primary winding as in an LC circuit?

[joestue]

any one ever run a capacitor in parallel with a the primary winding as in an LC circuit?

there is little point.
the capacitor can cancel out the inductive load of the air gap but it can't cancel out the harmonic load the iron presents to the alternator.
you might be able to cancel out half of the magnetising current at a given frequency, but as the freq rises the capacitor will suck up a lot of VARS

[electrondady1]

often transformers are required to supply several output voltages .
i believe these are called taps.
does each of these taps require a separate secondary coil or are the taps split off one larger secondary coil at the point where the required voltage is achieved .


 

[DamonHD]

AFAIK either is possible.

Rgds

Damon

[rossw]

often transformers are required to supply several output voltages .
i believe these are called taps.
does each of these taps require a separate secondary coil or are the taps split off one larger secondary coil at the point where the required voltage is achieved .

In litteral terminology, multi-TAPPED is a single winding with taps to access more or less of the turns.
Transformers also frequently have multiple secondary windings.

A transformer with one winding, lets say its norminally 20V, and has a tap at 15V, you can get 15V, 20V or 5V from it.

A transformer with one 20V winding, and one 15V winding, can give you 15V, 20V, 5V or 35V.

Multi-tapped windings are generally a low cost and efficient way to get multiple output voltage options.

Frequently, transformers are made with taps on the primary side too, for example in my part of the world for 220 or 240 V operation.