Transformer Impedence Matching.

[Lord Loh.]

Is it necessary to calculate the impedence matching in a simple step down transformer considering the line frequency ?

Will a transformer designed for 60 Hz fall out on efficiency at 50 Hz ?

How is the current rating of the transformer detirmined ? by the thickness of the wire or the number of turns ?

To make a step down from 220 to 110 one could either do it by having 2 turning on the primary and 1 on the secondary or 2000 on the primary and 1000 on the secondary....What is the difference ? efficiency ?


Please Help!!!

 

[Kral]

Lord,
Maximum power transfer occurs when the load (secondary) resistance , reflected back to the primary through the square of the turns ratio, is equal to the source resistance. This is not a consideration in power supply circuits.

Yes. Power transformers are designed so that the flux density at the design frequency is just below the point where the core material begins to saturate (The B vs H) curve starts to flatten out. As the frequency is lowered, the flux density starts to go into the saturation region, and the excitation (no-load) primary current becomes excessive. If you operate a 50 Hz transformer at 60 Hz, you suffer only a minor loss of efficiency due to increased core loss.

The current rating is determined by the wire size.

Selection of core size, number of turns, wire size, etc is a somewhat iterative process. For a given core size, the required number of turns is determined by the flux density at which you want to operate. Once this is determined, you calculate the number of turns required to achieve the desired flux density at the desired operating frequency. You then select the largest wire size that will fit into the core "window". If the maximum wire size is too small for the desired current rating, then you need to choose a larger core size, that allow you to get the desired flux densit with a smaller number of turns. Core manufacturers, such as Arnold, Magnetics, Inc, have design tables in theri catalogs that greatly simplify this process. Also, see "The Radio Engineer's Handbook", published by ITT.
Regrads,
Jon

 

[flatulent]

One thing to keep in mind on the AC mains is that each user does not want maximum power transfer. They would not know what to do with the MW or GW they would get. Each user wants a specific power at maximum efficiency of transfer from the generating station. This is why the load impedances are much higher than the generating source impedance.

 

[homefree]

Load,
The Farady's law is used to calculate AC-line transformer:

B=E*10^8/(4.44*Ac*F*N)

B is operating flux density, depends the core material, you may arbitary choose the value, for example, 0.8*Bmax of core is a good start for high power ones, and 1.2*Bmax of core is used for small power. However, it should be trimmed to meet your requirement.

E is your AC-line voltage, RMS.
Ac is core's cross section area, cm^2
F is your line frequency.
N is primary turns, if the E above is AC-line input.

You now have 3 parameters, ie: B, Ac, N to play around.

In practical, I will suggest to fix the B value, and adjust the Ac to find N.

And then, check core loss, wire loss, temperature rise, regulation, efficiency etc, until they all meet your requirement.

Use B to final/fine tune your design until core lose is prety close to wire lose.

It realy bothersome, and there are lot of software may help you to design such transformer.