help needed for engineering college project

[mahone]

hi all, we are doing a college project and need to design a step down transformer CAD only. what has me stumped is that we need to include the minimum resistance needed for the primary to be used on single phase 240ac.

can anyone help? if you can include the actual equation it would be a tremendous help. I remember that there is such a spec from the dim and distant past, but Google turns up nothing :-(

also because its design only and we don't need to build it, I need the figures as they would be in a "perfect world" in other words I don't need to take into account what adjustments I would need for the real world, I need the textbook calculations.

many thanks to anyone who helps.

 

[BobK]

Why would there be a minimum resistance?

Bob

 

[(*steve*)]

Why would there be a minimum resistance?

Of course there's a minimum resistance. One goal should be to achieve that minimum resistance.

Of course, that may conflict with other requirements so there may be some trade-offs and you might need to compromise.

That minimum resistance is 0 ohms.

 

[KrisBlueNZ]

The current drawn from the mains is not limited by the DC resistance of the primary; it's limited by the inductive reactance. X_L = 2 pi f L.

 

[BobK]

Of course there's a minimum resistance. One goal should be to achieve that minimum resistance.

Of course, that may conflict with other requirements so there may be some trade-offs and you might need to compromise.

That minimum resistance is 0 ohms.

When someone says what is the minimum resistance needed too ...? I assume he means that any resistance above that will be OK. That makes no sense in the context of the OP.

Bob

 

[duke37]

Should it be maximum resistance?

Does the project include power losses?

 

[mahone]

thanks all, no it doesn't include power losses, hysterisis, eddys or any of the factors involved in actual design. its purely a design exercise.

I remember some time ago when I built one as a hobby that the primary needed to be a certain gauge/length (also resistance/load) to work on uk mains without tripping the breaker or any other type of problems. we want to include it in the design parameters, and I can't for the life of me remember where I found the information :-(

also as another question can anyone tell me whether silicon grade or ferrite would be more suitable for the core?

 

[BobK]

As Kris already said, the current in the transformer primary is not limited by the resistance of the coil, but by the inductance. If you read otherwise, it was wrong, which is probably why you cannot find it now.

Bob

 

[duke37]

For mains frequencies, use 4% silicon iron. For high frequencies use ferrite.
The number of primary turns will depend on the size of the core and the inductive current should be perhaps 10% of the load current.

You will need to find a transformer design book.