[Amps] Transformers

[Will Matney]

*********** REPLY SEPARATOR  ***********

On 7/15/06 at 6:45 AM Tom W8JI wrote:

>> Why does Heathkit show in their manuals the open circuit 
>> voltage, and the full load voltage that
>> calculates to 15% sag (OCV / FLV )? They show this in 
>> several of their models.
>
>Manuals are written from other manuals, and the goal was to 
>always be conservative. Manual writers at Heath were a 
>completely different group than engineers. I don't think I 
>talked to a manual writer more than a dozen times.
>
>> I should have worded this different. No they don't 
>> saturate under load.
>
>That's right, you should have worded it much differently.
>
>>However, the
>> amount of iron is what controls the power output in watts. 
>> Not enough iron and you have
>> a high flux density, a higher magnetizing current, and 
>> more sag.
>
>That's not correct. That still implies flux density controls 
>maximum power.


Well Tom, that's not what the 15-20 or so books I've read on the subject says, nor what the
manufacturers of the iron lams say either. I can quote these if you want. Are all these authors
wrong on the subject?

The lesser the amount of iron in the core at a given power, the higher the magnetizing current is, 
and surely a higher flux density. Also, the very first step in designing a transformer is calculating its 
core size from the output power in watts it needs to deliver. That's a fact, and it's in every 
book on the subject! The amount of iron in the core in square inchers (or square CM) is 
exactly what determines the flux density and the amount or power in watts it's capable of.
If there's not enough iron in the core when the current is applied to the primary, it can staurate.
If there's not enough iron in the core under load, it will get hot, and the voltage regulation will
be terrible. I can't change the laws of physics, that's just the way it works.


>
>>>Maximum flux density, which means closest operation to
>>>saturation, occurs with NO load. As load is increased flux
>>>levels do not increase. Increased current, because of
>>>resistive losses in the primary circuit, actually causes 
>>>the
>>>transformer's flux level to decrease. When a transformer 
>>>is
>>>designed the highest primary voltage under no load is used
>>>to set flux density at a safe level.
>>
>> correct
>>
>>
>>>
>>>The actual mechanism inside a transformer is the secondary
>>>develops a counter-MMF. This opposing flux would reduce 
>>>flux
>>>density, but primary current increases in order to try and
>>>maintain the **same**  flux density. When mains and 
>>>primary
>>>resistances carry more current from increased load, the
>>>primary voltage drops slightly. This REDUCES flux density,
>>>moving the transformer further from saturation. Not closer
>>>to it.
>>
>>
>> correct
>
>The idea flux density sets maximum available power is not 
>correct.
>
>73 Tom 
>
>
>_______________________________________________
>Amps mailing list
>Amps at contesting.com
>http://lists.contesting.com/mailman/listinfo/amps

Will