I with steam boiler + generator theory

[khometmibro1]

Hi. My group and I did a practical the other day and we have to write a lab report on it using our data and the data of another group.

The steam boiler creates steam which is fed through a nozzle to a turbogenerator. The generator creates electricity, and this is fed into a loop with a potentiometer, like this:

http://i.imgur.com/auKVlzP.png

The strange thing is that for our group's data, the highest potentiometer setting gives the lowest efficiency. While for the other group's data, the highest setting gives the highest efficiency. AFAIK a potentiometer is simply a variable resistor.

What should be expected? If I know what to expect I will be able to understand who went wrong and where (probably). Thanks for the help.

 

[Hesch]

If you short circuit the load ( 0 Ω ), the load power will be measured 0 W, as the volt meter will show 0 V ( P = V * I ). Still power will be lost in the generator due to ohmic resistance in the coils, but these losses are internal, so the efficiency will be 0 %.

I vote for the other group.

 

[khometmibro1]

Fair enough, that makes sense. Any other replies welcome :)

 

[khometmibro1]

I have to ask another question: The efficiencies found both ours and other group's experiments are very small (<1%) while real steam turbines have much higher efficiencies (according to my googling)...

Would this have to do with the fact that our loads are also tiny, while real turbines would power whole cities?

 

[Hesch]

Would this have to do with the fact that our loads are also tiny, compared to the generators rated power

( My verbal alteration ).

Yes. In the generator there also are power independant losses such as friction and iron-losses. This means that if the rated power is not made use of, the efficiency will fall.

 

[khometmibro1]

Understood. Thankyou.