True sine wave generators

[jfm]

How does a generator make DC? I mean from a physics/engineering point of view?

I can understnad how a cheap geenrator doesn't give a perfect 50hz sine wave, but i can't see at all how they make DC from a rotating alternator. Please explain?

Or can you post me a link to a IC engine powered generator that makes (directly) DC?

 

[jfm]

Ah, ok, I can see the logic of that thanks Wiggo. If they don't govern the RPM then the outpout of the gen will be a pure sine wave but not 50Hz. I can see, as you say, that a cure to that is to rectify the AC into DC, then use an inverter to make the DC back into 50Hz AC. I'm amazed though that such a solution could make sense, given how cheap electronic control now is to get the engine running at a perfect 1500 or 3000rpm (to give 50Hz). Certainly the weight of a rectifier and an inverter would be more than a speed governor. But if that's what these gensets do then I'm happy to believe it and stand corrected (and quietly shudder at such poor engineering!). Thanks

 

[nimbusgb]

You could generate DC by using a dynamo rather than an alternator. You get pulsed DC or 1/2 wave rectified 'varying voltage' but its still DC nevertheless. With a large enough capacitor ( a Battery ) on the output you'd see DC.

 

[Garryt]

The article was actually about "generators" in the sense of mechanical rather than inverters.

You're right in that it wasn't obvious what it related to. I should have put this link http://www.mobilesatellitetv.com.au/power-generators.html in instead!

 

[jfm]

Ok but now we're getting into semantics. If you want to call a half sine wave from a dynamo "DC" that's perfectly fine (and I accept the current travels in one direction only) but what I meant by DC (as I hope was clear from the context of all this discussion) a flat line "proper" DC. Your comment that adding a high capacitance would smooth a pulsed voltage is perfectly true but that's not the point becuase you're adding another component

I think wiggo cleared up my doubts. He said (in effect) that these cheapie gensets actually make pure sine wave AC but not at 50Hz becuase there is not good governance of the engine RPM. To cure the non 50Hz problem they rectify the AC to DC then invert (with a non pure sne wave inverter) the DC back to AC, at pure 50Hz. That makes me shudder in terms of its engineering kackhandedness but if that's what these cheapie gensets do then I stand corrected/informed (and amazed!)

Much better to pay a few quid more and get an genset with an engine governed to 1000/3000rpm imho

 

[jfm]

[ QUOTE ]
The article was actually about "generators" in the sense of mechanical rather than inverters.

You're right in that it wasn't obvious what it related to. I should have put this link http://www.mobilesatellitetv.com.au/power-generators.html in instead!

[/ QUOTE ]

Thanks. The article didn't actually say how all this works but Wiggo's explanation al;ready answered that. It was a pretty poor article, for example they say a simple (non inverter) generator has 3-5% inaccuracy level in making the 50/60Hz frequency, which of course was right in about 1973 but is wrong in the world of electronic controls where +/- 0.5% is very easy to achieve

 

[dhobyb]

Ah Ha, spotted my mistake, what I should of said is that an inverter changes DC into a switched voltage which has the same properties as AC, but in no way does it replicate a true sine way, however the switched mode power supply within the appliance does not need a perfect sine wave, as it works on rectifing the voltage transitions. therefore a switch mode power supply does not need a perfect power source to convert AC into DC.
and I am going to stop now as this could end up in a very technical discussion on AC/DC theory. Needless to say if you purchase a generator, or even a battery and invertor, as long as they have CE marks you should be OK. ( and the beast is sized correctly)

 

[gary111160]

I know nothing about electrics, but I have run a red colour suitcase type kippor generator on my boat for the last two seasons. Cheap to buy and fairly quiet. It has run tv/dvd player, electric hob, microwave, laptop, toster, kettle, battery charger and immersion heater all without any problems. Not all at the same time I may add!

Cheers Gary

 

[Dave_Snelson]

Well I went and bought the.....

.....Kipor!

Coz its TSV and much quieter than the industrial cradle type gennies that are cheaper but more powerful.

The quietness was a big selling point.

Thanks to all for this debate - it was excellent and I got much more out of it than I bargained for. I am a more educated man today than I was yesterday (not difficult, I know!).