What determines the output of a generator at different RPMs?

[mellotango]

Hi,

I have a questions about generators and how they generate electricity. Why is that some generators generate large electrical power running at 3600 rpm, and yet some generate little electricity running at the same speed? And why are low rpm generators (e.g. 900 rpm or less) able to generate the same electrical power as a high rpm generator?

 

[Simon Bridge]

Depends on the number and strength of the magnets and the number and size of coils and the arrangement of each. Open some generators up and see.

 

[mellotango]

So let's say if I wanted to save costs and bought a small engine running at 3600 rpm, can one simply attach a large generator with many magnets, and large coils and generate huge amount of kwh? Seems to me it might not be a simple as that...

 

[AlephZero]

can one simply attach a large generator with many magnets, and large coils and generate huge amount of kwh?

No. If you could do that, you would have disproved the law of conservation of energy.

If your motor produces a maximum power of X kW, you can't possibly get more than X kW from the generator. In real life you will always get less than X kW, because the system will never be 100% efficient.

 

[Simon Bridge]

Sure you can build a generator as you like, you'd just have to find an energy source capable of driving it.

eg. the bigger the water-wheel, the more it can drive - but you cannot take more energy out of the stream than it can deliver. At some point you will get diminishing returns as the wheel gets too big for the stream to push.

You'll notice that your 3600rpm engine runs slower at higher loads? This is why engines are characterized by their power.

 

[mellotango]

I've been reading up more about this, and it seems torque is the issue. If a small engine is to drive large generator (both at 3600 rpm), it may not be possible as a small engine does not have enough torque to drive the large generator, despite both machines having a rated speed of 3600 rpm. As I discovered that power=torque x speed. Am I getting close?

 

[Simon Bridge]

@mellotango: it's related - if the big generator will generate 1000W at 3600rpm and the small motor only supplies 500W at 3600rpm, then the small motor will be unable to accelerate the generator to that speed. But it could probably run at less than half speed.

Torque is not the issue - it's conservation of energy: not just a good idea - it's the law.

@ahujajp: "Nigeria"? Anyway - the alternative to commercial radiators is to build your own. On the whole - you need to talk to an engineer in your own country, someone who can look at what you have and knows what is available. What you are asking is too general and is the sort of thing most of us would charge for.

 

[brewnog]

What's wrong with Nigeria?

Heat exchangers (using cooling towers as you say) are a common solution to avoid having to use radiators. Your engines won't like having water provided at 26deg so you'll need a heat exchanger between your cooling tower and your engine. You need to know the heat rejection from your engine (jacket water and charge cooler, if fitted). Your engine manufacturer will provide this information. You also need the engine coolant flow rate (also from the manufacturer). Then you can approach a local industrial cooling provider, but they'll need this information first of all.

 

[Simon Bridge]

What's wrong with Nigeria?

I don't know - never been. I get lots of emails from Nigerians who would like to borrow my bank account :)

 

[brewnog]

Well I've been (to deal with some generators as it happens). None of the Nigerians I met asked to borrow my bank account, and most of them were pretty ashamed that their national identity seems to be synonymous with fraud. Most of the Nigerians I met wanted to learn a bit more about generators...

 

[Simon Bridge]

Well I know but what can you do? If you met a guy who said he was Simon Bridge then he punched you in the face, and then another, the same, and another, and another, then I came along and introduced myself... well I would not be too surprised if you ducked. A little frustrated perhaps.

Similarly - when I get an electronic communication claiming to be from a Nigerian I have to double-take because there are just so many bad guys impersonating Nigerians for some reason. It is entirely possible that I have conversed regularly with actual Nigerians who have not let on because they want to avoid this reaction?

Admittedly it's not just "Nigeria" - every time a country likely to be obscure to Americans has some political trouble there's a flood of emails claiming to be from there. I blame the 1%

Interestingly, ahujajp's question seems to have found an engineer familiar with what is possible in Nigeria. I'm used to having more jerry-rigged solutions in NZ.

 

[Jobrag]

The speed of an alternator (generator of AC power) has nothing to do with the power out put but a lot to do with the frequency being generated, For a 50Hz system most alternators will run at 3000 or 1500 RPM for a 60 Hz system at 3600 or 1800 RPM ( you might get some monsters that run at 750 and 900).
The amount of power that you can get from the alternator depends on a few design constraints but the limiting factors are the voltage that you can generate, limited by the insulations between the windings and the current that the stator windings can cope with, limited by the cross section of the copper that makes up the windings.
The other limitation is the power of the engine (prime mover driving the alternator, if you had an alternator capable of generating 20 MW but connected it to a turbine that could only deliver 10 MW you would only ever get 10MW (or less) out of the alternator.