If I need to generate power at a specified frequency, then I need to make sure that the rotor of the generator rotates at a specified speed (rpm). But when I am rotating it with steam or water how do I control this speed? It seems to me that the mechanical forces that rotate the generator somehow has to balance to achieve this. How exactly is this done?

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    With some sort of throttle on the prime mover, adjusted so that power in equals power out (plus internal losses). – Dave Tweed Nov 8 at 13:35
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    I'm pretty sure that large generators attached to mains are forced into synchronicity by the influence of all the other things attached to mains; if the generator slows down any, there would be a motor torque applied by the mains power pulling it back towards synchronous speed, and likewise if it speeds up. This is a problem in itself rather than a solution, though, because if your generator starts running as a motor it's likely to tear itself or things attached to its shaft apart; it's not made to operate like that. – Felthry Nov 8 at 13:41
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    Wasn't the flyball governor on Watt's steam engine essentially doing this through mechanical feedback? – DaveInCaz Nov 8 at 17:58
  • The faster the generator tries to spin, the more current you could draw from it to make it slow back down. That requires a variable load of course. – immibis Nov 9 at 0:58
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    This is awfully broad for a question. In sum, your options are to either vary the incoming power, vary the load, or forget about frequency regulation altogether and do something else (use a DC load, a load which doesn't care about frequency, or use a frequency transformer). I don't think it's possible to reasonably pick an option given that you disclosed zero information about your system. – Dmitry Grigoryev Nov 9 at 8:25
up vote 18 down vote accepted

Electrically

Some systems do this electrically. The generator either generates DC, or the variable-frequency AC is rectified to make DC, and then an inverter makes the desired AC frequency. Common on more modern small wind turbines.

Mechanically

Other systems are mechanically controlled to get the desired frequency. The mechanism used would be called a governor. Most simple mechanical governors are not very accurate, so this would not be good enough for a grid connected device. It is also possible to make more accurate governors which work mechanically in a similar way to the paragraph below, these are commonly used on internal combustion engines.

With Feedback

Another approach, and probably the most common is to have some form of feedback. A microcontroller monitors the frequency being generated, and adjusts the mechanical system via some form of servo to get the right frequency. For example, it could open and close a sluice gate to adjust the water flow through a turbine. A more complicated system could adjust both a sluice gate and the turbine blades to keep the correct frequency while also varying the output power.

Grid Synchronous operation

In some cases, it might not be necessary at all. If you have a small wind turbine, connected to the mains grid near a coal power station, you could just hook it up and forget about it. The huge turbines in the power station will stabilise the grid frequency, and fix the rotation speed of the wind turbine. If the wind blows harder, you'll just get more current, and a slight power factor shift. Note that as more and more wind turbines get added, the folks who run the power station will get less and less happy about this, so the grid operator will eventually ban it.

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    It's probably worth mentioning doubly-fed induction machines here. They use an induction machine simultaneously as both a generator and a rotary converter to keep the output frequency constant with varying input frequencies. – Felthry Nov 8 at 13:45
  • @Felthry I didn't include them because I'm not sufficiently familiar with them. Feel free to propose an edit though. – Jack B Nov 8 at 13:47
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    I would, but Andy's answer already takes care of that. – Felthry Nov 8 at 14:11
  • Perhaps worth noting that in practice, multiple/many of these will be happening at the same time. Grid upsets will initially cause weird power flows/factors, feedback systems will respond after some delay, coarse level governors will apply/remove brakes if needed. – mbrig Nov 8 at 17:07
  • Small typo: inverted (inverter) makes the desired AC frequency – Hanky Panky Nov 9 at 8:16

If I need to generate power at a specified frequency, then I need to make sure that the rotor of the generator rotates at a specified speed (rpm).

No, that is not necessarily true. A lot of wind generators use a doubly fed induction generator (DFIG) and that can regulate frequency by controlling the rotor winding: -

enter image description here

They can convert power at one frequency to another frequency i.e. they can produce 50/60 Hz even though the rotor may be running too slowly. This is done by injection of an AC current into the rotor coils. The control system that acheives this may also be able to alter pitch angle of the turbine as another means of increasing or decreasing mechanical rotation speed.

For more information please read this EE answer.

But when I am rotating it with steam or water how do I control this speed?

There can be situations when there is too much rotational speed and although the DFIG can deal with this it is best to have a combined approach such as the pitch angle control shown in the above picture.

However, the inevitable bottom line is that if you have a plentiful source of mechanical energy and very little load demand at that moment, then you have to un-hook the generator from the grid. If you don't "have" a grid, then you have to have a back-up supply that can supply the low power demand and this usually means a diesel generator or solar power via an inverter.

That is what a governor is for.

The mechanical version is a device that will use centrifugal force or a blower to actuate the throttle/intake to slow down the engine when the speed climbs too much.

You can make it electronic with an RPM sensor and a electronically controlled throttle/intake.

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    Mechanically throttling the input with a brake generates heat and wastes input power.... – rackandboneman Nov 8 at 20:12
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    @rackandboneman he isn't talking about that, and that's not how governors work. – Harper Nov 8 at 21:23
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    @Harper to be fair some governors do work by using braking however for generators you would be adjusting the mechanical energy generated instead of wasting it. – ratchet freak Nov 9 at 1:27

In my (limited) experience of generator design, you have to look at multiple factors:

  • Speed of mechanical input (turbine, wheel etc)

  • Power of mechanical input

  • Output voltage

  • Output current

  • Output power (which depends on voltage and current, but you usually want to maximise this, at a peak power point)

In many cases what you want to maintain is a constant voltage output, which will vary with the electrical load; higher current would make the output drop.

In your set up, you only care about the speed of the shaft (for whatever reason that is). There are two ways to do this: control input power or control output power.

If you know that your mechanical power will always be more than the output power, you could use a governor or similar, which will limit the shaft power to keep the speed constant. This will control the input power in a simple manner.

If you can't guarantee that your mechanical input is higher, you'll need to limit your output power in some way. I have done it where we controlled the output current via a PID feedback controller to keep the shaft speed at fixed values. But that was in a DC system, where we had a large battery to push the current into during high supply times, and draw from during lower supply times.

This isn't the way it is done, but it's a way it could be done: vary the load in order to modulate the speed.

You can imagine a (potentially very large) auxiliary load bank in addition to the normal load, with the input power source sized so that it will provide at least enough power for the normal load under all conditions. This isn't completely unthinkable for hydro setups, since the head is the usual determinant. Then, as the load and the input vary, the auxilliary bank is controlled so that the total demand on the generator produces mechanical loading on the input which maintains the output frequency at the desired point.

And no, this isn't a serious suggestion, but it could be done.

  • So the auxiliary bank could be used to pump some water back uphill? Sounds stupid, but isn't that the same as charging a battery? – amI Nov 8 at 19:01
  • @amI: It's similar, but pumped-storage hydroelectricity is a smart choice because it costs far less than the batteries that would be required to store the same amount of energy. – davidcary Nov 8 at 23:56
  • Storage is good, but you would never power a battery charger off the battery itself (pump hydro using hydro power). I guess the aux load would need to be a 'controlled' customer (that is willing to accept <100% duty). – amI Nov 9 at 6:13

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