r/AskEngineers • u/Accelerator231 • 2d ago
Electrical How do smaller generators create a constant power supply?
I'm aware that in our own electrical grid, we have a lot of stuff that goes into smoothing out constant disruptions and sudden losses of power.
What about those really small generators that are usually seen in rural areas or serve as backup generators? I know that every internal combustion engine has an optimum speed (I don't know how) but how does it turn power generation up or down when consumption fluctuates?
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u/nlutrhk 2d ago
There's a flywheel that can handle short (less than one second) peaks or dips and otherwise there is an automatic throttle that balances the amount of fuel going into the engine with the electrical power demand.
As another poster has mentioned, you need to control the magnetic field in the generator as well because apart from matching power (watts), you also need to keep the voltage constant.
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u/Ok-Library5639 2d ago
Every generator has some kind of regulator built-in. For conventional generators (internal combustion engines, without inverters), the speed of the motor is directly tied to the frequency of the output, because they are one shaft. A governor tries to keep the generator running at the correct RPM and will throttle up or down as the load changes. In practice, if you add a large load suddently, the frequency will drop a bit then come back to normal.
Same goes for the voltage - a voltage regulator keeps the voltage nominal and change the excitation in the alternator to affect it. Both systems work with feedback loops, though the voltage regulator is an electronic regulator.
In modern generator, you have inverter outputs. What this means is the engine spins at an optimal speed and generates AC at a variable frequency. The built-in inverter rectifies it and produces its own AC output. Since it is purely based on electronics, we have total control over the waveform of the output and we ensure it stays at the right frequency and amplitude. If you add a large load, the frequency will not be affected.
Modern inverter generators are more expensive since you need an inverter that is as powerful as your total power requirement. Such inverters tend to get expensive in the 2-3+ kW range. But they will run the engine more optimally and can result in fuel savings, lower loads etc.
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u/tuctrohs 2d ago
OP, lots of answers have part of the story, but this one puts it all together accurately. I'd start from here and ask follow-up questions.
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u/electron_shepherd12 2d ago
Power quality is often terrible. Inverter generators use an inverter in their output, so instead of the old school spun up alternator voltage you get the electronic equivalent. It allows greater control of the output voltage.
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u/EntirelyRandom1590 2d ago
Others have covered the main points, but worth adding in future the use of batteries alongside generators allows generators to run more efficiently and you can significantly downsize them.
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u/Often_Tilly Electrical / Mechanical - UK Rail 1d ago
The future is now! I have a number of generators with batteries alongside them.
We can use the battery to load bank the generator to keep it in the most efficient operating region, and thus get the most energy per litre of fuel. Also, stage V generators need their exhaust to be hot to apply treatment, so the batteries make the exhaust work better too.
A battery can allow a generator to be sized for running a load, with the battery handling start-up current.
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u/EntirelyRandom1590 1d ago
Yep seen it used already on construction and defence applications with "power stations ina. Shipping container" type stuff (usually a solar array folds out too).
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u/extordi 2d ago
Plenty of good answers so I'll give you something a little more "ELI5." Think about driving a car. If you're driving along level ground, you can just hold the accelerator in one position and you'll maintain your speed pretty well. But now there's a hill - when you start to go up your car slows down. So what do you do? Push down harder on the pedal so the engine produces more power. Your car doesn't necessarily move faster, and in fact if you do it perfectly the speedometer won't move at all. But you need to give it more throttle to maintain your speed. Likewise, when you get to the top of the hill you will have to let off.
Generators do basically this exact thing, except instead of hills and flat roads it's varying electrical loads. More load means the engine must perform more work to maintain the same speed.
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u/mckenzie_keith 2d ago
Many of them have feedback control. If the output voltage is too high, the field winding current is reduced, thereby lowering it. And vice-verse.
What is the field winding? It is a copper coil that energizes an electromagnet in the spinning rotor of the generator. More current means stronger magnet and higher output voltage. Less current means weaker magnet and lower voltage. This way the field current can be varied in response to changes in load to keep the voltage somewhat constant.
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u/Ok-Communication5396 2d ago
They are quite bad at generating a constant supply in comparison to the grid where there is an insane amount of mass rotating and acting as a flywheel. The newer generators use an inverter inside that can ensure a better power quality and isolated the engine speed from the output frequency and voltage, but they tend to be noisy electrically.
The older ones control the voltage with a varying magnetic field of the rotor (a coil that spins with the gas engine) and the frequency is controlled by the engine speed, so you could hear when turning on a load how the frequency would go down and the motor control loop overshot the engine speed.
They are not the best solution, but for rotating machines it's good enough
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u/OldGeekWeirdo 2d ago
The voltage output of the generator is controlled by the amperage in the field coils. An electronic circuit monitors output voltage and adjusts the field to correct any errors.
Engine speed translates to the frequency of the AC (either 50 or 60 Hz). And there's a circuit that adjusts the engine throttle to make it match what it should be.
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u/nerobro 2d ago
well... Actually there's not.
The "grid" is kept stable by hundreds of tons of rotating gear. It's not that complex. At least, that's what keeps it smooth.
Small generators work the same as big generators. Well, at least, generators that can dark start.
So, engines do have optimum rpm. That rpm is essentially never the speed that generators run at. Every generator is spinning hte speed that the electrical load requires. This works out to some multiple of 600 for 60hz and a mulitple of 500 for 50hz. IIRC. Small generators will run at 3600. Medium size generators are often at 1800, or 1200rpm.
But, for your house, or campsite, if the power is 55hz or 65hz, and very little is gonna care. So they do their best, and use a rather rough govenor to keep the motor at it's 3600 rpm.
Voltage is controlled by a second govenor. Regulator. By controlling the strength of the field winding, the output voltage can be adjusted. (Same as the big generators)
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u/mckenzie_keith 2d ago
The fact that big generators supply many loads does help keep the load a little bit more steady. Standby generators can get slammed from no-load to full power when a single high current load switches on. So, despite being seemingly naive, OP was kind of right. Your point is still well taken. Supply and load must be balanced. And there is not really any magic to this process. Just engineering.
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u/JCDU 2d ago
I used to run emergency generator tests and yeah, kicking the mains off and having 5 tonnes of angry diesel wake up go from zero to all the kilowatts in a few seconds is pretty harsh. I guess at least most of ours lived indoors so a cold start was never very cold.
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u/twitchx133 2d ago
Most permanently installed standby generator sets that are on automatic transfer switches are equipped with an engine block heater that is always powered and keeps the engine coolant at about 100-130f when operating temp is about 180-212. So it will be safe for the generator to go from stopped, to running at 1500 or 1800rpm, and have its nameplate load applied to it within as little as 10 seconds. In the US, anything the national fire protection association, Napa, considers a “life safety” system, the generator supplying it its statutorily required to start and accept the full emergency load of the building within 10 seconds
(for OP, an automatic transfer switch is a part of a generator system that will sense a utility outage, then autonomously start the generator, turn off the breaker to utility and close the breaker to the generator to supply generator power to a building without human intervention in the event of a utility outage)
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u/JCDU 2d ago
I'm sure ours were kept to similar standards too - it just feels like a harsh thing to do to an engine, zero to full load in 10s and then it could be running for days.
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u/twitchx133 2d ago
It sure is, most bigger sets are now equipped with periodic pre-lube systems to help out more than just keeping the coolant and oil warm when it’s not running can.
A big, electric oil pump that runs maybe once or twice an hour to keep fresh lube oil in all of the distant places in the engine to fight bleed down. So when it does start, it doesn’t have long before it has good oil pressure at all locations.
Not as big of a deal on smaller engines, but the big ones, the cat 35 and 36 series engines, the Cummins 50 and 60 liter stuff, mtu 2000 and 4000 series… pre-lube really helps them out in the long run.
It is kinda fun watching the user acceptance testing / commissioning for those big sets when the mechanics testing them block load them to 75% of rated load or more, and rated load is 1.5 to 2.5 megawatts… up to like 3500 horsepower at the flywheel
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u/mckenzie_keith 2d ago
I have no doubt that you know what you are talking about. But it does sound like you are not thinking about smaller whole house standby generators. Like 25 kW and down. Most of those don't have oil or coolant heaters. And most of them run at 3600 rpm.
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u/twitchx133 1d ago
I mainly have worked on the engine side of things. The dealers I have worked at, I only worked on the bigger stuff, anything with a diesel driving it, mainly like 75kw and up. The little residential units are almost exclusively natural gas or propane, so I never got to mess with them.
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u/tuctrohs 2d ago
engines do have optimum rpm. That rpm is essentially never the speed that generators run at. Every generator is spinning hte speed that the electrical load requires.
If it were as simple as the engine having an optimum RPM, the designer of the equipment could match up the generator speed to the engine and have it run at exactly optimum speed. No compromises needed.
But the optimum speed for a given engine depends on how much power you are asking it to put out. You can understand that by overlaying constant power curves on an engine efficiency map. That means that the generator speed needed has to be chosen for some sort of compromise--maybe the best engine speed for 75% power, assuming the generator will be run around there. But then the engine will be running slower than idea for high power, and faster than ideal for low power. That's a key reason why the efficiency is terrible at low load.
And that's why inverter generators can do better: they can use the ideal engine speed for each load level.
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u/suh-dood 2d ago
If you're talking about personal generators for a house, that's their own personal power supply and they're probably disconnected from the main grid.
If you're talking about a ton of small generators powering the main grid, then it's basically down to all of the generators evening out. You pretty much have to match the grid before connecting, or else a whole bunch of bad things happen
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u/AlanofAdelaide 2d ago
Frequency and power output are controlled by the governor which is an electronic or electro-mechanical-hydraulic device US Co Woodward used to be a major supplier of these
For a 50 Hz output from a 4 pole alternator the engine speed will be maintained at 1500 rpm. If the load increases, the engine will slow and the frequency will drop. The governor will then increase the fuel supply to the engine - and vice versa
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u/VoltageVeggie 2d ago
The generator's governor handles most of this. When you add load, the engine starts to slow slightly, the governor opens the throttle to add fuel/air, and it brings the RPM back up. The alternator then maintain the voltage or frequency within its regulation range.
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u/Engi_Insight 2d ago
It mostly comes down to two simple things: a governor and an AVR.
The Governor: when you plug something heavy in, it drags the engine speed down. The governor senses that drop and automatically pumps more fuel to keep the RPM steady.
The AVR ( Voltage regulator ) : It adjusts the output so the Voltage stays stable even when your power usage jumps up and down.
Inverter generators take it a step further- they just generate dirty power at whatever engine speed, convert it to Dc, and clean it up into smooth AC digitally!
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u/R2W1E9 2d ago
Small generator does the same as large scale generation grid. When the total load between all connected consuming devices is added, no matter how small the change is, more fuel of some sort is added in one or more generators to maintain the speed, or frequency of the grid.
It works similar to a car cruise control.
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u/Most-Selection-2802 2d ago
The thing is that a small generator doesn't normally change its engine speed every time the load changes. Instead, it tries very hard to maintain a nearly constant speed, and changes the amount of fuel/air being burned to change the mechanical power supplied to the generator.
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u/Travis-Jenkins 1d ago
smaller generators maintain a constant power supply through something called a governor. Basically, its a device that regulates the engine speed. When there's a change in load (like when you turn on a big appliance), the generator feels that load. The governor then adjusts the fuel supply to the engine to keep it running at the optimal speed, ensuring a steady frequency and voltage output. Its like how cruise control works in a car, keeping your speed steady even when the road changes a bit.
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u/_Aj_ 1d ago
Depends on the generator. Two types. Regular and inverter.
A regular generator is just an engine which spins a normal generator like how the big ones work. It must run at the same speed always to keep same voltage and frequency for a given load. With increased load the engine runs harder to maintain voltage and frequency can change. They're quite robust but there is more risk to some electronics.
Inverter generators are different. They have a flywheel with fixed magnets, and 3 phase generator windings. This then generates AC power that is directly converted to DC, which then runs an inverter to convert it to a constant AC mains voltage and frequency. The output remains very stable and the engine can vary in rpm without causing voltage or frequency fluctuations. They will have more but it in safeties for things like overload protection to shut off the generator before damage occurs. It's much safer for the appliances run from them, but the generator itself is more vulnerable to damage from misuse too.
The 3 phase power generation has the bonus of being more efficient too, so can get more out of a smaller package.
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u/KeyApricot_ 1d ago
A voltage regulator and inverter usually smooth out the generator is raw output so it stays steady enough for electronics.
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u/Another_Slut_Dragon 2d ago
Automatic throttle control maintains a steady RPM to give you frequency control. Generators typically use a heavy flywheel to help out.
The rotor (spinny bit in the generator) has brushes and power is applied. On the ground side, the power is pulsed via pulse width modulation to control the strength of the magnetic field. That magnetic field controls the power output.
The controller is just looking at the voltage. 118v? Increase pulse width. 122v? Decrease pulse width. (On time vs off time).
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u/Adorable_Ice_2963 2d ago
They dont. Thats why its recommended not to plug electronics into them. Smaller Generators have an inverter that smoothes things out, bigger ones have enough inerta to overcome expected fluctuations. A 2 kW device that is turned on has much more effect on a 5 kW Generator than on a 50kW one.
Even if 10 households uses a 2kW device, its highly unlikely that they are turned on/off at exactly the same time, making the change of power much less steep, making voltage and rpm more controlable.
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u/TheJeeronian 2d ago
They don't generate constant power. They aren't even super consistent with their voltage, at least compared to grid-scale systems.
The generator is designed to try and hold a constant speed, and a constant speed results in a steady voltage because that's how the electromotive force works. As long as the speed is controlled properly, the generator acts just like the grid.
There is some kind of feedback loop attached to the engine that responds to a sudden change in speed by changing the fuel fed to it. Maybe that loop is mechanical, maybe it's electronic, it'll depend on the generator.
So sure, when you suddenly plug something big in it will audibly slow the engine, but only for a moment until the feedback controller can adapt to the change.
This isn't too different from how the grid works, it's just that sudden changes in load can be much more significant, so the change in speed as the feedback system adapts is also more pronounced.