Hi, a simple, maybe a silly, question about electricity, for example generated by a Car alternator, what happens to the power generated that is not being consumed by the users, like lights or Radio etc. If for example the Generator, generates 500Watts, but the users, use only 300Watts, what happens to the exess?
Thanks
Most of it goes into the car’s battery, some will become energy waste (heat).
Car batteries are a nifty thing: they’re rechargeable, so whenever you’re running the motor they get charged, and when extra energy is needed (for example to start the car) or when you need electricity without running the motor, the battery provides that stored energy.
No, that isn’t it. If the user doesn’t turn much on and there is little demand for power, the alternator doesn’t generate as much. It will be easier for the engine to turn it, too.
It’s both, actually. The battery doesn’t charge by itself.
I’ll side with Napier. Alternators have a device called a voltage regulator. This keeps the voltage at a fixed level (around 13.5 volts). When you use an accessory it draws current causing the voltage to fall. The voltage regulator then calls on the alternator to deliver more power to get the voltage back up. When you turn off the accessory the voltage regulator calls for less power from the alternator. The alternator recharges the battery in the first minute or so after you start the car. After that all its power is going to your ignition and accessories.
In the US domestic electric generation business, watts produced must match watts consumed very closely. If the plants on the grid are producing more energy than what is being consumed (usage + losses), then the frequency will get too high and your fancy electronics will expldoe (OK, not quite - but they can be ruined. Other bad things can happen too.). Current regulations set target frequency to be within plus or minus 0.05 Hz of 60 Hz (with lots of caveats and exceptions).
This means that utilities have to respond to consumer demand for electricity on a basically instantaneous basis.
Of course, we under- or over- generate on purpose sometimes to fix the time on your clocks (no, I’m not kidding).
http://www.ornl.gov/sci/btc/apps/Restructuring/ORNLTM200341.pdf
The alternator is always turning and generating electricity as long as the engine is running. Excess electricity is sent to ground.
No no no. A 500W alternator with a 300W load generates 300W. There is no ‘excess sent to ground’. The rating on an alternator is it’s designed capacity, not it’s output.
That’s somewhat of an oversimplification, in that generation of AC requires paying attention to power factors, but in a car used in the OP, the power factor is a known and static quantity because the power is rectified into DC for distribution.
Alternators generate voltage as a function of shaft RPM, and that can be as high as 50 volts at an engine’s redline. The voltage regulator’s job is to hold the usable output voltage constant (or nearly so), which in practice is about 14 volts DC after rectification. Power supply circuitry in the electronic modules will knock the input power down to 12 volts for consumption; other stuff like lights don’t really care. FWIW, car batteries won’t charge if the terminal voltage is less than 13.5 volts.
Remember, an alternator produces voltage, not current (bear with me). Current is drawn from the alternator by the load (i.e., the things that need power). At a certain RPM, an alternator will produce a certain voltage. If you add a load, it requires more torque to spin the alternator, which must be kept at a minimum speed. As you add more and more loads, the engine must work a little bit harder to maintain the speed of the alternator. If you have an older car, then while at idle you can probably hear/sense the engine adjusting itself any time you activate a high-power device, say, your power windows or rear defroster.
If you have toy DC generator, try spinning it with your fingers. Next, short the outputs together, and try spinning it with your fingers. You’ll then have a very good idea of what I describe above.
Excess power that is not used or stored is simply not generated. The alternator is easier to turn for the engine and takes less power automatically as a property of electromagnetism.
Through the rubber tires, I presume.
You’re kidding, right? Yeah, you’re kidding.
In case you’re not rubber is an insulator and would make for a very poor ground. The chassis of the vehicle is used as the ground. As others have explained this is not a factor in determining the load on the alternator at any rate.
I’m not sure what you are referring to with the bit about the alternator must be kept at a minimum speed. In a car, it’s the engine that needs to be kept at a minimum speed or it will stop working. The alternator’s power will drop if its speed drops though, so maybe that’s what you are saying.
The first part of that is exactly right though. If there is no electrical load on the alternator, then it will spin freely. The more electrical load is connected to the alternator, the harder the alternator will be to turn, requiring more mechanical force from the engine.
If you have an older carburetor style car where the idle is controlled by a set screw, then you’ll hear the RPM of the engine get lower as you turn on more electrical things. A modern car controls the idle via the engine computer, and will increase the RPM to its desired value as the alternator loads down the engine.
A car’s electrical system (battery + alternator) functions pretty much as a voltage source, meaning that it will hold the voltage constant and the devices attached to it determine how much current is drawn. The alternator voltage is higher than the battery voltage, so there will always be some current flowing through the battery. This charges the battery, and once the battery is fully charged the current mostly gets wasted as heat, though some of the energy will go into splitting the water in the battery’s electrolyte into hydrogen and oxygen.
The more devices you turn on, the more electrical load will be drawn from the battery and alternator. If you draw 300 watts (watts = voltage x current) from an alternator rated for 500 watts then there is no problem. If you try to draw 600 watts then the alternator’s wires will overheat and the alternator will be destroyed.
Most generators function similarly. If you have a portable gas or diesel generator for example, it will basically be a voltage source and the load will determine how much power is actually drawn from the generator. There is no “extra” power generated, except for waste heat that comes from the fact that the wires aren’t superconductors.
The one exception to this is when you start tying generators together like the power company does. Once you do this, the generators all become locked together, and once you have more than a couple of generators then each individual generator ceases to be able to have a major effect on the line itself. If you try to make the generator spin faster, all it does is provides more power to the “grid”. If you try to make it spin slower, then it provides less power, until it actually becomes a motor and starts drawing power from the grid. Adjusting the frequency of the grid becomes a bit tricky, because you can’t just speed up or slow down the individual generators. The power company does adjust the frequency though. It may vary a bit during the day, but in the long run you can run a clock with a synchronous motor off of the grid and it will be guaranteed to keep the right time. It may be off by a few seconds during the day, but months or even years later it still won’t be off by more than a few seconds. In order to regulate the frequency this accurately, the power company has to coordinate its generators to maintain a precise control of the frequency.
This is pretty apparent when you have a small portable generating set. When you switch on a high current load the governor opens the throttle, causing an obvious change in the exhaust note.
As for minimum speed, it is not unusual for motorcycles not to be able to supply any power at idle. BMW airhead twins were notorious for this.
That is what’s known as ‘sarcasm’.
How can the chassis of the car be a ground when it is not in actual electrical contact with the ground? Wouldn’t it just build up charge until it asplodes or something?
I was trying to isolate the alternator from the car. The alternator requires a minimum speed to maintain its voltage outlet. In any case, your description was much better!
That’s why I like the non-Americanism “earth” so much. You’re right, it’s not a ground in the true sense; it’s just the return current path. Or current source, depending on if prefer that holes flow or electrons flow. More formally, I’d call it “chassis ground” to distinguish it from earth ground.
So does that mean the chassis is basically connected to a wire somewhere that goes to one of the terminals on the battery? (Or one end of the alternator?)
Yup. go out to your car and find the negative battery cable. follow that cable it will either be bolted to the chassis, and there will be a (heavy) wire connecting the chassis to the engine, or it will connect from the battery to the engine, with the wire going to the chassis, or in some cases it will split and go to both the engine and the chassis.
This is done as a wire saving measure. If you did not use the chassis and body as a ground it would take twice as much wire to wire the car.
the chassis is the wire that connects one terminal of the battery, alternator, and all else electrical together. in the USA the negative terminal is connected to the chassis.