Where does the power go ?

[DougSpindler]

You then turn off half of the motors.  The generator is still spinning at the same speed producing the same amount of energy because it does not know the motors are turned off and you need less energy.  For that split second the wires are conducting far more energy than what you need.  What happens to that excess energy?  Where does it go?

You have to think of it like this: the generator is "pushing" electricity into the grid.

Imagine you are pushing someone along on a bicycle, but they have the brakes on. You are pushing really hard, but they are only moving slowly. Suddenly they let off the brakes. You are still pushing really hard, but the resistance has suddenly dropped. You are pushing much harder than you really need to. What happens to all your excess energy?

Exactly  At the moment the brakes are let of the amount of energy you are pushing with has to go somewhere so you fall forward and hit the ground.  That extra KE you aren't using to push is instantaneously converted to PE (since you have no resistance) than as you begin to fall it's converted to PE (as you are falling) then it's converted to heat and sound as you hit the ground.

This is exactly what I've been saying would happen with the generator. 

 

[DougSpindler]

I thought we agreed if the generator is spinning faster voltage would increase.

Yep.

And if spinning at twice the speed there were by 4 X the kinetic energy.

If the turbine spins at twice the speed, the turbine will have 4x the kinetic energy, yes.

My question is where is all of that extra energy going?

The energy is not going anywhere. A faster spinning turbine has more kinetic energy, just like a fully charged battery has more chemical energy in it. Energy doesn't have to go places, you can just have a system with lots of gravitational potential energy (fully filled dam), lots of chemical potential energy (charged batteries, a pile of coal), or lots of kinetic energy (a rapidly spinning heavy thing), and that's just fine. The energy can stay where it is, it's not a flow (like power) and so it doesn't have to flow, let alone flow to any particular place.

I think you're confusing energy and power? Energy is measured in Joules, and power is measured in Joules per second (also known as Watts). There's a recent EEVBlog video that goes into some detail on this, you really need to have this concept down before you can discuss subtle things like an imbalance in the power going into/out of a turbine leading to a change in the kinetic energy of the turbine (and that being the end of the story and just fine) -- just like an imbalance in the water flow going into/out of a bucket leading to a change in the quantity of water in the bucket with no need to "explain" where all that "extra water" "goes".

I will disagree with you.  The moment the load is removed from the circuit the generator "doesn't know" and will still be sending the same amount of energy/watts in the wires.  Similar concept in plumbing with pipes hammering.  The hammering is how the excess energy is being dissipated.  What's the equivalent to pipe hammering with the generator.

[DougSpindler]

Not sure we agree on the definitions or use of words in the same way.
Are you saying a short does not cause or result in an an over current event?
A short circuit is one where electrons/current flow through an unintended path with very no or very little resistance.

A short circuit is when the current takes a shorter path than it is intended to take. Hence "short" circuit. A short may be high resistance or low resistance, but it is true that a low resistance is commonly assumed in casual speech.

An over current situation is again what the words describe. It is a situation when the current is over what it is intended to be, or over the maximum designed for or allowed for. Hence "over" current. Usually an over current situation will be protected against by tripping a fuse or circuit breaker.

I was taught electrocution is what happens when a person becomes a conductor either intentionally or accidentally accompanied with an external flow of electrons/current through the body.

Electrocution is to be killed by electricity passing through the body. If you don't die it is not electrocution.

Not sure what the cherry picker was made out of.  I'm surprised if you understand electricity why you don't understand why the two guys in the cherry picker were being electrocuted.

If the cherry picker was made out of an insulator like GRP, then no current would flow at all and the people riding on it would be safe. This is the case with any cherry pickers that might be used near overhead wires.

If the cherry picker was made out of steel then its resistance would be very low and as soon as it contacted the wires it would cause a short circuit and an over current situation (see above) and the breaker would trip, cutting power to the wires. In the unlikely event that the breaker didn't trip, then the low resistance of the steel would prevent any differential voltage being high enough to cause harm. (Unless the unfortunate person was between the high voltage wires and the cherry picker. In that case they would not only be electrocuted, they would be instantly incinerated.)

If a live wire falls on the ground firemen are taught to keep both feet in contact with the ground and shuffle away from the live wire.  If they take a stride the resistance of the ground can be high enough current will flow up one leg and down the other electrocuting the fireman.

Just a quick footnote: it is quite unlikely a human can be electrocuted in this way, because the electrical path up one leg and down the other does not pass through a human's heart. In addition to which, the fireman is probably wearing heavy insulated boots. It is sensible to take the proper precautions and avoid unnecessary risks, but the actual risk is not that great.

Can you give me an example of a short circuit with high resistance?

[timb]

Not sure what the cherry picker was made out of.  I'm surprised if you understand electricity why you don't understand why the two guys in the cherry picker were being electrocuted.  This is something firemen are taught with live wires.  If a live wire falls on the ground firemen are taught to keep both feet in contact with the ground and shuffle away from the live wire.  If they take a stride the resistance of the ground can be high enough current will flow up one leg and down the other electrocuting the fireman.  I remember hearing of three police horses being killed in Florida because of a live wire in a field.  The resistance of the ground between the legs of the horses was enough that the current flowed through the horses killing them.

This is what I was taught.  Not say I'm right, but then again I've been looking up the definition of the words to make sure I'm using them as properly. 

I do want to thank you for questioning me.  It's required me to do a fair amount of researcher to confirm what I saying is correct.

I’m surprised why you think they *would* be electrocuted.

If the metal arm of the cherry picker touched the wire, there would be no path through the men in the bucket to ground, right? Hence they would be safe.

If just the (fiberglass or composite) bucket touched the live wire, there would be no current flow because the bucket is an insulator, and again they would be safe.

If the bucket was metal and touched the wire, they *may* be safe so long as they didn’t touch the sides of the bucket.

The only case in which they’d be harmed is if they touched the wire in a metal bucket, attached to an uninsulated truck (the type that uses hydraulic pistons to lower metal feet into the ground, for stability). In that case the current would go through the person, base of the bucket, down the arm, over the metal skin of the truck and down the steel feet into the ground.

This is all assuming they touched a “low voltage” wire, like the type attached to wooden power poles. The high voltage transport wires attached to the tall metal towers are a different story, but those are generally out of the reach of cherry pickers anyway. (Those are serviced via helicopters! The technicians fly right up to them and use an insulated bar to attach a cable between the copter and wire, to equalize the potentials.)

As for live cables on the ground, I’ve also heard it’s best to shuffle or hop with both feet together when moving away from a fallen cable. However, that’s a completely different scenario to the cherry picker one. (The potential difference surrounding a live wire  that’s fallen on the ground drops off rapidly as you move away from the wire, so walking with a large stride could cause current to flow through you.) There’s a PDF out there with diagrams and images that talks about all this. It’s been posted on the forum before, but I can’t seem to find it at the moment.

[timb]

I thought we agreed if the generator is spinning faster voltage would increase.

Yep.

And if spinning at twice the speed there were by 4 X the kinetic energy.

If the turbine spins at twice the speed, the turbine will have 4x the kinetic energy, yes.

My question is where is all of that extra energy going?

The energy is not going anywhere. A faster spinning turbine has more kinetic energy, just like a fully charged battery has more chemical energy in it. Energy doesn't have to go places, you can just have a system with lots of gravitational potential energy (fully filled dam), lots of chemical potential energy (charged batteries, a pile of coal), or lots of kinetic energy (a rapidly spinning heavy thing), and that's just fine. The energy can stay where it is, it's not a flow (like power) and so it doesn't have to flow, let alone flow to any particular place.

I think you're confusing energy and power? Energy is measured in Joules, and power is measured in Joules per second (also known as Watts). There's a recent EEVBlog video that goes into some detail on this, you really need to have this concept down before you can discuss subtle things like an imbalance in the power going into/out of a turbine leading to a change in the kinetic energy of the turbine (and that being the end of the story and just fine) -- just like an imbalance in the water flow going into/out of a bucket leading to a change in the quantity of water in the bucket with no need to "explain" where all that "extra water" "goes".

I will disagree with you.  The moment the load is removed from the circuit the generator "doesn't know" and will still be sending the same amount of energy/watts in the wires.  Similar concept in plumbing with pipes hammering.  The hammering is how the excess energy is being dissipated.  What's the equivalent to pipe hammering with the generator.

We’ve given you the answer numerous times.

The generator is powered by steam. There’s a controller that operates the valves that control the flow of steam. They monitor the generator’s output. When the load is removed from the generator the output will start to rise, the controller will sense this and close the steam valves a bit, causing the generator to slow and the output to come back down. This all happens in the blink of an eye.

That’s a control loop. It works no different to an Op-Amp.

Think about it. I have a simple op-amp follower circuit (no gain), and I apply 5V to the + input and attach a 100 Ohm load to the output. Now the output is at 5V. If I remove the 100 Ohm Load, the output voltage will briefly spike by a few millivolts before returning to 5V. Where did the power go? Nowhere. The op-amp used negative feedback to close the internal “steam valve” (a transistor) in order to keep the voltage at the setpoint.

Is this making sense yet? We really can’t make it much simpler.

[timb]

Can you give me an example of a short circuit with high resistance?

Let’s say I have a circuit only capable of 1uA output current. If I place a 1MOhm resistor on it’s output, it’s essentially a short circuit.

[rs20]

I will disagree with you.  The moment the load is removed from the circuit the generator "doesn't know" and will still be sending the same amount of energy/watts in the wires.

All I can say is, you're asking a vast number of interesting questions, but you need to focus your attention a bit. There are all sorts of different time scales on which generators and loads balance out:
- Milliseconds: Speed-of-light causality limit, which you seem to have brought up for the first time
- Short term (ms): Lights in your neighborhood burn brighter, with the vast rotational inertia of all motors and generators in the country hold frequency nearly steady
- Medium term (100ms - multiple seconds): Turbine flow control in power stations is adjusted
- Long term (integrated/infinity): Less coal burned or more water still in dam.
- Even longer term (years): Power stations getting built and decomissioned.

Now, when you're talking about instants of time so short that the speed of light does not allow a power source and load to even communicate let alone match current, you're getting into the realm of transmission lines. You'll note that the linked webpage there notes that the study transmission is (typically) restricted to "radio frequency, that is, currents with a frequency high enough that their wave nature must be taken into account". This wouldn't normally refer to 50 Hz power, but the huge size of transmission networks, and the newly introduced question of a device being turned off instantaneously (a hard edge with unbounded frequency content) makes transmission line theory necessary to answer your question with the sort of rigor that you seem to be demanding.

The rest of us on this thread are simply assuming that the load and generators are communicating and matching currents practically instantaneously, an assumption which a) simplifies the thought process massively and b) does not introduce any practically relevant errors when answering the question "what happens when I turn my TV off". I'd strongly encourage you to allow yourself to make this assumption, understand things within this simplified framework, and then start studying transmission lines if you're still bothered by the "spooky action at a distance" aspect. I mean, you can start developing formulae for the positions of individual electrons if you want, but none of us are going to hold your hand through that process! We're electronics people, not physicists.

Similar concept in plumbing with pipes hammering.  The hammering is how the excess energy is being dissipated.  What's the equivalent to pipe hammering with the generator.

That's because the water has inertia. The corresponding concept in electronics is inductance. Inductance of power lines is totally irrelevant when answering the question "what happens when I turn my TV off", except that power lines have both distributed inductance and distributed capacitance, which means they form... a transmission line. But as mentioned previously, you shouldn't think about this aspect unless you really want to. Allow yourself to assume that the load and the generator are connected by ideal, inductance-free, superconducting, short wires first.

[rs20]

Just for sake of a analogy, if you were explaining to someone how a dam "knows" to stop "sending" water when someone turns off a tap, and they asked what happens at the instant the tap is turned off, the dam doesn't know yet, where does the water go... the fully rigorous answer is that the pressure builds up behind the turned-off tap, and that high pressure then travels, as a sound wave, all the way back to the dam. After a few reflections back and forth, the system eventually settles into a new equilibrium. Does this explanation fully satisfy all the rules of causality and no spooky-action-at-a-distance? Why yes. Is it a useful or practical explanation? Nope. Would it even be used by professional water pipe planning people? Not even. Much simpler, and practically identical to reality, to assume that the dam instantaneously stops sending the extra water.

Btw, transmission lines vs wires is like sound waves in pipes vs standard hydraulics.

[IanB]

Exactly  At the moment the brakes are let of the amount of energy you are pushing with has to go somewhere so you fall forward and hit the ground.  That extra KE you aren't using to push is instantaneously converted to PE (since you have no resistance) than as you begin to fall it's converted to PE (as you are falling) then it's converted to heat and sound as you hit the ground.

Lol! I'm rather assuming you wouldn't be that clumsy 

What should happen is that as the bicycle runs away from you you put a foot out in front to catch yourself and you take an extra pace or to control your speed.

[timb]

You know, thinking about this question a bit more, there’s two critical things I think @DougSpindler fails to grasp: A generator can run freely without any load and power isn’t what he thinks it is.

Think about this, I have a propane based whole home generator with automatic switchover. When it starts up, the engine is spinning the generator windings just fine even though they aren’t connected to an electrical load. At this point the generator is producing 120VAC@60Hz and putting out 0A. Suddenly, a solenoid throws an A/B switch and my entire house is connected as the load. Now, you can hear the engine connected to the generator briefly whine and slow down a bit. At this point it might be producing 110VAC@58Hz and putting out 30A. Within a second or so, the control loop inside the unit increases gas flow to the engine, to compensate for this newly added load. Now we’re back to 120VAC@60Hz. After a few minutes, say my Air Con kicks off, decreasing the load on the generator, so now the voltage jumps to 125VAC@61Hz until the control loop slows the engine.

Now, I know what you’re thinking, at the end there the “power” jumped up, right? Well, no. The voltage may have gone up five volts, however the current would have gone down an equivalent amount, keeping the total power draw the same.

So, while voltage may fluctuate as I add and shed loads to my generator, the overall wattage used will stay the same (since an appliance will use less current at a higher voltage and vice versa).

Keep in mind this is different for the typical power distribution network, as there are so many generators spread out over such a large area that as loads are added and shed it’s basically imperceptible to the home user.

[jmelson]

*More specifically, synchronous machines do.  Shorting slats are simply shorted turns on the armature, which act to slow any change in its magnetic field (which is an electromagnet, so the field is supposed to be static anyway).  Note: I haven't studied modern generators per se -- their design may vary.  Anyway, even if this isn't the exact solution used, the same effect is required, however it's achieved.

I've had the opportunity to study a few GE power house alternators in a repair shop and before installation.  Very interesting stuff.  So, I'll describe the alternator that went into the Calloway County nuclear plant in Missouri.  The rotor was a single piece of steel about 15 feet long.  The active rotor section was, as I recall, about 18" diameter and about 6 feet long.  It fits within the stator iron laminations with just a few thousandths of an inch gap.  The rotor is a solid piece of steel, no laminations, and has a pair of rectangular-spiral grooves machined into it to form the poles.  The magnetic field is produced by a winding of about a dozen turns of a copper bar fitted into the groove.  The bar is about 1/4" thick and 2" wide.  The groove plunges deep under the winding to allow one end of the bar to pass from the center to the outside, where it is connected to the exciter. The bar is insulated with some kind of paper or fiberglass material inserted into the slot first.  The rotor winding is fed something like 10,000 Amps, with about  100 V drop across it.  Beyond the active rotor section, the shafts were somewhere between 9 - 12" diameter, with big flanges on one end where the turbine was attached.

These alternators are sealed as best as they can be (with a rotating shaft passing through) and filled with hydrogen gas.  There is a water-hydrogen heat exchanger to cool the hydrogen.  The stator laminations have channels in them for the hydrogen to pass through and pick up heat.

Jon

[DougSpindler]

Can you give me an example of a short circuit with high resistance?

Let’s say I have a circuit only capable of 1uA output current. If I place a 1MOhm resistor on it’s output, it’s essentially a short circuit.

How is that a short circuit?  Can you provide a reference source which describes the scenario you described as a short circuit?

Here's what I was taught.
A short circuit is an abnormal connection between two nodes of an electric circuit intended to be at different voltages. This results in an electric current limited only by the Thévenin equivalent resistance of the rest of the network which can cause circuit damage, overheating, fire or explosion. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, for the purpose of voltage-sensing crowbar circuit protectors.

In circuit analysis, a short circuit is defined as a connection between two nodes that forces them to be at the same voltage. In an 'ideal' short circuit, this means there is no resistance and thus no voltage drop across the connection.

[IanB]

Let’s say I have a circuit only capable of 1uA output current. If I place a 1MOhm resistor on it’s output, it’s essentially a short circuit.

How is that a short circuit?  Can you provide a reference source which describes the scenario you described as a short circuit?

Well, you provided your own reference yourself below:

In circuit analysis, a short circuit is defined as a connection between two nodes that forces them to be at the same voltage. In an 'ideal' short circuit, this means there is no resistance and thus no voltage drop across the connection.

Let's imagine the 1 µA flowing through the 1 meg-ohm resistor. By Ohm's law, the voltage difference across the resistor will be 1 V. Now in practical terms, if the two nodes are only one volt apart, they are effectively "at the same voltage". Especially since a circuit capable of only 1 µA output is likely to produce thousands of volts in the normal mode of operation. Compare 1 volt to thousands of volts and the two nodes may as well be at the same voltage. So it's a short circuit.

[T3sl4co1l]

I've had the opportunity to study a few GE power house alternators in a repair shop and before installation.  Very interesting stuff.

[snip]

Cheers!

So, that solid iron rotor will have pretty good eddy currents induced in it by harmonics, so that should count for something.

Tim

[T3sl4co1l]

Let's imagine the 1 µA flowing through the 1 meg-ohm resistor. By Ohm's law, the voltage difference across the resistor will be 1 V. Now in practical terms, if the two nodes are only one volt apart, they are effectively "at the same voltage". Especially since a circuit capable of only 1 µA output is likely to produce thousands of volts in the normal mode of operation. Compare 1 volt to thousands of volts and the two nodes may as well be at the same voltage. So it's a short circuit.

Could you please read a textbook?

Thanks,

Tim

[IanB]

Could you please read a textbook?

OK 

Where do you think I made an error?

[T3sl4co1l]

Could you please read a textbook?

OK 

Where do you think I made an error?

Thanks for asking --

Let's imagine the 1 µA flowing through the 1 meg-ohm resistor. By Ohm's law, the voltage difference across the resistor will be 1 V. Now in practical terms, if the two nodes are only one volt apart, they are effectively "at the same voltage".

[citation needed]

Especially since a circuit capable of only 1 µA output is likely to produce thousands of volts in the normal mode of operation.

Making assumptions about a completely unspecified source.

Compare 1 volt to thousands of volts and the two nodes may as well be at the same voltage. So it's a short circuit.

TIL the voltage sense divider on a high voltage supply is definitely a short already, so it won't matter if I jam a screwdriver into it.

Another example: I was working on a circuit a few days ago with uA bias currents on BJTs.  Vbe is a mere 0.5V at this level.

I guess that whole circuit was accidentally a double short, then...
</sardonic>

But seriously, 1V isn't even so low that you can't do anything with it.

Tim

[IanB]

OK, I guess context is important.

We could imagine an electrostatic machine with an output current of 1 µA that in normal operation would charge up an output terminal to a high voltage. If we connected this output terminal to ground with a 1 meg resistor, then the output wouldn't generate any voltage at all. In that context it would effectively be shorted.

I accept that this example needs to be stated for it to make sense otherwise everything is open to interpretation.

Besides that there are cases of sensitive electronic circuits where contamination on the circuit board can lead to stray currents that compromise operation of the device. Maybe that's not a "short" circuit, but it is an unintended current path.

[T3sl4co1l]

OK, I guess context is important.

We could imagine an electrostatic machine with an output current of 1 µA that in normal operation would charge up an output terminal to a high voltage. If we connected this output terminal to ground with a 1 meg resistor, then the output wouldn't generate any voltage at all. In that context it would effectively be shorted.

This is actually a very sneaky, and -- probably unintentionally -- contrived situation!

Most static generators work on positive feedback.  Kelvin dropper, Wimshurst machine, those kinds: they operate on induction from the existing charge.

If you "short out" the output, the gain also goes to zero, and the output -- well and truly -- is shorted.  But it's also not 1uA (say) anymore.  Which makes it kind of disingenuous to say so!

A triboelectric or otherwise powered generator (like a van De Graaf generator -- often, static is "sprayed" onto the belt with a separate, lower (still "high") voltage, electronic supply), delivers constant charge (roughly speaking) and so cannot be shorted out with any load other than zero ohms.

A short is simply zero ohms -- to imply otherwise, whether in context or not, is simply not good usage!

Tim

[rs20]

I'm not sure this thread is too healthy -- we've got one person perpetually asking for more information at twice the rate that he is able to process it; another person taking on the challenge of providing a bulletproof definition for the ultimately fuzzy and colloquial concept of a "short-circuit", and another person poking holes in the previous attempts by piling on the interesting specifics of real-world static generators. I'm not sure if anyone is winning here.

Regardless, I, for one, have no quibble with the claim that a nominally CV power supply running at 1% or even 10% of its nominal output voltage due to a overcurrent condition is a power supply that (at the very least, colloquially) is being "shorted". Regardless of any oxidative reactions that might be happening in the vicinity.

[timb]

A short is simply zero ohms -- to imply otherwise, whether in context or not, is simply not good usage!

Tim

So, you’re saying if I have a 1uA current source with a, say, 10V compliance voltage and I place a 10, 100 or even 1kOhm resistance across it, it wouldn’t be considered shorted? Who knew!

[Ice-Tea]

A short is simply zero ohms -- to imply otherwise, whether in context or not, is simply not good usage!

Tim

So, you’re saying if I have a 1uA current source with a, say, 10V compliance voltage and I place a 10, 100 or even 1kOhm resistance across it, it wouldn’t be considered shorted? Who knew!

I would consider it heavily loaded

[DougSpindler]

Interesting in how in over a hundred years of using electricity we still can't agree on the use of words and terms such as short-circuit.

[dr.diesel]

Interesting in how in over a hundred years of using electricity we still can't agree on the use of words and terms such as short-circuit.

IMO a short circuit is simply an unintended path or a bypass, it certainly doesn't require zero ohms to happen.

[DougSpindler]

You know, thinking about this question a bit more, there’s two critical things I think @DougSpindler fails to grasp: A generator can run freely without any load and power isn’t what he thinks it is.

Think about this, I have a propane based whole home generator with automatic switchover. When it starts up, the engine is spinning the generator windings just fine even though they aren’t connected to an electrical load. At this point the generator is producing 120VAC@60Hz and putting out 0A. Suddenly, a solenoid throws an A/B switch and my entire house is connected as the load. Now, you can hear the engine connected to the generator briefly whine and slow down a bit. At this point it might be producing 110VAC@58Hz and putting out 30A. Within a second or so, the control loop inside the unit increases gas flow to the engine, to compensate for this newly added load. Now we’re back to 120VAC@60Hz. After a few minutes, say my Air Con kicks off, decreasing the load on the generator, so now the voltage jumps to 125VAC@61Hz until the control loop slows the engine.

Now, I know what you’re thinking, at the end there the “power” jumped up, right? Well, no. The voltage may have gone up five volts, however the current would have gone down an equivalent amount, keeping the total power draw the same.

So, while voltage may fluctuate as I add and shed loads to my generator, the overall wattage used will stay the same (since an appliance will use less current at a higher voltage and vice versa).

Keep in mind this is different for the typical power distribution network, as there are so many generators spread out over such a large area that as loads are added and shed it’s basically imperceptible to the home user.

@Timb, you are following me. 

Let's take your scenario but add a second generator.  What happens if the second generator is 108 degrees out of phase with the other generator.  I'm guessing here.....   But I would assume there would be almost no current flowing to you home and all of the current would be flowing between the generators effectively resulting it a short and probably a fire (exothermic oxidative reaction) from over heating.

Second question.
What if the generators used permanent magnets and the speed of the generator was constant.  IF the generator was connected to a load let's say 1,000 watts are being generated and consumed.  When the switch is opened and current flow stops does that mean the generator has stopped producing 1,000 watts?  I would not think so.  The generator is still spinning at the same speed, the magnetic field is the same.  So where does that 1,000 watts go?