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I know that this question might sound very stupid for +90% of you, but my brain doesn't really get the electricity generation.

The thing that I don't understand is what does a generator produce, voltage or current when it spins? Sometimes I see that a generator can produce x amount of potential and y amount of current.

I'm starting from the basics of electricity. A spinning/moving magnet makes the electrons move in one direction, generating a potential due to the change in kinetic energy, right? So if one would connect a multi-meter at both terminals, one would read the voltage. I did this and it's easy to comprehend. Now things start to confuse me.

Sometimes, one can see on the specifications that a motor can deliver a certain current and this is what I don't understand. The current is related to the potential through Ohm's law, right? In other words, the current is dependent only on the resistance of a load (or wire) and the potential. Also, the current flows only if there is a voltage gradient/drop. Why is there the current specified on generators like this one?

What confuses me is this intrinsic combination of voltage-current on the generator. Can anyone help me to understand this? Thank you and sorry for dropping the IQ of this group.

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    \$\begingroup\$ does a hydroponic pump produce pressure or flow? \$\endgroup\$ Sep 3, 2017 at 16:14
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    \$\begingroup\$ Pressure. So if one adds the flow value on the specs, it means that that value is the maximum flow the pump can reach, right? \$\endgroup\$
    – Physther
    Sep 3, 2017 at 16:17
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    \$\begingroup\$ Some related links: electronics.stackexchange.com/questions/15444/… And [understanding alternator ratings][1]. The latter makes it very clear that the numbers refer to specific measurements under test conditions; actual values will vary in real circumstances. [1]: lifewire.com/understanding-alternator-output-ratings-534785 \$\endgroup\$
    – pjc50
    Sep 3, 2017 at 16:37
  • \$\begingroup\$ @Physther So how come when I don't put a hose on the output to contain the water, I get lots of flow but no pressure? \$\endgroup\$
    – user253751
    Sep 4, 2017 at 5:35

6 Answers 6

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Internally, a generator produces voltage proportional to its speed.

However, that doesn't mean a constant voltage always comes out of a generator run at a fixed speed. The windings and other parts of the generator have some electrical resistance. To a first approximation, you can think of a generator as a voltage source proportional to speed, with a fixed resistance in series.

When current flows as a result of a load on the generator, two things happen:

  1. The current causes a voltage drop across the generator's internal resistance. The voltage you get out of the whole generator is its internal voltage minus this voltage drop.

  2. the current thru the generator causes a torque opposing the shaft rotation. This is how generators don't magically create power. The input power is torque times rotation speed. The output power is volts times amps. As the amps go up, the output power goes up. However, this causes the torque on the shaft to go up, so the input power goes up too. The input power is always at least as much as the output power. Physics doesn't give you a free lunch, and generators obey the laws of physics.

For example, let's say a particular generator when spun at 60 Hz (3600 RPM) produces 50 V RMS with no load. No load means the current is 0, so the voltage drop across the internal resistance is zero. Therefore, at no load you get to see the actuall internal voltage the generator generates.

Let's further say that the internal resistance is 2 Ω. If you connect a 75 Ω load to the generator, the internal voltage source now sees the internal resistance and your load in series as its load. That means the load on the internal voltage source is 77 Ω. The current is (50 V)/(77 Ω) = 649 mA. The drop across the unavoidable internal resistance is (649 mA)(2 Ω) = 1.3 V. You therefore get 48.7 V at 649 mA.

The total electrical power the generator is producing is (50 V)(649 mA) = 32.5 W. Of that (1.3 V)(649 mA) = 844 mW ends up heating the generator and you actually get (48.7 V)(649 mA) = 31.6 W out. The backwards torque on the shaft created by the 649 mA times the shaft rotation rate is 32.5 W plus whatever it takes to overcome the mechanical friction.

If you were to short the output of the generator, the maximum possible current would flow. However, you don't get any power out since the voltage is zero. All the power produced by the generator goes to heating its internal resistance. The resistance on the shaft would be high. If you managed to continue turning the generator at the same speed anyway, you'd be putting a lot of mechanical power in, and all of it would turn into heat in the generator. For many real generators, this would overheat and destroy them in a relatively short time.

Added

The above discussion was for basic generators. Apparently some are interested in discussing automobile alternators. Those are nowadays complete units with rectifiers, regulators, and feedback, to which the above doesn't apply. It does apply to the bare generator inside a car alternator, but you don't have access to that directly, and its workings are actively modified and obscured by the controller.

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    \$\begingroup\$ This is a great answer! Thank you so much! It makes so much sense now. \$\endgroup\$
    – Physther
    Sep 3, 2017 at 18:31
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    \$\begingroup\$ -1/2 pt , technical errors on term " generator" and -1/2pt for non-relevant examples and no mention of field current controlled current source output. \$\endgroup\$ Sep 3, 2017 at 19:03
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Sometimes, one can see on the specifications that a motor can deliver a certain current and this is what I don't understand. The current is related to the potential through Ohm's law, right?

Not in the way you think.

Yes, if you connect a lower resistance load the current will increase if the generator's output voltage remains constant. This is shown by \$ I = \frac {V}{R} \$.

However, there are a few limits including details such as the maximum current carrying rating of the windings. The motor can supply more and more current but the winding temperature will increase as you do and at some point you will burn them out. A rule of thumb is that every 10°C rise in running temperature halves the winding life.

In other words, the current is dependent only on the resistance of a load (or wire) and the potential.

Yes, as explained above.

Also, the current flows only if there is a voltage gradient/drop.

Yes. A difference in voltage or "potential" between two points will cause current to flow. It's a bit like two water tanks connected by a hose. Water will flow until the levels are the same. Current will drop off as the height difference decreases.

Why is there the current specified on generators like this one?

It is the design limit for the output.


Update: I missed the "this one" link in the question.

That looks like a car alternator so the situation is a bit more complicated. To charge a car battery will require > 14 V so the output is a nominal 12 V but actually more like 14 V. If you monitor the voltage on a car battery while someone revs the engine from idle to medium you might see the voltage rise. (You will also see the headlights brighten noticeably.) That alternator has a regulator on it which will adjust the field winding current to maintain the voltage close to the specified output - probably 14 V or so.

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  • \$\begingroup\$ Thank you very much! So basically, when I look at a generator that has its current specified on it, it means that that's the maximum amount I can ever achieve with it, right? And that is only because of its winding/construction and it is independent of what is connected to it. Did I understand it correctly? \$\endgroup\$
    – Physther
    Sep 3, 2017 at 16:13
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    \$\begingroup\$ Yes. You can safely run any load up to the rated current. Some generators will allow a short overload - maybe to start a large electric motor which draws a high current on startup - for a short time. This will be detailed in the machine specifications. \$\endgroup\$
    – Transistor
    Sep 3, 2017 at 16:19
  • \$\begingroup\$ And a last question. About that alternator... The specifications say 12V, 160A. I imagine that the voltage would go to 12V, and then the only thing that will change would be the current, reaching a maximum of 160A. Is that correct or will the current reach 160A only when the voltage is 12V? In other words, can there be different currents for the same voltage drop? \$\endgroup\$
    – Physther
    Sep 3, 2017 at 16:24
  • \$\begingroup\$ "About that alternator..." Which alternator? Add a link to the datasheet into your question. \$\endgroup\$
    – Transistor
    Sep 3, 2017 at 16:25
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    \$\begingroup\$ @Physther A thinner wire could have been wound inside your example alternator. Many more turns would fit the same internal winding space. The alternator would be rated for higher voltage, and lower current. \$\endgroup\$
    – glen_geek
    Sep 3, 2017 at 16:25
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In vehicles, there is a difference between generators and alternators.

First, what is shown in the photo, is not a (DC) "generator" rather it is an AC alternator which includes a diode bridge (3 phase, 6 diodes) to rectify into DC.

Let's make sure you understand this first.

Originally generators were used with commutating windings with brushes making contact with rotor windings to alternate the AC voltage back to DC.

Due to brush arccing failures in the late 60's these were redesigned with Alternators that use copper slip rings to provide DC to the rotor instead, which we call self-excited since it can use the rectified output voltage to excite the rotor current so that the rotating flux can effectly increase the output AC current amplified by the work done to rotate the pulley and regulated by the output voltage to 14.2V with a regulator that has a fixed low voltage reference.

Next we know that motors/generators and alternators produce a "no load" EMF voltage proportional to RPM, but in this case the regulator controls the rotor (rotating DC) "Field current" to also regulate voltage as RPM changes. However below a certain RPM, no amount of field current can raise the voltage, this only gives current gain.

So RPM creates the voltage while field current automatically regulates the output current determined by the internal voltage regulator and load current (Ohms Law, Vout= Iout(load)*R(load)). So the field current naturally reduces with rising RPM at a fixed load.

However we know that the load also affects voltage by impedance ratio, so the source impedance must be lower than all expected loads except for the starter motor, which is a much lower impedance but like a generator it has a commutator with heavy copper brushes.

So the Alternator produces both voltage and current regulated by RPM and field current, so we say it generates power that depends on load. Since the available torque is high from a serpentine belt, it may be undervoltage below 500 RPM but be able supply full power at 1200 RPM or so using engine RPM scaled to alternator RPM by pulley ratios.

In short (no pun intended as that would blow the 6 diode bridge at high RPM) the alternator is a voltage regulated current controlled current source at fixed RPM that supplies all the current needed to charge the battery and other loads. It must be suitably sized for current as the battery ESR determines the max current to raise the voltage to 14.2V and the battery ESR reduces with rising CCA capacity (when new) needed to turn a big truck starter motor.

side effects from mismatched alternator and battery

Although a vehicle, once running can operate without a battery, the alternator must have the current capacity at correct regulated voltage voltage and current to meet expected demands. The biggest load is a an undercharged battery. When batteries age each cell becomes more mismatched and the weakest cell can boil the electrolyte from over voltage from excess alternator current to reach 14.2V so instead of 14.2/6=2.366V per cell, anything 10% higher will age the battery rapdly. So installing a bigger alternator on an old battery, can result in battery failure quicker than normal. Also as diode bridges get hotter and age slightly with higher ESR a fresh battery with higher CCA rating (and lower ESR result) can increase the stress on the alternator bridge and fry it in ar least one phase so the alternator reduces its capacity from 3 phase to 2 or 1 phase and can no longer keep up to worst case load current causing the headlamps to dim at normal idle.

Lets consider a new 1000 Amp CCA rated battery that when warm has CA rating of 1200A. This means the ESR is (12.5v-7.5V)/1000A= 5 mΩ from the std test at drop to 7.5V and Ohm's Law. So what is the max current to raise the battery from 12 V (undercharged) to 14.2V ? Again from Ohm's Law, (14.2-12)V/5mΩ= 440 amps !! Fortunately the diodes can handle brief over current periods and the weak alternator could fry its diodes , while a stronger one might need more RPM to raise the voltage to 14.2 until the charge level rises.

Thus the alternator current and battery CCA are designed for each vehicle to increase longevity at the least cost by careful selection of matching diode ESR and battery cell ESR mismatch from aging. This is why they sometimes dont last long after replacing one or the other. This is aggrevated by excessive V regulator settings and poorly sized or maintained batteries. (warped plates from brief shorts), excess ambient temperature baking in the sun in Arizona. etc etc.

Sorry for the long answer, I hope this raised your IQ on Alternators. It is more complicated as the control loop is a current controlled current source (CCCS) with a Voltage Reference to get an output of 14.2V +/-0.1. This results in it becoming a regulated Voltage source. The other answers at this time, do not mention this at all.

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  • \$\begingroup\$ Thank you very much for the nice answer! Definitely raised my knowledge on Alternators. \$\endgroup\$
    – Physther
    Sep 4, 2017 at 12:38
  • \$\begingroup\$ ESR on batteries, diodes are not constant but with constraints in current, v, time, temp can be made linear constant. Batteries are just a bank of caps in series/parallel, with a no charge threshold voltage with large kF *ESR =T constants, with memory (// effects) but can be constrained for either transients or steady state to make estimates. \$\endgroup\$ Sep 4, 2017 at 12:44
  • \$\begingroup\$ The votes are not an indicator of content, but the capacity to understand of the readers. \$\endgroup\$ Sep 4, 2017 at 12:48
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A generator, operated into a short, produces the maximum current; that current inside the generator coils will act to oppose the field provides by the "magnets" and implements a feedback system by means of flux bucking, flux opposition, thus partial flux cancellation.

That partial flux cancellation limits the internal "B" field of the coils.

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This answer may offend some folks but that is not my intention.

I am with you. The so called experts, the one with credentials, the know hows are at fault. The knowledge authorities, they are more interested in how great they are, in how much they know and the attention they crave spitting out big words at you than they care in you knowing. If they did, you will know it instantly on day one. And by the way don't be surprised to find out that themselves don't know. They have simply learned to mimick and repeat what they have simply memorized.

Unfortunately the world as it is to have a good teacher.....nope don't look for it. To find a true caring teacher is a needle in a haystack.

In plain English it looks like this: What is voltage, what is current and what is frequency and what it looks like when you look in the mechanism called a generator, alternator or whatever you want to call it.

When you rotate a conductor in magnetic field, here is what is involved.

1. How fast are you turning the conductor per second round and round. That in itself determines the frequency and nothing else.

2. Now the voltage is determined by how strong the magnetic field is. Increasing or decreasing the magnetic field is the easiest. It takes more effort to turn that conductor in a strong magnetic field ( strong magnet) compare to a weak mangetic field ( weak magnet) if you want to keep the same frequency, right? If you don’t keep up with your frequency it will of course lower. If you want to keep up with the same rotation then you have to work harder to turn the conductor. And this extra effort increases the voltage. Another name for voltage is what they call amplitude. Amplitude is how high the waves are. And frequency is the number of up and down of the waves per second. But don’t get caught up in fancy words… Just because someone uses fancy words doesn’t mean they actually know what it really means. ( Just remember that and don’t be fooled). Now you can see that with strengthening or weakending the magnetic field you determine the voltage. So you can have high frequency but very little voltage or you can have low frequency and high voltage and you can have high frequency and high voltage. [ Oh by the way you can also get more or less voltage if you increase or decrease the length of the conductor also. However of course that is impractical because you have to redesign the motor. It is simpler to manipulate the strength of the magnet. More current through an electromagnetic, produce more strength and less current produce less magnetic field strength]

3. Current. What on earth determines current. Current is the number of charges moving or getting displaced. But where? in the conductor itself, the wires if you will. The bigger, the thicker the wire the more charges can move through it. The thinner the wire the less charges can move through it. So if your conductor has thick wires, you have more current, If your conductor is made of thin wires, less current can through it. ============================================= So why did I use the word displacement up there? Charge displacement that is?. It is because charges don’t move through conductors like water moves through a pipe. In AC charges/electrons just viberate back and forth and end up going nowhere. Therefore current is not how many charges move like water and of course that means distance, that is how far or what distance they move. No sir. When something move you have to ask at what distance. So charges or current of chages if you will is not measured by how far they travel, they are measured by how many of them passes at a certain point per second and that is it and nothing more. In DC charges move about an inch in hour. Oh jee, now what? Then what moves as electricity ( whatever that is, is pretty fast). Electricity are charges and electric energy… now that is something else. And here we go. Now.=====================================

4. What is electric energy? It is none of the above, it is neither frequency, nor voltage and not current either. Electric energy is electromangetic waves moving through empty space and not inside of wires and in one direction. …….. It is this propagation that transport the frequencies, the volts, and the amps from the source to the load. And no matter which way the charges move. And no matter what the frequency is, it has no effect to the speed. However the electromagnetic waves do transport those three values. Those values do not move from one place to another in and of themselves. No sir. Photons are the deliveryman and those are the parcels it delivers. And it is transported close to the speed of light. That is why when you turn on your switch your light comes on instantly. Boom. AND THAT IS IN PLAIN EASY TO UNDERSTAND LANGUAGE. DO NOT GET BAMBOOZLED BY FANCY LINGOS. Electric energy, the real thing, the stuff we are after move through empty space outside the wires. And there you have it. Cheers.

[[ I do not want to confuse you but between you and I, electric energy, that is the energy itself is not photons, is not current, is not voltage, it is not frequency. Photons deliver it, charges play role, so does volts ( pressure or potential difference) and frequency simply shapes the photons so that this invisible stuff called energy can be transported. NO ONE KNOWS WHAT ENERGY IS AND YET IT IS REAL. NOW DO YOU FEEL BAMBOOZLED OR INSPIRED AND FASCINATED]]. You be the judge of that.

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    \$\begingroup\$ Just because someone uses simple words doesn’t mean they actually know what it really means either. For example, "1. How fast are you turning the conductor per second round and round. That in itself determines the frequency and nothing else." It also determines the voltage. "... spitting out big words ..." Engineering has a vocabulary, as does any branch of science, and these facilitate clear and unambiguous communication. [Edited by a moderator.] \$\endgroup\$
    – Transistor
    Aug 8, 2021 at 22:44
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Here, I will even add more stuff for you too look at. Having said all that and so putting it all together, it looks like this.

A generator does not produce so called electricity, it does not produce charges. The entire wires, all the conductors, and the generator itself and everything around us is full of charges and that is a fact. A generator is simply a pump you can say. Just as a water pump does not produce water, a generator does not produce electricity either. It pumps electricity you can say. Electricity is already there. Electricity is another word for charges.

Now while water is pumped out of the system, out of the pipes basically; on the contrary with electricity charges stay in the system right where they are. If any charges leave from any particular place, the same amount flows right back. Round and round it goes like a flywheel. It is a closed system.

After it is all said and done. Photons, electromagnetic waves are produced and is absorbed, gone at the delivery checkpoint at the load if you will. At the source they arise from the electromagnetic field that surrounds the charges. When energy is worked into the field... what energy?

The work you put, turning that rotor, the conductor inside the magnetic field, disturbes the electromagnetic field, this causes the field to separate and it becomes what is called electromagnetic waves.

It is these waves that tranport that work you had put in. It simply transports your effort, your energy, which is made of combination of your inspiration, designing the system, all the sources, be it water or wind or you peddling it or whatever that cause the rotor, the conductor to turn and so on.

Once that energy is transported, the electromagnetic waves is gone, disspears, dies, it goes back into the field. However that energy moves on and takes on different forms. It is heat, it is mechanical, it is light, it is this, it is that. It has many names, it was not created, nor destroyed, We give it different names just to talk to each other. It is the stuff of the stuff. It in itself has no name. It is abstract and real all in one, that is energy.

Here is one for you. And the experts can correct me if I am wrong. No one has seen a photon, a photon of light if you will or for that matter a photon of anything. We don't see light, not by our eyes nor by an electron microscope. No such thing. Then what on earth are we seeing. What is seeing.

Check it out, when you see a wall, you are not seeing photons. What we see is what they call reflection. But did you ask reflection of what?

Aha, stay with that and see what happens.

It is not reflection of photons, it is meaningless. If that is true then you simply ask, " You mean I can't see the photons that go into the wall but I can see its relfection, the photons that come out of the wall?". It is meaningless.

So read this: Photons deposit the energy on the wall as it gets absorbed. And so photons sacrifies itself to deliver the goods you can say. What we see is the energy deposited by the photons. SEEING IS ENERGY. That nameless energy. Whatever photons that is not absorbed by the wall, reflected that is,termed as the color you see, no you don't see that, what you see is the imprints ( deposited energy) that is left behind and it is the experience called color.

Memories, experiences in whatever form, color, thoughts it is the stuff of energy. Be it in the computer, the supposed memory it stores, a knot in a rope or whatever other forms....the stuff itself is energy.

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