Alternator question - Page 1
Forum section: electronics / general

author: nix85
author_rank: Regular Contributor
timestamp: July 09, 2019, 01:59:30 pm
content: This apparently simple question has been troubling me for years and altho i asked number of "experts" none had even a remote idea what the answer is.To summerize, we know that as magnetic pole approaches the coil, in the coil is induced a field which opposes it (Lenz) and as it receeds field that attracts it, as shown in this vid.Now, if we take a look at an alternator, let's take this single phase S16 model from Mecc Alte which has 2 rotor poles and like 20 (or more) stator coils.How does this configuration produce regular AC?At any point in time rotor magnets are approaching number of stator coils and receeding from same number of stator coils, clearly inducing opposing voltages.But this is not what happens so i am clearly missing something out.What's the catch. Please answer ONLY IF YOU KNOW.I don't need more confusion about this.

author: nix85
author_rank: Regular Contributor
timestamp: July 09, 2019, 02:21:26 pm
content: The model above is a 50Hz alternator and must be turned at 3000 RPM to produce 50Hz.How are these stators wound, is there really 20+ coils there or it only appears so..?Another example of single phase alternator, 2 rotor poles, how many stator coils??

author: Miyuki
author_rank: Frequent Contributor
timestamp: July 09, 2019, 02:29:10 pm
content: It is called Distributed winding, it is one coil spread in multiple slots to improve geometry and optimize coil, space and core ratio.And also affect waveform.For more info Search : What is Concentrated and Distributed Winding

author: nix85
author_rank: Regular Contributor
timestamp: July 09, 2019, 02:39:05 pm
content: Quote from: Miyuki on July 09, 2019, 02:29:10 pmTnx a lot, as i assumed.

author: nix85
author_rank: Regular Contributor
timestamp: July 09, 2019, 02:50:33 pm
content: Another quick question. In video above it is shown that as magnetic pole passes the center of the coil, polarity in the coil switches.But in this old military video, it shows just the opposite, as you can see in the screenshot.Which one is correct?

author: nix85
author_rank: Regular Contributor
timestamp: July 09, 2019, 03:15:41 pm
content: To me it seems old military vid is correct afterall.Or rather, they are both correct for different config they show.

author: nix85
author_rank: Regular Contributor
timestamp: July 11, 2019, 06:21:33 pm
content: That video above is wrong, voltage changes direction at midpoints, not at the poles, as shown in this video.I hope i did not case confusion to someone as that canadian video did to me.

author: soldar
author_rank: Super Contributor
timestamp: July 11, 2019, 07:12:25 pm
content: Note that the voltage induced is proportional, not to the magnetic flux traversing the coil, but to the rate of change of the magnetic flux. In other words to the first derivative of the flux.

author: nix85
author_rank: Regular Contributor
timestamp: July 12, 2019, 12:20:17 pm
content: ANOTHER INDUCTION QUESTIONIn alternator for example, as magnet sweeps across the coil voltage is induced.What i find strange is if coil is wound as in picture below, both sides of the coil (left and right side relative to the magnet) see flux in same direction moving towards them.This is supposed to induce voltage in the same direction which shuld cancel the overall voltage.This is the very reason why in dc generator oppsite sides of the coil move in opposite direction in the same magnetic field, to induce opposite voltages on each sides of the coil.What's the catch.

author: nix85
author_rank: Regular Contributor
timestamp: July 12, 2019, 12:48:28 pm
content: In other words, if we sweep a magnet along the side of the coil like this, is voltage induced?I tried it and it is, the same voltage as when i sweep it across the coil (horizontally in the pic below).How come?

author: nix85
author_rank: Regular Contributor
timestamp: July 12, 2019, 01:36:31 pm
content: Also if we compare this part of the video below with what happens in DC generator, isn't what we see contradictory. In DC generator to get voltage across the coil, each side of the coil is spinning in opposite directions inside the same field, thus inducing opposite voltages on each side.However in video below when coil is placed at the exact middle of the wire, so that upper part of the coil sees flux in one direction and lower in another, there is no voltage induced on the coil.https://youtu.be/g_HD_17-Msw?t=95

author: nix85
author_rank: Regular Contributor
timestamp: July 12, 2019, 01:55:15 pm
content: If we look at DC generator, pic below, each side of the coil is moving in opposite direction inside same magnetic field to induce opposite voltage on each side of the coil.If we then look at this induction video, we can see if coil is placed at the exact middle of the wire with oscillating current, voltage on the coil is 0 and speaker explains this as "flux in the upper section is opposite to flux in the lower section and thus they cancel out".https://youtu.be/g_HD_17-Msw?t=91But if we look at the direction of induced voltage as i sketched below, we see the induced voltage is also opposite on each side of the coil, just like in DC generator example.Why then is there no induced voltage?

author: Red Squirrel
author_rank: Super Contributor
timestamp: July 12, 2019, 04:17:59 pm
content: One thing I always wondered actually is how do they keep the voltage constant? For example you have an alternator that needs to produce 120v at 60hz. It needs to spin at a constant speed to maintain the 60hz, but as you put a bigger load on it, there would be a voltage drop, how is the voltage kept at exactly 120v no matter the load? Normally you would speed up/speed down but since you need to keep the frequency the same you can't do that. Or do generators always have a variac too to regulate voltage as needed?

author: soldar
author_rank: Super Contributor
timestamp: July 12, 2019, 04:55:25 pm
content: Quote from: Red Squirrel on July 12, 2019, 04:17:59 pmA generator producescurrent, not really voltage. The current produced is proportional to the magnetic flux so the field current is adjusted to maintain current at whatever level is wanted.

author: nix85
author_rank: Regular Contributor
timestamp: July 12, 2019, 05:24:00 pm
content: Automatic voltage regulator.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 12, 2019, 05:32:54 pm
content: Quote from: Red Squirrel on July 12, 2019, 04:17:59 pmThe voltage stays the same as you add load... but the alternator gets harder to turn the more current you draw, so whatever is driving it has to work harder just to keep it at the same RPM.

author: soldar
author_rank: Super Contributor
timestamp: July 12, 2019, 06:10:50 pm
content: Quote from: SilverSolder on July 12, 2019, 05:32:54 pmThe voltage stays the same because there is a regulator adjusting the field current (and a battery to serve as a buffer).The voltage (in reality the current) is controlled by the regulator.

author: nix85
author_rank: Regular Contributor
timestamp: July 12, 2019, 06:15:46 pm
content: Quote from: soldar on July 12, 2019, 06:10:50 pmDo you (or anyone) know the answer to this.If we look at simple induction example below, both sides of the coil (left and right side relative to the magnet) see flux in same direction moving towards them. This is supposed to induce voltages in the same direction in opposite sides of the coil which should cancel out but do not.It would be even better if coil was square shaped but you get the picture. I asked this at yahoo answers too, just one dared to answer and all he came up with is "it's due to Maxwell's equasions". LOLIn case you did not understand the question, imagine left and right sides of the coil were just two vertical wires not connected. Look at the direction of the induced voltage, it's in the same direction. Now we connect them, obviously, they cancel out, but they don't.What's going on.

author: Red Squirrel
author_rank: Super Contributor
timestamp: July 13, 2019, 12:56:35 am
content: Quote from: soldar on July 12, 2019, 04:55:25 pmThat's kinda what I figured, sort of like a solar panel. The more you draw the lower the voltage goes, more or less. So how is the voltage kept the same, as you need the voltage to be the same no matter how much current is being drawn.I'm speaking about AC alternators, like power plants or portable generators. Or do those all have regulators, like a variac that is constantly adjusting? Or something more sophisticated that slightly moves the magnets a certain way to change the field?For DC I imagine something like a car alternator also has a SMPS regulator built in to always output 13.5v once the current reaches a certain point? The speed at which it turns matters less, so I imagine if one were to build a DC generator you can also alter the speed.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 13, 2019, 01:52:02 am
content: Quote from: soldar on July 12, 2019, 06:10:50 pmIt is the voltage that is controlled in a car alternator - there is also a current limit (to prevent overload) but it is not the primary controlled parameter. You can run some cars without a battery at all and it will still stay nicely around 14.5V (not recommended with newer cars, do not try this at home, etc.). In the bad old days, this was all implemented with buzzers and relays...The engineering problem/challenge in a car is that the engine RPM is not constant, so the regulator has to keep changing the field current to keep the voltage constant despite changes to engine RPM. (as a first approximation). It is a real b!tch at idle speed where many alternators struggle to keep up with the load... especially smaller alternators on motorcycles etc.

author: amyk
author_rank: Super Contributor
timestamp: July 13, 2019, 03:31:19 am
content: Quote from: nix85 on July 12, 2019, 06:15:46 pmThe field strength is increasing on one side of the coil, and decreasing on the other. In the exact middle, the sum of the induced currents cancels, but at any position other than the middle, they do not, and you will see that the midpoint is where the current goes to zero and then reverses direction.I recommend reading this book, it goes into a lot of detail about electric machines:https://archive.org/details/hawkinselectrica01hawk

author: soldar
author_rank: Super Contributor
timestamp: July 13, 2019, 08:48:21 am
content: Quote from: Red Squirrel on July 13, 2019, 12:56:35 amA basic regulator is extremely simple. Below a certain voltage the field current is max and the alternator output is at its max. As the voltage rises above a certain point the field current is reduced by the regulator which means the output current is reduced proportionately.Just like a linear voltage regulated power supply does not regulate voltage directly but it has a feedback loop that controls the current allowed through the series transistor and by controlling this current it maintains a voltage at the output. Both a linear power supply and a car generator are basically current sources which have a feedback control loop designed to maintain an output voltage.A generator can be thought of as an amplifier. The output is the input (field) amplified. And the power is supplied in the form of mechanical power instead of electrical.I tinkered with my boat's regulator so I could manually control the output current. By shorting the zener on the bottom right to ground the transistor will never limit the field current and the alternator will always give max current regardless of voltage.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 13, 2019, 11:42:52 am
content: The voltage generated at the output terminals of an alternator is directly proportional to the speed of the generator and the flux; V = K * Flux * speed * turnsSo if the RPM is constant, and the field winding current is constant, the voltage at the output will also be constant.

author: soldar
author_rank: Super Contributor
timestamp: July 13, 2019, 01:30:29 pm
content: Quote from: SilverSolder on July 13, 2019, 11:42:52 amI could be mistaken but I believe this is not correct. I believe the current would be maintained not the voltage.The alternator in my boat with the regulator disabled outputs about 55 A which goes into the system, part being used and part into the battery. If consumption is over 55 A part will come out of the battery. If consumption is under 55 A then the battery will be charged regardless of voltage and it can be overcharged that way. it will go up to whatever voltage it needs to maintain current. I have seen up to 18 V with a bad battery.What do you think would happen if the battery is accidentally disconnected? (a) Voltage is maintained at ?? volts or (b) voltage spikes and damages a ton of instruments?Ask me how I know the answer.

author: soldar
author_rank: Super Contributor
timestamp: July 13, 2019, 05:05:57 pm
content: I think most people do not realize the basic essence of all electro-magnetic machines.Let us leave aside for now pure inductances and flyback coils which store energy over time and only consider machines that transform energy directly, in real time. Transformers, generators, motors, all fall into this group. None of them store energy. At any given moment energy in equals energy out (minus losses).Energy in creates magnetic flux that is at all times counteracted by the flux created by the output side.A transformer does not need more core to handle more power. A lot of people seem surprised by this simple fact.It needs more copper wire which then necessitates a bigger core. (This is a huge simplification, obviously.)Take a regular ideal voltage transformer. There is a small current to keep the core magnetized. Neglecting this the magnetic flux does not change at all when the current changes on primary and secondary. That is the whole principle and basis for the working of a transformer. Flux created by one side is instantly counteracted by flux created by the other side.But flux is created by current, not by voltage. A transformer, a motor, a generator arecurrentmachines. All currents must create zero flux. Flux created by all inputs and outputs must add up to zero. And, again, it is current that creates flux, not voltage.In a voltage transformer the ratio between primary and secondary voltages is a consequence of applying a voltage source to one side but the rule remains that the sum of fluxes created by currents remains zero (neglecting magnetizing current). The secondary takes current which causes flux which is immediately counteracted by current in the primary.In an electrical rotary generator, at a given speed, the output current is proportional to the input field current. Set a given speed and field current and you get a fixed output current which, in a vehicle goes into the battery which, absent the regulator, sets the voltage.The regulator diminishes the field voltage so that the battery voltage does not exceed the floating setpoint.My (smallish) alternator outputs about 55 A at max output. If the wire from the alternator to the battery is increased in length and resistance the current will remain the same while the voltage at the alternator increases a bit.

author: nix85
author_rank: Regular Contributor
timestamp: July 15, 2019, 10:46:12 am
content: I just want that we are all clear on this, is this video correct or not, i see many people claim the same. As magnet approaches the coil field is induced to counter it (lenz) and as it starts to move away induced field flips now attracting the magnet (lenz), just like when we shove the magnet into and out of the coil.https://youtu.be/bht9AJ1eNYc?t=2742Well, video is right if we consider field first cuts the left side of the coil producing one polarity and as it crosses the center and starts to move away cuts another side of the coil producing opposite polarity.

author: GlennSprigg
author_rank: Super Contributor
timestamp: July 15, 2019, 12:04:50 pm
content: Unless I'm severely missing something in all these responses, the O.P. seems to be missing something...'Diagrams' like that do NOT correctly indicate the effects ofInductive Reactance, (andmotion).Consider how the 'brushes' in say a 'Motor' are offset so many degrees, to reduce commutator 'sparking' ??Or how/why a motorized "Threading Machine" designed to work in 'both' directions, has the 'brushes' setat precisely 90-deg, so as to equalize the lesser 'sparking' in both directions... but so suffers both ways...If you don't understand 'current' phase-shift, then we can do no more ??

author: soldar
author_rank: Super Contributor
timestamp: July 15, 2019, 12:33:07 pm
content: Yes, I am not even sure what the question is.QuoteYes, but how are the coils wired?Because this is like saying in a two cylinder engine when one cylinder is going up the other one is going down so how can anything move? Well, because the rods are 180º apart on the crank.Or maybe I am just not understanding the question.Take a generator with a single pole and spin a permanent magnet inside. For every complete turn of the magnet this creates in the coils a complete cycle. Seems pretty simple and obvious.Now put another two poles so that they are all 120º apart and you have three phases.So when the magnetic field is moving away from one phase it is moving towards the next phase. So what? The magnetic field is offset by 120º and so are the voltages generated. So what? If you short two phases then you have a problem but you don't do that.The next step is to build a machine with a multiple of poles such that a single mechanical rotation is two or more electrical cycles.Or maybe I am just not understanding the question.

author: nix85
author_rank: Regular Contributor
timestamp: July 15, 2019, 01:29:24 pm
content: You two are clearly missing something, everything has been cleared up in my last post.

author: soldar
author_rank: Super Contributor
timestamp: July 15, 2019, 01:48:00 pm
content: Quote from: nix85 on July 15, 2019, 01:29:24 pmWell, I am sorry but I really do not understand what it is you don't understand.I really do not see it as complicated. Take a single pole and it is not difficult to understand. When the N field recedes from a coil the S Field is approaching and vice versa. Yes, one is approaching and one is receding but they are contrary signs and "pushing" in opposite directions. That is the whole foundation of the machine.Now take a three phase generator. It is just three generators in one machine. You could take three single pole generators on the same rotating shaft and get the same result. Whatever happens in one phase is exactly what happens to the next phase with 120º delay.I am sorry. I can't explain it any better.I think this may be something like themissing dollar riddlewhere you are just analyzing the question in a way that leads to confusion.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 15, 2019, 02:42:10 pm
content: Quote from: soldar on July 13, 2019, 01:30:29 pmIt is definitely not a good idea to disconnect the battery on any alternator charging system newer than the 60's, since the regulator performance without a battery is poor (not helped by various electronic boxes connected to the system reacting to the fluctuations in various ways, potentially making the problem worse, inducing oscillations, spikes, etc.)...It is the job of the regulator to keep the system voltage at 14.5V (or so). In that system, it is the job of the alternator to supply enough current to make that happen. So, when controlled by a regulator, the alternator output is indeed a boring constant 14.5V and the only thing that ever changes is the current coming out of it.However, in the case of an alternator on its own, with no regulator, and no load connected, the voltage you would measure at the terminals is simply proportional to speed and field current.

author: larrybl
author_rank: Regular Contributor
timestamp: July 15, 2019, 04:12:04 pm
content: Quote from: nix85 on July 09, 2019, 02:21:26 pmThat looks a lot like the one I am working on.

author: Red Squirrel
author_rank: Super Contributor
timestamp: July 15, 2019, 05:50:48 pm
content: Interesting so there is two sets of coils, a stationary one and one that spins? I always figured the spinning part would just be magnets. Or are those coils acting as the magnet and just require seed power to get it going then it powers itself?

author: soldar
author_rank: Super Contributor
timestamp: July 15, 2019, 05:54:54 pm
content: Commonly the field is created by a coil. That way you can control the output by controlling the strength of the field (by controlling the current).

author: Red Squirrel
author_rank: Super Contributor
timestamp: July 15, 2019, 06:34:09 pm
content: Ahhh I see so that's how the output voltage would be regulated. Now where does the power to power the coil come from initially, I assume it takes from battery?

author: soldar
author_rank: Super Contributor
timestamp: July 15, 2019, 06:59:02 pm
content: Yes, look at the schematic I posted earlier in this thread.It is often possible to start an alternator generating without a battery because there is often a bit of remaining magnetism in which case it immediately multiplies. You just need a tiny bit of current or magnetism to seed the process.

author: tautech
author_rank: Super Contributor
timestamp: July 15, 2019, 11:16:44 pm
content: Quote from: soldar on July 15, 2019, 06:59:02 pmMuch less so for an alternator however a generator (dynamo) that uses a field winding, then yes, residual stator pole magnetisim is what kicks generation off.Generators that are new or been rebuilt sometimes need polarizing to get them to function and this is a simple process in which the field winding is momentary flashed from B+ to start charging. Normally this only needs be done once.However it depends on the regulator circuitry and type where sometimes the regulator has NO ignition key connection to power the field winding and in this case residual pole magnetisim initiates charging.....never seen this principle used for alternators and would be interested to know if it is.

author: larrybl
author_rank: Regular Contributor
timestamp: July 15, 2019, 11:22:24 pm
content: A common No-Output issue can be resolved by simply plugin in an electric drill and spinning it. My Generator / Alternator doesn't have a battery so I assume it is self exiting. PS I bought this at a junk sell for @25.00 usd.

author: Circlotron
author_rank: Super Contributor
timestamp: July 15, 2019, 11:28:51 pm
content: Because of the low residual magnetism in an automotive type alternator rotor, the rotor winding needs to be "tickled" to get it going. After it is up and going the rotor current is supplied by the three relatively small field feed diodes. Usually an initial current is fed to the rotor via the dashboard charge warning light, and if this light ever goes open cct the alternator may perhaps not begin charging. Naturally the light won't come on to let you know...

author: GlennSprigg
author_rank: Super Contributor
timestamp: July 17, 2019, 10:37:22 am
content: Quote from: Red Squirrel on July 15, 2019, 05:50:48 pmThe 'term' they use is 'Field Excitation' to get it started ! although as others have said, SOMETIMES a bit ofresidual magnetism is enough on smaller systems to act like a 'boot-strap' ......I remember once, about 40-odd years ago, (working with our then power-authority), that due to a catastrophicsystem failure, the State lost ALL power, everywhere! As a result, there was NO power to Excite the windingsin the main plant Generators!! After running a small manual isolated diesel generator, they got a system backupplant on line, (no load yet), which then connected to the Main Plants large generators to 'excite' their Fields.'Residual' magnetism didn't/wouldn't cut it with these huge machines !!NOTE: The 'backup' plant (for the middle stage of this process), was also needed apart from 'Excitation', to drivethe (something-like... I've forgotten...) 1000-HP Jacking-Oil pumps, that 'lifted' the huge turbines so many thou,before the turbines could start to turn in the huge bearings. Interestingly too, these huge jacking-pumps werestarted 'D.O.L.' (Direct On Line) giving only a short 'blip' to the huge generators!

author: SilverSolder
author_rank: Super Contributor
timestamp: July 17, 2019, 11:10:17 pm
content: I guess it's a completely different world, when machinery becomes that big!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 17, 2019, 11:56:24 pm
content: Quote from: nix85 on July 12, 2019, 12:20:17 pmConcentrating on the wires may make this more confusing. Current induced comes from changes to the field penetrating a loop of wire. If you draw the fields at multiple points during the passage of the magnet you will see that the fields penetrating each loop of the coil have the same polarity, even though they may be weaker at the far end of the coil. Thus the current induced in the end loops has the the same polarity, and thus the induced voltages are the same polarity. When connected together those voltages add.This explanation glosses over some important details. For extra credit think about the fact that when connected together the currents in both loops must be the same, in spite of the different magnitudes of the field at the two ends. How is this resolved? As usual, this is left as an exercise for the student. As your Yahoo informant stated the math all works out when applied properly.

author: nix85
author_rank: Regular Contributor
timestamp: September 03, 2019, 08:53:05 am
content: I already cleared up the issue on another thread, there was no need for your reply but ok.What i noticed in general about you so called humans, you like to SOUND smart, but you don't care much about really BEING smart.You usually take 10 paragraphs to explain what can be PERFECTLY explained in 1 sentence.Be more effective, be more intelligent.