A science question from a newbie

[madmardagan]

Hello, I have been ready the posts here and have been working on my own wind generator. I have what may be a simply answered question but one that has stumped me when explaining it to my kids. I have constructed my rotors and stator and tonight built my rectifier. When turning by hand I am easily getting 35+ volts (it's a 48 volt stator). But the moment I put a light to it as a load, the rotors are dang near impossible to turn. What is the mechanics behind this? Is this what also happens once the wind gen is up in the air and connected to a battery bank or is there something I am just missing here altogether? I must say it was facinating and completely unexpected. I've read about cogging but wow.

Thanks in advance for your posts!

[TomW]

Simply put, That resistance to turning is power being transferred from your hand into the load.

Tom

[wooferhound]

 A generator should spin freely without a load, as you start drawing power off of the genny it will start to be harder to turn and will get harder to spin as you take more Amps from the output. In fact, we will short out the generator to protect it from high winds. shorting the genny pulls the maximum power from the output causing the maximum drag to a turning motion.

 I am a little unclear on the cause but I'll attempt to explain it.

Without a load there is no current flowing in the coils. As you start pulling power from the genny and current flows in the coils they become electromagnets operating with AC voltages. Unfortunately the electromagnets that the coils create the same polarity as the magnet that is approaching the coils creating a North against North arrangement and trying to repel the approaching magnet.

 Multiply this effect by the number of coils and magnets in your generator and it can be a quite strong drag with high current outputs. This is why we talk a lot about stalling the blades as there is no drag while not producing electricity, and the drag increases as the blades speed up and more and more current is flowing creating ever increasing drag.

[Flux]

Don't confuse normal loading with cogging.

Cogging is an issue associated with iron cored alternators when the magnetic reluctance is not constant. With no load on it it turns in a series of lumps ( like badly meshed gears, hence the term cogging).

What you are experiencing is purely the load reaction and whether it feels lumpy or smooth, it is not cogging. There is no iron in an airgap alternator ( it can't cog by definition).

If the thing didn't become difficult to turn when on load you would be looking at perpetual motion. Whenever you change energy from one form to another this takes place.

You are just converting your work ( mechanical energy) into electrical energy in the alternator and then into heat and light in the lamp. Man can only produce just over 100W for any length of time so it becomes hard work to produce enough energy to light a modest bulb. This gives you some idea of the power produced by the wind turbine blades in a good wind. Even a small machine say 10ft can produce several horsepower but man can not manage 1/10th hp for more than a few seconds unless you are a cyclist or trained for man powered flight.

Best way to look at it from the conservation of energy point of view. You are converting work to other energy forms.

Flux

[tecker]

When the magnets align for charging the coils in a axil flux (air core stator ) the charge rises .As that phase is released ie the magnets move to null the current moves through the load .For a moment a flux is developed with the opposite  pole as the magnet in the center hole and the same pole as the approaching magnet.So for one half phase  all the coils in the phase that is conducting are trying to bond with the center hole  magnet and repulse the magnet that is  approaching.  As the current is relaxed the charge is reversed etc etc.

[imsmooth]

Why is it harder to turn when there is a load?    When there is no load there is no current.  There is just a potential: an electrical "pressure" ready to drive electrons.  When you have connection (load) current flows.  As the magnet moves over the coils (the magnetic flux induces the voltage allowing current flow), the current flowing through the coils sets up its own magnetic field in a direction that opposes the magnetic field which induced the current.  You use the right hand rule to get the current direction for a generator; you use the left hand rule for a motor.  

Let's say you have a N-S pair such that the magnetic field is going NS into the computer screen.  Let's also say the wire is moving relative to the field from right to left.  Setting up the right hand rule the first finger points into the screen (north going south away from you); the second finger point in the direction of the force causing the motion (the wire is moving right to left, or you could say the magnetic field is moving left to right); the thumb is pointing up, which is the direction of the current.

Now, using the left hand rule for a motor (current traveling in a wire within a magnetic field to cause motion), we see something interesting.  The thumb points up in the direction of the current; the first finger points into the screen (away from you) in the direction of the magnetic field.  We see that the second finger points left to right.  This is the direction of the force on the wire.  The force wants to push the wire the opposite  direction of the force that induced the current in the first place.  This is why the rotor gets harder to turn.   The greater the force pushing the rotor the greater the force pushing back against it as long as current is flowing.

This is an important point.  If the force was in the same direction as the force that induced the current we would get free energy.  You would simply have to start the motion of the wire through the magnetic field and then there would be a force created that would continue to push it through in the same direction.  Alas, this is not the case, and much like friction, there is a force that will always try to stop the motion.  Hope this helps.

[ghurd]

When it is just spinning, it is doing no work but spinning.

It is doing powerful work when the bulb is connected, so it is harder to turn.

Connect a smaller wattage bulb and it will be easier to demonstrate to the kids.

Maybe a 7W 120V night light bulb?

G-

[spinningmagnets]

Clearly air is not electricity, but something that happened with an air compressor once might be useful to ponder.

A neighbor had an air compressor. It had electric start on a small gasoline engine. He was annoyed that it was very hard to start it when there was half the max pressure in the storage tank.

He would drain all the air from the storage tank whenever he was done using it, and then he could later easily start it, but had to run it a long time to get the tank up to full pressure (which of course was quite noisy)

I recommended installing a "T" and two valves on the output of the compressor. By closing off the air-tank when it was still half-full, and opening the output of the compressor to the air, the compressor could be spun up easily without any load on it. Once the gasoline engine was running, the half-full air tank load could be slowly applied as the open exhaust route is slowly closed off.

Posters here who are more experienced than me have stated that it is very important to match the generator to the load. And also to match the turbine to the generator. It seems that if you are able, a larger turbine is almost always better than a smaller one, but its seems to me the most important question to start with is "what kind of wind do I have?"

PS I have been here over a year, and I am STILL learning new things !

[dsmith1427]

Think of it in terms of Newton's third law: For every action there is an equal and opposite reaction.  Also, remember there is a physical relationship between electricity and magnetism.  The rotating magnetic field of the rotor generates a current in the coils of the stator.  The current in the coils produce a magnetic field (much like the electric magnet we built as kids with a nail, wire, and battery).  The current in the coils is in opposition to the magnetic field of the rotors.  When the generator is free spinning you are measuring electric potential energy.  The very little internal resistance to the system.  When you added the light bulb, you created a load for the system which the current and magnetic field in the stator is trying to resist.  The blades of the turbine "harvest" the power of the wind and the rotor and stator converts the wind power to electrical energy which is converted to electrical power when it is connected to a battery bank (load).  Massachusetts Institute of Technology offers their Physics classes (classical mechanics - Electricity and Magnetism) on line at the following web site:

http://ocw.mit.edu/OcwWeb/Physics/index.htm

You can download them as a podcast or watch them on YouTube.  I finished classical Mechanics and I am on lecture 28 of Electricity and Magnetism.  The lectures are very entertaining and informative.  Part of the lecture is dedicated to demonstrating the theory.  Don't be intimidated by the math.  If you had a calculus class, you can follow.  If you did not have a calculus class, it is still worth watching the lectures because the professor, Dr. Lewin, is more interested in teaching concepts then solving complex math problems.  Anyway, I hope this helps!

Don

[dsmith1427]

"The current in the coils is in opposition to the magnetic field of the rotors."

"should be "The magnetic field generated by the current in the coils is in opposition..."

[CmeBREW]

Hello,

    What voltage and wattage rating is the light bulb you hooked up?  And did it light up?  It sounds like you hooked up something like a 12v 65W headlight to the 48V alternator. This would take more Torque to turn since it is only 12v and has to turn slower rpm.

On the other hand, if it were a typical 120v 100Watt light bulb, it would take MUCH less torque to turn -- but more rpm speed to partly light the bulb.

I learned a lot from my pedal generator experimenting.

I recall turning by ONE hand It was difficult to even get 3 amps (40 watts into a 12v batt).  

But using both hands to pedal I could reach around 5 amps for a minute.  But pedalling with both legs is a LOT easier to get 6-8 amps for a good amount of time.  

A person's legs are at least TWICE as strong as one's arms, is what I concluded.

-Also, There is a remote slight chance you wired a coil backwards. This would still light up the 12v light bulb, but at the same time magnetically brake the alternator making it harder to turn, and vibrate badly as you are turning it.

 

[Flux]

"Also, There is a remote slight chance you wired a coil backwards. This would still light up the 12v light bulb, but at the same time magnetically brake the alternator making it harder to turn, and vibrate badly as you are turning it."

It would certainly make it vibrate as the load would be mostly single phase. It wouldn't make it harder to turn, opposing coils just don't produce anything, they don't cause a braking torque, it would be easier to turn for a given load with reversed coils as the voltage would be lower for a given speed.

Flux

[CmeBREW]

Wow,,, thank you for the correction FLUX. I'm not sure where I got that wrong notion.

I was thinking the voltage AND current would be inducing in the opposing (wrong) direction in the backward coil, and this would cancel some voltage and cause some magnetic braking.  Thanks for clearing that up for me.  Sorry for being incorrect about that statement.

So It seems the backward hooked coil would produce negetive (or;opposing polarity) voltage which subtracts from the over-all produced voltage potential WITHOUT trying to reverse current in that coil.

So, evidently then, if I made a single phase alternator and hooked up every other coil incorrectly, it would cancel ALL voltage and NO current would flow even with the output wires shorted together.  But at least it would be easy to turn!

 

[mdntdncr]

"but at least it would be easy to turn!"

hahaha . . that really cracked me up.

Okay . . so what happens with this genny if he does hook it up to a battery storage bank.  Would it turn more easily? or would it depend on how much power the battery has already stored?

[tecker]

Cut in

[Madscientist267]

The generator would be easy to turn up until the point at which it is producing a potential equal to the battery. Any attempt to increase the speed of the rotor beyond that will result in resistance to rotation, since current is starting to flow through the coils. A phenomenon called "cut in".

[madmardagan]

I want to seriously thank each of you for posting your replies. They were all very helpful, and raised other questions. One last thing, in a follow up to the last. If it is easier to turn the generator when connected to a low battery until it meets the resistance of a full charge, then what would happen if the gen was tied to the grid? Could someone explain "cut in" a little more?

Thanks

[wooferhound]

Cut-In is the point at which your genny will start charging the battery, or the battery voltage. when your genny is producing less than battery voltage it will spin freely, but when it starts producing more voltage than the battery it will start being loaded since it will be charging the battery at that point. The voltage will not go up with an increase of RPMs after that, the voltage will be locked to the battery voltage after that, but the mechanical drag will increase and the Amps will start going up.

The battery will always try to keep the voltage going into it, at the voltage level of the battery itself. The battery will be sitting at 13 volts, and your Wind/Solar power will be 22 volts open circuit. As soon as you connect your open circuit Wind/Solar power to the battery, POOF ,everything is now running at 13 volts. It pretty much doesn't matter what voltage you want to charge the battery with, Your genny may be making 65 volts open, but connect it to the battery and then everything is at the battery voltage.

What happens to all that extra voltage? It's converted to amps and that is where you start to worry about your stator burning up. Once your wind genny reaches Cut-In (battery voltage) the measured volts will be the battery voltage which should be increasing slowly as it is charging. As your genny speeds up past cut-in the voltage doesn't increase, but the amps do. Under most circumstances you don't have to worry about the voltage of the device you are connecting to the battery, your worry will be with the ability of the charging device to deliver the Amps without burning up.

When Charging a battery try to keep the current going in to less then 10% of the capacity of the battery. For Example: a 100 amphour battery should be charging at 10 amps or less.

[Madscientist267]

It sounds like you're confusing a couple of things here.

Cut in refers to the point at which a generator begins to actually produce power.

Remembering Ohm's law is critical to understanding this.

The pertinent formula for this particular concept is P=IxE

Power (Watts) = I (Current) x E (Voltage, or potential)

A spinning generator with nothing connected to it produces a potential proportional to the speed at which it is rotating, but does not produce any power, because there is no current flow since the circuit is not complete.

In order for current to flow, you must have a complete path, and the potential to push this current. Voltage is pressure, just like in an air tank. Current is the number of electrons passing through a conductor.

Combined using Ohm's law, this is the Watt; the measurement of how much work is being done. In the case of a light bulb, it is how much energy it takes to get the filament hot enough to get the rated amount of light out of the bulb.

Exerting energy makes you tired because it's more work than doing nothing right?

Therefore, with no current flow, no work is being done, and no energy is used. Until you reach cut in, no current is flowing. The shaft of the generator is easy to turn.

At cut-in, the generator's shaft will begin to provide significant resistance to being turned any faster.

With that said:

The level of charge on the battery determines the cut in voltage. Any time the generator would have a higher open terminal voltage (OTV) than the battery, the generator will see a load, and thus suddenly become more difficult to turn at that RPM.

If a low battery is connected to a generator, the generator will reach cut-in at a lower RPM than it would if the battery were full.

This happens with a purely resistive load as well (such as a light bulb), but gets overlooked because the generator will always have a higher potential than the light bulb when it is turning. This is why no matter how fast you try to spin the generator with the light attached, it is always more difficult than if there is nothing connected to it. A light bulb always tends toward a potential of zero between the terminals, since it has no way of storing energy electrically.

Explaining grid tie gets hairy; as it would have several other mechanisms in place that would obfuscate cut in, but the same basic principles still apply.

In a nutshell, if the generator is not spinning fast enough to make a high enough voltage, the grid will remain above it, and the generator will never see a load. Once the generator is spinning fast enough to produce a higher potential than the grid, current would flow, and the generator is said to have reached cut in.

Fair warning, however: There is much more to consider than this when dealing with the grid, however, such as frequency, phase, syncronization, and so on, so please don't try any experiments like this with the live grid, lest you don't value safety and the like. AC is a vastly different animal than DC, particularly when combining power sources.

Steve

[spinningmagnets]

I'm sure I'll probably explain this wrong, but I think it will be helpful. Electrcity flows in the direction of least resistance. Diodes are a type of electrical check-valve that only allow flow in one direction.

If there are no diodes inbetween the battery and wind-gen, when winds are very slow the turbine will "motor" meaning it will draw energy from the battery and spin faster than the wind, like a fan.

If the wind then kicks up and has enough force to overcome whatever the battery voltage is, then the turbine will begin pumping electricity into the battery.