# Designing a Generator

## Main Question or Discussion Point

How do I determine magnetic spacing when designing a generator which has magnets placed evenly in a circular pattern so that each magnet do not affect one another when they are going to be spun around a wheel full of coil.

How do I determine the Volts generated?
How do I determine the magnetic field of a magnet if I am given the pull force in lb?

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To get a value for the voltage try the equation:
V= (closed loop integral of)(vXB)*dl where V=voltage v=velocity cross product B=magnetic field strength in Tesla and increment of lenght (dl).
Here is a video of equtions that will help.
http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/VideoAndCaptions/detail/embed11.htm" [Broken]
The most efficient generator I've ever seen with coils and permanent magnets is one nicknamed the "phi" generator. This is seen in the attachment I have included. The reason it is so powerful is because the magnet is never unevenly attracted toward the iron/steel core anywhere along the rotating path thus the magnetic drag losses are kept to a minimum. Also all of the magnetic field lined are contained within the radius of the generator thus using more magnetic flux of the magnet.

3. Force can be accounted for by the lorrentz force F=q(E+vXB) (q-charge E-electric field F-force) but B~1/r^2 meaning the magnetic field is a 1 over the distance squared phenomenon (in permanent magnets). So the maybe this fact can be used to determine the magnetic field strength.

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Thank you for the reply... The problem is my background is mechanical engineering, and I've hardly done electric courses... so I am not familiar with these formulas... I found this formula on the physics form.

V = - N A (2 pi RPM/60) B

where N = number of turns on the armature
A = area of coil on the armature
RPM = armature rotation speed
B = DC magnetic field
V= volts out

Do you think I can use this formula to determine the voltage? if I have a coil of R radius and l length and the thickness of the coil is r. If so how come the parameters of the coil aren't incorporated such as the length, R and the r? I am designing a Generator, I know the magnetic field of a the magnet because I am choosing the magnets, and I would be choosing the coil as well. I would also know the rpm.

How to design a generator?

Does anyone know how to design a generator or know any website that shows calculations that must be conducted to design a generator for example magnet spacing between magnets, the distance the magnets should be away from the coil...I was wondering if anyone knows any website that helps you to design a generator and equations that needs to be used to determine the voltage generated.
Thank you.

There are many types of generators, beginning with a dynamo (ac output from stator winding with voltage proportional to RPM), a generator (DC output from rotor coil via a commutator), a regulated dc generator (dc output from rotor coil with voltage feedback to stator), alternator (usually 3-phase output from stator winding with dc excitation on rotor coil via slip rings. Frequency output proportional to RPM).

What is your need, RPM, voltage, current? Do you have a rotating permanent magnet rotor?

Bob S

There are many types of generators, beginning with a dynamo (ac output from stator winding with voltage proportional to RPM), a generator (DC output from rotor coil via a commutator), a regulated dc generator (dc output from rotor coil with voltage feedback to stator), alternator (usually 3-phase output from stator winding with dc excitation on rotor coil via slip rings. Frequency output proportional to RPM).

What is your need, RPM, voltage, current? Do you have a rotating permanent magnet rotor?

Bob S
This is for my thesis project and I got like 15 days to finish it off ...I am looking to do a DC generator...I ve done research and understand how it works...I want to know how to do calculation for a similar generator that was created by the person in this website http://www.otherpower.com/17page1.html if you don't mind looking at it quickly...My design is similar to that....with permanent magnets in two rotars which are going to rotate around a rotar with coil...I just want to know how to conduct calculations such as Voltage, and other calculations if there are any...Please help...Thank you

This design is a three-phase alternator (with 16 magnets on rotor and 12 coils on stator)? This produces ac voltage and requires rectification to produce dc. This a BIG project to finish in two weeks.

Bob S

This design is a three-phase alternator (with 16 magnets on rotor and 12 coils on stator)? This produces ac voltage and requires rectification to produce dc. This a BIG project to finish in two weeks.

Bob S
Hey, I am not going to build it, I am just want to know how to calculate the voltage that can be produced...is there any formulas that I can use to determine the voltage and other useful information such as current, also how much resistance is created if the magnets are close to the stator as compared to it being further apart...Thanks

Correct me if I am wrong. There are twelve coils. They are connected as three sets (meaning three phase) of four coils in series. The polarity of connecting the coils is important. Because there are sixteen magnets on the rotor, the output frequency is eight times the revolution frequency. So to get the output voltage per phase, get the area of the iron in one coil A, the peak B in this iron, the dB/dt = ωBmax = 8ω0Bmax, the number of turns N per coil. So the peak output voltage per phase might be

Vpeak = 4·A·N·8·ω0·Bmax

where ω0 = 2 pi RPM/60. Does this sound right? Please check.

The importance of getting the magnets close to the coils is to minimize the magnetic reluctance, which becomes important when there is current in the coils (back emf).

Bob S

berkeman
Mentor

Correct me if I am wrong. There are twelve coils. They are connected as three sets (meaning three phase) of four coils in series. The polarity of connecting the coils is important. Because there are sixteen magnets on the rotor, the output frequency is eight times the revolution frequency. So to get the output voltage per phase, get the area of the iron in one coil A, the peak B in this iron, the dB/dt = ωBmax = 8ω0Bmax, the number of turns N per coil. So the peak output voltage per phase might be

Vpeak = 4·A·N·8·ω0·Bmax

where ω0 = 2 pi RPM/60. Does this sound right? Please check.

The importance of getting the magnets close to the coils is to minimize the magnetic reluctance, which becomes important when there is current in the coils (back emf).

Bob S
Do you have a source in which you've got the formula? Also you've mentioned B as the magnetic field in the iron of the coil...But what about the magnetic field of the magnets? Do they not play a role in any calculations? Also how do I determine the B in the coil if I do not know the I.
Also if I were to find the I, the current that is induced through the coil, do I simply add a resistor of an assumed variable e.g. R=1 omhs and just simply plug in to I=V/R?

Thanks for your replies...really appreciate it...

Do you have a source in which you've got the formula? Also you've mentioned B as the magnetic field in the iron of the coil...But what about the magnetic field of the magnets? Do they not play a role in any calculations? Also how do I determine the B in the coil if I do not know the I.
Also if I were to find the I, the current that is induced through the coil, do I simply add a resistor of an assumed variable e.g. R=1 omhs and just simply plug in to I=V/R?
I based my formula and estimate on Faradays Law of induction:

V = ∫E·dl = -(d/dt)∫B·n·dA

This equation determines the output voltage and not output current. The magnetic field B (Tesla) is continuous (div B = 0), unlike H, the magnetization (amp-turns per meter), so the B inside the coil iron is entirely determined by the magnet (remanent) field and the external magnetic circuit. The voltage in my calculation (guess) is the no-load voltage output V0 based on Faraday's Law, hence there is no current in the coils. The current output should be ideally I = V0/R, but air gaps in the magnetic circuit (as well as the coil resistance) will have a big effect on the actual output voltage.

Bob S

I tried to formula you've given me, but the answer I am getting is unreasonable...maybe its because of the air gaps in the magnetic circuit...Should I multiply my answer by a factor of 0.8 compensate for all those things? Or maybe a factor of 0.5?

In the formula i estimated in post #9 (please check)

Vpeak = 4·A·N·8·ω0·Bmax

be sure to use A (square meters), N (turns per pole), ω0 (radians per second), and Bmax (Tesla). What are your values and what do you get for an answer?

Bob S

In the website I posted earlier http://www.otherpower.com/17page1.html when they tested out their generator for one coil and one rotor (consisting of 16 magnets) they generated 2.4 Volts @ 70 rpm.

Their values were:
A= 0.01824 m^2 (Area of the coil; theirs was 6 inches diameter)
N=79 per coil
RPM=70
B=0.2 (I found a magnet similar to theirs; approixmated to be this value)
I am getting a value of 67.6 Volts

Then when they put both the rotors on; they got At 70 rpm now, with this one coil, 5.4 Volts AC

The area inside this coil in the thumbnail is 0.001 square meters at most, not the 0.018 square meters you are using.

Bob S

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so if I were to multiply the V x 2 should give me the voltage of one coil rotating around the two rotars am I correct? And if I were to multiply this voltage by 12 (because there are 12 coils) should give me the Vo which is the peak ideal peak voltage for the entire system right?

Thank you so much for your help...you've helped a lot

so if I were to multiply the V x 2 should give me the voltage of one coil rotating around the two rotars am I correct? And if I were to multiply this voltage by 12 (because there are 12 coils) should give me the Vo which is the peak ideal peak voltage for the entire system right?
You will have to check the details, but I think in the generator you refer to, there are four coils each wired in a three-phase output circuit, so twelve coils total on three separate circuits. Please review the detailed magnetic design of the generator, and check my formula for mistakes.

Bob S