Measuring Amps on output coils

[Low-Q]

Hi

I have built a wheel with 16 magnets around it much like the bedini SG. I put a coil on the bottom with no other electronic components just to see if I could get anything from the coil. I have connected my mulitimeter to the coil. When i spin the wheel by hand I may get about 10 milliamps but it is not constant. However when i spin the wheel fast I get no reading at all! Only when the wheel is moving slowly can i get about 30 miliamps. Can someone explain what I am doing wrong. Once the wheel just about stops turning the wheel will start to speed up again. I guess this is the energy in the coil doing this?

I am on a very steep learning curve, so would appreciate any advice.

thanks

Nath

First advice: Provide all neccesary information.
Example of neccessary information:


Coil details -> Number of turns, coil hight/outer-inner diameter, wire gauge, inner resistance of the coil, iron core/air core
Distance between coil and magnets
Angular velocity of the magnets
Magnets orientation, assambly details.
Magnetic flux crossing through the coil measured in Gauss
Voltage readings over the coil.
Load details - whatever you use, bulb, resistor, short circuit. Provide resistance values.


An unloaded coil will not cause any current flow at all. You must load it. Also, you get AC out of the coil. Not DC. Check your instrument what setting you've choosen.


What's most interesting here, is the voltage x ampére product. I mean Watt or Joule pr./second - applies only on pure resistive loads. Ampére alone does not carry energy. Voltage alone does not carry energy.


Vidar

[nathanj99]

Because as the magnet approaches the coil, the flux will increase quickly in one direction, reach a maximum rate of change, and then the rate of change will fall to zero very close to where the magnet makes its closest approach.  That makes a voltage pulse in one direction.  As the magnet moves from its closest approach the reverse process occurs and a complementary shaped voltage pulse forms in both time and amplitude.Use a resistor that has a low inductive impedance compared to the resistance at the signal frequencies you are measuring.  Your basic frequency with 16 magnets will be:  rpm*0.267.  I don't know how sharp your rise times will be, so let's just take a gross stab and say that they are  about 1/350th the magnet to magnet interval.  So, the signal frequency content will be significant up to around RPM*0.267*350/0.35 ~= rpm*270.  I am going to venture that you are staying under 2000rpm, so I expect the frequency content will be mostly under 500kHz.  You would like jwL @ 500kHz < 0.1*R_CSR.  So, a tabulation:

L < 0.314uH*R

R = 100 Ohms, L < 31uH, 5W resistor safely handles 170mA rms
R = 10 Ohms, L < 3.1uH, 5W resistor safely handles 0.5A rms
R = 1 Ohm, L < 310nH, 5W resistor safely handles 1.7A rms
R = 0.1 Ohm, L < 31nH, 5W resistor safely handles 5A rms

These resistors from Digikey have practical inductances down around 3nH, and they are affordable, handle a decent amount of power, and are 1%:  http://www.digikey.com/product-search/en?pv1=1323&pv1=1025&pv1=1028&pv1=103&pv1=2358&FV=fff40001%2Cfff80482%2C1c0002&k=wne&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25.

You should solder sense wires to each side of the resistor, right at the body.  You can tightly twist those wires together and take them away to your oscilloscope probe, or DMM.

Wow thanks Mark. But seriously, that went way over my head. Maybe i should just quit at the woodwork stage . I just cannot get your equations and calculations. I dont really have much hope at this do i?

Nath

[sm0ky2]

Ampére alone does not carry energy.
Vidar

Could you provide an example of where Ampere exists, without voltage?

[Low-Q]

Could you provide an example of where Ampere exists, without voltage?

In superconductors. Current flow has nothing to do with voltage.
In the oposite case, you can have voltage, but no current (Ampéres).
Normal conductors have loss due to resistance. This resistance will cause a voltage drop when current flows through it.
Therfor you might assume that you cannot have a current flow without applying voltage.


For example: You only need current to make a magnetic field.
Permanent magnets have this current going on on an atomic level, but there is not voltage involved.
Also the reason why permanent magnets cannot provide free energy, because the product of current and voltage is zero.


Vidar

[tinman]

In superconductors. Current flow has nothing to do with voltage.
In the oposite case, you can have voltage, but no current (Ampéres).
Normal conductors have loss due to resistance. This resistance will cause a voltage drop when current flows through it.
Therfor you might assume that you cannot have a current flow without applying voltage.


For example: You only need current to make a magnetic field.
Permanent magnets have this current going on on an atomic level, but there is not voltage involved.
Also the reason why permanent magnets cannot provide free energy, because the product of current and voltage is zero.


Vidar

Current flow has everything to do with voltage. If you have no potential difference,then current will not flow. The voltage differential may be very small,but there has to be one. I must also ask how current flows in ferrite magnet's,as the ferrite is non conductive,and even if it were,you would have nothing but a dead short. A dead short with current flowing through it means extensive heat-->PMs do not get hot sitting on the bench.