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### Author Topic: Secret Of Back EMF  (Read 49663 times)

#### MarkE

• Hero Member
• Posts: 6830
##### Re: Secret Of Back EMF
« Reply #45 on: August 17, 2014, 01:38:00 PM »
MarkE.. please correct me if I'm wrong , but if the voltage developed during inductive kickback of coil depends only on the time of decay, and if voltage is EMF or is caused by EMF, then why we are not talking about EMF if I saw equations stating that EMF generated depends on the rate of change of current ?
The rate at which current builds or decays in an inductor is a function of the voltage across the inductor and the inductance:  di/dt = V/L.  This fundamental relationship is the basis of all hard switching switch mode power converters motor drivers.
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That is my fisrt question.

Then, secpnd question : if voltage or EMF / or the EMF itself is the result of inductive kickback then why we never see a kickback of large current instead of voltage ?
Magnetic fields resist current changes.  The voltage changes as needed in order to maintain the current at its instant value.  As the voltage changes, the current begins to change at a rate that depends on the voltage.
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Isn't that the EMF or voltage according to Ohm's law the force generating current across resistance ?
If there were no magnetic field that would be correct.  As there is a magnetic filed  the equation becomes more complicated.  In addition to I*R, there is L*di/dt.
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What limit us to use some weird coil configuration during inductive kickback to get more amps instead of bigger voltage then the original power supply can do?
A transformer readily translates the voltage or current up or down as desired.  Flyback devices, be they power supplies or your ignition system do exactly that.
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Or in other word : why I didn't saw video of Tesla coil generating lots of amps on secondary ?
Energy is conserved and he did not have gigaWatts for his input power supply.  Had Tesla had the use of a 1GW power plant then I think he probably would have built things that threw out thousands of amps and hundreds of thousands of volts.
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Impossible ?
Not if you have enough source power and enough materials to build a big enough transformer.  The Pacific Intertie transforms power to 500kV and 3000A.
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EMF is just a force, why it is limited to generating voltage spikes during inductive kickback ?
It isn't so limited.  BEMF is easily visible when it generates a big spike.  But it is present in virtually every electronic device that you use daily, including the wires that connect digital signals between ICs.
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#### TommeyLReed

• Hero Member
• Posts: 554
##### Re: Secret Of Back EMF
« Reply #46 on: August 17, 2014, 02:09:57 PM »
HI Mark,

I'm doing other experiments, but I can show a basic DC motor with a commutator.

As the motor runs at higher speeds with no load, there won't be any BEMF, but as the motor load down the EMF is created and higher voltage is pump into the capacitor at lower amps.

I aslo can do another experiment showing a large coil with .6 ohms resistance  being pulse with a PWM at o volts and a few amps, yet I can get very good output with BEMF at 30% modulation on the PWM setting.

You can call the this action aflyback, but it is not the same a BEMF? Flyback are transformer not a single coil like my experiments.

Tom

#### MarkE

• Hero Member
• Posts: 6830
##### Re: Secret Of Back EMF
« Reply #47 on: August 17, 2014, 02:38:37 PM »
HI Mark,

I'm doing other experiments, but I can show a basic DC motor with a commutator.

As the motor runs at higher speeds with no load, there won't be any BEMF,
Tom, a classic demonstration of generator BEMF is to connect two small DC motors together.  Spin one and the other spins due to the generator BEMF of the first.  The diode I labeled D1 in your circuit blocks generator BEMF from flowing through your capacitor / switch circuit.
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but as the motor load down the EMF is created and higher voltage is pump into the capacitor at lower amps.
As you load down the motor, more current is carried in the windings and the more magnetizing energy there is in them to discharge into your right hand circuit.
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I aslo can do another experiment showing a large coil with .6 ohms resistance  being pulse with a PWM at o volts and a few amps, yet I can get very good output with BEMF at 30% modulation on the PWM setting.

You can call the this action aflyback, but it is not the same a BEMF?
It is BEMF.
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Flyback are transformer not a single coil like my experiments.
Single coil flyback circuits are relatively common.  They are a convenient way to generate negative voltage supplies.
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Tom

#### ltseung888

• Hero Member
• Posts: 4363
##### Re: Secret Of Back EMF
« Reply #48 on: June 25, 2016, 12:08:29 PM »
HI Mark,

I'm doing other experiments, but I can show a basic DC motor with a commutator.

As the motor runs at higher speeds with no load, there won't be any BEMF, but as the motor load down the EMF is created and higher voltage is pump into the capacitor at lower amps.

I aslo can do another experiment showing a large coil with .6 ohms resistance  being pulse with a PWM at o volts and a few amps, yet I can get very good output with BEMF at 30% modulation on the PWM setting.

You can call the this action aflyback, but it is not the same a BEMF? Flyback are transformer not a single coil like my experiments.

Tom
Tom,

Use a DSO to capture the waveform as in the "ufo propu" thread.

#### dieter

• Hero Member
• Posts: 938
##### Re: Secret Of Back EMF
« Reply #49 on: June 27, 2016, 04:19:03 AM »
Isn't that funny, so many interpretations.

I agree, the CEMF that drags the generators rotor when a load is attached, is not what is generally meant by the term "BEMF-Spike" or IK. As a motor and a generator are basicly reversible, the CEMF in a motor is the higher current dissipation when a mechanical drag is attached, or when it starts up. But there is also usually a non-optimal situation in which some partially  interaction between rotor and stator is opposing the rotation. This could be considered CEMF as well. Tesla did some designs to reduce that.

Inductive Spikes, often termed BEMF too, are diffrent, in that they are the result of a collapsing field. It is important to understand that a low impedance coil can collapse very quickly and therefor generate a much higher voltage. As the voltage at the coil is now much higher than at the positive supply contact, the current will naturally flow from high voltage to liw voltage, which is back, and will probably fry some fragile transistors etc.

If Tommy for some reason managed to abruptly stop current supply to a coil, due to some diodes, he may indeed cause spikes, even if the power supply seems to power the motor without interruption.

But one thing tinman said made me wondering: "in inductive kickbacks is no free energy" ... I doubt that. A high pression or tension (voltage) pulse is caused, which is able to pull electrons out of an external medium, given a situation in which freely / easily moveable electrons are available at the ground side of the collapsing coil. With the right frequency one may pull HV at high amperage that way, because current at HV has inertia, which results in arcing when separating electrodes, as is known.

#### darediamond

• Full Member
• Posts: 178
##### Re: Secret Of Back EMF
« Reply #50 on: August 31, 2018, 08:31:01 AM »
At Resonance, Back E.M.F is SPECIAL.

My coil of 940inches when tuned to it resonant frequency uses maximum of 8W (24V/8W) to generate 560Vdc X 36mA.

#### darediamond

• Full Member
• Posts: 178
##### Re: Secret Of Back EMF
« Reply #51 on: August 31, 2018, 09:44:48 AM »

You do not seem to understand Tom at all.

First of all know that there is Electrity in the Air and Earthcrust or Material Ground.
When you energised a coil at High Frequency, you are on your way to Overunity or taping cosmic power.
Now,energise or pulse that coil at it resonance frequency you get more power out than in FOR SURE WHETHER THE COIL IS OF HIGH RESISTANCE OR LOW RESISTANCE.

But the best is to tune an high resistance coil to it Resonance's frequency or better pad the coil so as to enable it to get pulsed at high frequency.

You can further boost the back e.m.f by collecting the power in a Series-Paralled capacitors the exactly matches the Back E.M.F Voltage. That is provided your back emf voltage is 500V, just get 25v 4700uf or 10000uf DC caps and get 40 pieces of High Voltage High Frequency Diodes like HER208 or FR607.
Connect the caps is series with Jump Wire. Thereafter, solder the forward of the diode to the positive terminals of the capacitors in series and solder the backward part of the diodes to the negative points on the serially linked caps.

Now, parrraled the diodes points that are negative
Then parallel the points that positive.

What you have done is to leverage the power in High frequency to amplify the Back emf current. If you connect 20pieces of 10000uf Caps in Parallel, the charging period will be high. But now that you Series the caps to 1000uf at 500V,  it will take low time to get repeatedly charged and recharged.

Your best bet is reduce the capacitance further by making sure you use thin wire to make your coil and thus generate at resonant frequency of the high resistance coil High voltage.

Let say you generate 2500v back emf, that means you need maximum of 100pieces of 25Vdc rated electrolytic caps and 400pieces of H.V H.F Diodes.
Let say the each cap is 10000uf.In series, that makes 2500v at 100uf. And this level, the amassed current will be high as the capacitance is  now further lowered by 100times.

Back E.M.F recirculation, high pulsing frequency, high resistance coil and capacitor trick makes the magic Power in BEMF.