Secret Of Back EMF

[MarkE]

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.

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.

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.

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.

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.

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.

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.

[TommeyLReed]

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]

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.

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.

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.

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.

Tom

[ltseung888]

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]

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.