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Electric vehicles => Electric bikes => Topic started by: TommeyLReed on August 12, 2014, 08:16:28 PM
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Hi All,
I throught it would be a great test to show some basic back EMF test on this pulse motor. This is free extra energy while you add input energy.
https://www.youtube.com/watch?v=6BnHNdMzW2I
Tom
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Tommey that's a nice video, but the conclusions are incorrect. The energy recovered from the motor BEMF through the rectifiers into the capacitor bank all came from your power supply. The motor operates as a generator that you tap through your diode / capacitor network when the MOSFET is off. When you shorted the capacitors the generator worked against the small resistance of the diodes and the wires, slowing down a lot each MOSFET off interval. That's why the motor ran slowly. When you removed the short, the capacitors charged until a new equilibrium point was reached that balanced the power supply output energy each cycle against the electrical and mechanical losses. Instead of heating the wires as with the short, much more of the power from the power supply went into mechanical work by spinning the motor faster against the friction in the system. If you put a resistor or some other circuit that does work across the capacitor bank, then for the same power supply settings and MOSFET duty-cycle, the motor will slow down.
An experiment that you can do that I think is educational would be to put a fixed value power resistor, RSERIES, between the power supply and the motor. Set the power supply to some fixed voltage with the current limit greater than: ILIMIT >= VSUPPLY/RSERIES, and then vary the load across your capacitor, keeping track of the load resistance, load voltage, and motor speed. See what load value gives the most total electrical plus mechanical power.
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Hi Mark,
First of all there is no PWM involve in this design.
This works only off the RPM with a optical transistor and 4 incoder on the disk at every 90deg.
The change in rpms is do to the fact BEMF is not running back to the coil.
The fee energy is coming from the BEMF of power input coil.
The motor speed up while the BEMF is collected, it slow down when shorted out and increase load.
Tom
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Hi Mark,
First of all there is no PWM involve in this design.
This works only off the RPM with a optical transistor and 4 incoder on the disk at every 90deg.
In the video you showed a board connected to an optical interrupter switch that drives a MOSFET that chops the motor current. And from your description above, the MSOFET chops the power supply connection to the motor. You are just using the rotation angle instead of a fixed frequency to drive the chopping / PWM. Once the motor is going some relatively constant speed the MOSFET turns on and off at regular intervals and is for most intent and purpose acts same as a fixed frequency PWM chopper.
The change in rpms is do to the fact BEMF is not running back to the coil.
Actually, that is backwards. The motor runs slowly when a lot of the energy built-up during the MOSFET ON time gets dumped into the external circuit during the MOSFET OFF time. The motor runs its fastest when there is no external circuit other than the power supply. Every last bit of energy that leaves the motor during a given MOSFET OFF interval gets replenished during the MOSFET ON time.
The fee energy is coming from the BEMF of power input coil.
If you measure carefully, you will find that there isn't any free energy. All of the energy that can be accounted for in the external circuit and work performed by the motor spinning against the friction load and then some can be shown to come from the power supply.
The motor speed up while the BEMF is collected, it slow down when shorted out and increase load.
Absolutely, that is correct for the reasons stated above.
Tom
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@ Tommey
First you need to know that it is not the backEMF you are collecting in your cap's-you are collecting the inductive kick back from the inductors within the motor when the mosfet opens.
BackEMF(counter EMF) dose the opposite to what you state,in that backEMF is what brings the motors current draw down-not raise it as you suggest. Back EMF is the voltage, or electromotive force, that pushes against the current which induces it.If there is no back EMF,the current draw will be very high.The more back EMF you have,the lower the current draw is
To explain a little better-Lets say you are supplying your motor with 10 volts @ 1 amp,and the voltage going to the inductors in your motor is very close to 10 volts. The backEMF (once the motor is up to running speed)may be say 7 volts.This means that the voltage across the inductors is only 3 volt's.So it is taking 10 volts @ 1 amp to maintain that 3 volt's across the inductors of the motor. If there was no back EMF,then the voltage across the same inductors will be very close to 10 volt's,and it would take a lot more amp's(current) to maintain that 10 volts across the inductors.A simple way to see this is when you first start an electric motor,and there is very little backEMF.You will see a high current draw(start up current)-same as when you place an electric motor under load.
So back EMF is needed to bring current draw down-it dosnt raise the current draw of an electric motor.
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Hi Tinman,
I built many motors and even have 19hp DC motor. BEMF it created due to starting a motor under load. When motor speed is full speed, less if any BEMF is created.
The reason speed controller fail is the increase of BEMF when motor are at the stall point.
I can also prove this with a transformer load using DC pulse. This way you just might understand how BEMF really works.
This is what BEMF does using a coil to recover BEMF. This motor is being power by BEMF only!
https://www.youtube.com/watch?v=2xLwBaub4lc&list=UUp3mD3EJromKns3YpglKdpA (https://www.youtube.com/watch?v=2xLwBaub4lc&list=UUp3mD3EJromKns3YpglKdpA)
Tom.
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Hello all,
If you think Back EMF is not something special, look what happens when you use a transformer and DC pulse.
https://www.youtube.com/watch?v=Ty5CLgjmd4c
Tom
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Tom, Tinman is correct. Take a small DC motor such as from a toy. Measure the winding resistance with a DMM. Then hook the motor up to a power supply with a current sense. Lock the rotor. turn on the power supply to a low voltage and measure the current. It will be V/R. Unlock the rotor and as the motor comes up to speed the current goes down. The net current through the motor is: I=(VSUPPLY - w*KGENERATOR)/RWINDING. Since the motor torque is: I*KTORQUE = (VSUPPLY - w*KGENERATOR)/RWINDING*KTORQUE.
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Hi Mark,
Clearly you don't understand my test, I know that. I was showing the effect of the diode when it cancels out the BEMF by grounding it..
As I place the diode in the motor, the BEMF is shorted out, this increase load while slowing down the motor a little.
When I take the diode out and feed the bemf into the capacitors the motor increasee with speed and load drops, that was my point of this test.
This test show the grounding out BEMF with a diode on the +/- create a greater load then without it.
That was my point on the video!
Tom.
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Hi Mark,
Clearly you don't understand my test, I know that. I was showing the effect of the diode when it cancels out the BEMF by grounding it..
As I place the diode in the motor, the BEMF is shorted out, this increase load while slowing down the motor a little.
When I take the diode out and feed the bemf into the capacitors the motor increasee with speed and load drops, that was my point of this test.
This test show the grounding out BEMF with a diode on the +/- create a greater load then without it.
That was my point on the video!
Tom.
Tom, there are two sources of BEMF in a motor: BEMF induced by motion which is the generator BEMF, (flux changes across the conductors), and BEMF caused by changes in the current path to the motor windings. When you switch your MOSFET ON or OFF the latter is at work. When the MOSFET turns ON the current in the motor windings does not change instantly. The motor winding inductance produces a BEMF in response to applied voltage changes that decays with time. The current changes from its previous value following an inverse exponential towards a limit value of the difference between the power supply voltage and generator BEMF divided by the winding and circuit resistance. When the MOSFET switches OFF inductor BEMF develops to maintain the winding current. When it builds high enough your diode conducts and curent continues into your capacitor and whatever you have connected across it. If the voltage on the cathode side of your diode is less than the generator BEMF, then current builds-up towards a limiting value of: (w*KGENERATOR-VDIODE-VCAPACITOR)/RCIRCUIT. When you remove the short across the capacitor, the capacitor ultimately charges to the point that it does not load the generator BEMF.
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Rather than "shorting" the input to the capacitors,
why not disconnect the capacitors instead?
By shorting the kickback the magnetic field which
is producing it is prolonged within the motor. It
may be that is what is reducing the motor speed and
causing a higher than normal current flow.
In some pulse operated motors a diode is placed
across the motor leads to "short" the kickback
which actually increases motor efficiency by
prolonging the current produced magnetic field.
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Ok. I really WANT somebody experienced to clear the difference and explain this topic.
For my simple mind there are two effects in play : let's call the first one " backEMF" rather COUNTER-EMF and the second one rather FLYBACK SPIKE. Am I right ?
CounterEMF is what I see as effect of self-inductance motors coils and I heard it limits the voltage applied to motor coils, because it is in oposite to applied power source voltage polarity.
Is this is correct then :
1. How is that everytime it fights against applied power ? Is that a matter of arrangements of coil ?
2. What would happen if counterEMF in motor coils will be very small ? Would that burn out wires inside motor ?
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Hopefully this diagram will clear things up.
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Ok. I really WANT somebody experienced to clear the difference and explain this topic.
For my simple mind there are two effects in play : let's call the first one " backEMF" rather COUNTER-EMF and the second one rather FLYBACK SPIKE. Am I right ?
CounterEMF is what I see as effect of self-inductance motors coils and I heard it limits the voltage applied to motor coils, because it is in oposite to applied power source voltage polarity.
Is this is correct then :
1. How is that everytime it fights against applied power ? Is that a matter of arrangements of coil ?
2. What would happen if counterEMF in motor coils will be very small ? Would that burn out wires inside motor ?
BackEMF(counter EMF) limits the voltage in the motor coils(as you call them forest),and thus limits the current. The lower the Back EMF,the higher the current draw.
Inductive kickback(or fly back) is when a powered inductor has it's power source cut off quickly(becomes open circuit).The current wants to maintain its flow as the magnetic field around the inductor collapses. Now while the voltage potential inverts from powered to non powered across the inductor,the current flow remains in the same direction. This is the very reason the Bendini ssg has the charge battery negative hooked to the run battery positive,as the positive becomes the negative potential when the inductor becomes open(transistor becomes open).
From what i can make out in Tommeys setup(a bit hard without a schematic),is the reason the motor bogs down when he shorts out the caps,is because he has created a continual loop in the motors inductors(coils).This being the case,the current has no where to go but back into the coils,and this would maintain the magnetic field around the coils,and in turn would be what is bogging the motor down,and drawing the extra current. Inductive kickback(fly back) is not free energy,but is what is left over from the supplied power that the motor(and all its losses) didnt use(convert).
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Hello all,
If you think Back EMF is not something special, look what happens when you use a transformer and DC pulse.
https://www.youtube.com/watch?v=Ty5CLgjmd4c
Tom
Tom-you made an inverter.
And it's inductive kick back or fly back-not back EMF.
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Hi All,
This is the basic design on the video.
This version replace the motor with a transformer.
https://www.youtube.com/watch?v=Ty5CLgjmd4c (https://www.youtube.com/watch?v=Ty5CLgjmd4c)
Tom
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Tom, here is your drawing with some annotations that should explain things.
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Hi Mark,
Yes that is the design, I did it in a simple drawing.
Now the question is, do you still think this is not BEMF?
As you ground out the capacitor, the two diodes run in series to positive and negative post of the coil, this shorts out the BEMF when kick back happens.
Like many transformer using dc pulse this is needed to protect the transister from being destroy from BEMF high voltage spikes.
My other question is why would the motor slow down, when I just short out the BEMF?
I believe it's from a negative pulse that pulls the motor magnet field backwards.
Question anyone?
Tom
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Hi Mark,
Yes that is the design, I did it in a simple drawing.
Now the question is, do you still think this is not BEMF?
As you ground out the capacitor, the two diodes run in series to positive and negative post of the coil, this shorts out the BEMF when kick back happens.
Like many transformer using dc pulse this is needed to protect the transister from being destroy from BEMF high voltage spikes.
My other question is why would the motor slow down, when I just short out the BEMF?
I believe it's from a negative pulse that pulls the motor magnet field backwards.
Question anyone?
Tom
Tom, nothing has changed. The capacitor shorted or not provides a path for the magnetizing energy in the motor winding when the path through the MOSFET opens. The capacitor charges cycle by cycle until the leakage through the capacitor and the diodes is enough to dissipate the magnetizing energy dumped into the capacitor each cycle. When the switch is closed, the energy dissipates much more slowly than when the capacitor is charged up to some voltage. That means that when the switch is closed more energy goes to the motor and it has more torque. If the motor is slowing down with the switch closed, then the diodes to the capacitor are leaking badly. In that case, generator EMF is driving durrent through the leaking diodes and switch.
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;)
Tom, nothing has changed. The capacitor shorted or not provides a path for the magnetizing energy in the motor winding when the path through the MOSFET opens. The capacitor charges cycle by cycle until the leakage through the capacitor and the diodes is enough to dissipate the magnetizing energy dumped into the capacitor each cycle. When the switch is closed, the energy dissipates much more slowly than when the capacitor is charged up to some voltage. That means that when the switch is closed more energy goes to the motor and it has more torque. If the motor is slowing down with the switch closed, then the diodes to the capacitor are leaking badly. In that case, generator EMF is driving durrent through the leaking diodes and switch.
;)
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Hi Mark,
Yes that is the design, I did it in a simple drawing.
Now the question is, do you still think this is not BEMF?
As you ground out the capacitor, the two diodes run in series to positive and negative post of the coil, this shorts out the BEMF when kick back happens.
Like many transformer using dc pulse this is needed to protect the transister from being destroy from BEMF high voltage spikes.
My other question is why would the motor slow down, when I just short out the BEMF?
I believe it's from a negative pulse that pulls the motor magnet field backwards.
Question anyone?
Tom
Tom, you can approximate shorting out the motor by for example putting a really good diode across it. The BEMF then takes on a low value as it only has to rise enough to make the diode forward conduct. That gives the motor the highest average torque during the MOSFET off interval. The higher the voltage that is across the motor: D1 anode to D3 cathode, IE the greater the BEMF, the faster that current in the motor winding decays, the lower the average torque, and the slower the motor runs against any constant load compared to the "shorted" condition. If the motor is slowing down when you close the switch, then that means that you are diverting power when the MOSFET is on due to leaky D1 and D3 diodes.
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Tom have you tried to recycle the kick back
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First,could you resize down images ?
I see that you all state that backEMF during coils magnetizing current (mosfet ON) is the same backEMF which charge capacitor bank.Am I right ?
I , oppositely think it is two different effects. First is due to self-inductance of motor coils , second one is due the LC oscillations between self-inductance of coils and distributed capacitance of wires.Because voltage is rised expotencially in short time it looks like a spike called flyback spike, but in fact it is ringing down oscillation.
What do you think ?
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Forest, if you are addressing me, I use a 72dpi resolution which is OK for 12 point type and for B size pictures stays below the 1280 pixels that Stefan asks. The image file sizes are generally well under 100Kbytes. The alternatives are breaking up the pictures or making them fuzzy.
Because only one of the switches is active, only the magnetizing energy, IE energy stored as I2LWINDING/2 goes through the D1/capacitor or switch/D3 path. That energy builds-up when the MOSFET is ON and discharges when the MOSFET turns OFF.
The energy that is stored in the winding capacitance and other parasitics is next to nil compared to the magnetizing energy, and can be safely ignored with no loss in accuracy. Ringing can be seen when the magnetizing inductance gets completely discharged. Then the very small remaining energy can slosh back and forth between the inductance and capacitance.
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Tom have you tried to recycle the kick back
Dave, in order to recycle the magnetizing energy back into the power supply an active switch is needed on both sides of the motor.
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First,could you resize down images ?
I see that you all state that backEMF during coils magnetizing current (mosfet ON) is the same backEMF which charge capacitor bank.Am I right ?
I , oppositely think it is two different effects. First is due to self-inductance of motor coils , second one is due the LC oscillations between self-inductance of coils and distributed capacitance of wires.Because voltage is rised expotencially in short time it looks like a spike called flyback spike, but in fact it is ringing down oscillation.
What do you think ?
No-the only one that claims this is Tommy.
What charges the caps is the collapsing magnetic field around the inductor,when the mosfet becomes open. When you have a low resistive load placed on the inductive kickback,the magnetic field collapses slowly(motor bogs down),when you have a high resistive load,the magnetic field collapses much more quickly(caps have high internal resistance)-motor dosnt bog down so much.
The simple SSG pulse motor is a good learning tool,and you can see all this happen on a scope when you place different resistive loads on the kickback output.
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Hi TinMan,
Please explain where the high voltage comes from?
If this is a simple load on the motor then why would the caps reach 50v-100v?
Tom
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Hi TinMan,
Please explain where the high voltage comes from?
If this is a simple load on the motor then why would the caps reach 50v-100v?
Tom
Hi Tom
I am glad you asked,as the answer will show you why it isnt the backEMF you are collecting. The back EMF voltage is always less than the supplied voltage to the coil(coils) in the motor. I think you were supplying 19 volts from your power supply?,so the back EMF voltage would be less than that. This means that the 50-100 volts in your cap could not be from the back EMF.
When you power up an inductor(coil),a magnetic field is built around the inductor(this im sure you know).When the power flowing into the inductor is abruptly disconected(mosfet opens),the magnetic field around the inductor collapses.The speed that this field collapses is directly related to the load applied to the inductive output-(in your case,caps),and the voltage produced by the inductive kickback is directly related on how fast that field can collap's. So if you have a very low resistive load on the kickback output(say 10 ohms),then the maximum voltage reached may only be say 12 volts across that 10 ohm's-but the current will be high and flow for longer period of time, due to the magnetic field collapsing slower. If you have say a 100 ohm load on the inductive kickback output,then your voltage across that 100 ohm load may be 70 volts,but the current flow will be lower,and for a shorter period of time,because the magnetic field collapses faster.
As your caps have a high resistance value in your setup,you can achieve a high voltage from the inductive kickback.If you place a 10 ohm resistor across your cap's,the voltage will drop,but the current will rise,and your motor will bog down.
Look at your inductive kickback as a generator hooked to a motor-the only difference in your case ,is that the generator and motor are all one device. But as you load the generator with heaver loaed's,you will see your motor bog down-just as in a nomal generator/motor system.
If you like,i can take the time to build a pulse motor,and make a video showing you on a scope how all this happens.
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Hi TinMan,
Please explain where the high voltage comes from?
If this is a simple load on the motor then why would the caps reach 50v-100v?
Tom
Tom the energy that builds up in the motor magnetizing inductance has to be dissipated. The capacitor alone stores energy with little dissipation. Voltage builds up until something gives: the capacitor and/or the diodes so that the magnetizing energy gets dissipated each cycle.
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Hi Tom
I am glad you asked,as the answer will show you why it isnt the backEMF you are collecting. The back EMF voltage is always less than the supplied voltage to the coil(coils) in the motor. I think you were supplying 19 volts from your power supply?,so the back EMF voltage would be less than that. This means that the 50-100 volts in your cap could not be from the back EMF.
When you power up an inductor(coil),a magnetic field is built around the inductor(this im sure you know).When the power flowing into the inductor is abruptly disconected(mosfet opens),the magnetic field around the inductor collapses.The speed that this field collapses is directly related to the load applied to the inductive output-(in your case,caps),and the voltage produced by the inductive kickback is directly related on how fast that field can collap's. So if you have a very low resistive load on the kickback output(say 10 ohms),then the maximum voltage reached may only be say 12 volts across that 10 ohm's-but the current will be high and flow for longer period of time, due to the magnetic field collapsing slower. If you have say a 100 ohm load on the inductive kickback output,then your voltage across that 100 ohm load may be 70 volts,but the current flow will be lower,and for a shorter period of time,because the magnetic field collapses faster.
As your caps have a high resistance value in your setup,you can achieve a high voltage from the inductive kickback.If you place a 10 ohm resistor across your cap's,the voltage will drop,but the current will rise,and your motor will bog down.
Look at your inductive kickback as a generator hooked to a motor-the only difference in your case ,is that the generator and motor are all one device. But as you load the generator with heaver loaed's,you will see your motor bog down-just as in a nomal generator/motor system.
If you like,i can take the time to build a pulse motor,and make a video showing you on a scope how all this happens.
The generator BEMF is always less than the supply voltage. The motor winding BEMF is a flyback voltage limited only by the external circuit and the breakdown voltage within the motor. Recirculating the motor winding energy speeds the motor up by supplying more average current and therefore torque. As there is no specific DC current path provided for the winding energy, the capacitor charges up until something starts leaking badly.
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As there is no specific DC current path provided for the winding energy, the capacitor charges up until something starts leaking badly.
I would say that the cap charges up until it has reached the maximum output of the flyback value.This is of course taking into account resistive lossed and the likes-unless that is what you are refering to MarkE as leakage ?. Of course there will be some leakage in the cap,and this can be seen as the voltage slowly dropping in the cap,if left sitting with a charge in it. This can also be seen as the internal resistance in the cap bleeding off the charge.
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I would say that the cap charges up until it has reached the maximum output of the flyback value.This is of course taking into account resistive lossed and the likes-unless that is what you are refering to MarkE as leakage ?. Of course there will be some leakage in the cap,and this can be seen as the voltage slowly dropping in the cap,if left sitting with a charge in it. This can also be seen as the internal resistance in the cap bleeding off the charge.
There is no maximum value until something breaks. The magnetizing energy integrates on the capacitor until the voltage is high enough that the energy gets dissipated either through a resistance, or a clamping circuit, or leakage across the capacitor. Many have designed circuits where flyback energy from a switched inductor broke things. It's blown up a lot of John Rohner's controllers.
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Many people just think they know the answers!
Inductive components like motor winding resist sudden changes in current. That's because the magnetic field caused by the current needs time to build up or decrease. That means that when current is flowing and this is suddenly cut off, the winding will try to maintain that current, and becomes a power source generating a voltage to be able to do so. It gets its power from the built up magnetic field.
Since the winding is now a power source instead of a consumer the voltage is reversed for the same current flow direction. That also explains how the voltage on a coil can become higher than the power supply: instead of subtracting the voltage over it you add it to the power supply. That's why you need a flyback diode on for instance a relay coil: the diode will allow the back emf to flow back to the power supply without damaging the switching transistor.
When a current flows through a conductor it generates a magnetic field around the conductor. with that being said in a solenoid the exact process take place, The magnetic fields around each turn on the coil link with the rest of the other fields on other turns to form complete loops around on the out side and the inner core of the coil. These line of flux will determine the polarity and strength of the solenoid. No matter how tight are the turns there will be flux lines that will always remain around each turn, these smaller flux lines will induce a current in the coil when there is an applied voltage(these currents that are induced are known as Eddy currents). But when these currents are induced they will be in a opposite direction with the applied current and since it is in a counter direction therefore it is known as the back EMF.
Counter-electromotive force
From Wikipedia, the free encyclopedia
Jump to: navigation, search
The counter-electromotive force also known as back electromotive force (abbreviated counter EMF, or CEMF)[1] is the voltage, or electromotive force, that pushes against the current which induces it. CEMF is the voltage drop in an alternating current (AC) circuit caused by magnetic induction (see Faraday's law of induction, electromagnetic induction, Lenz's Law). For example, the voltage drop across an inductor is due to the induced magnetic field inside the coil, and is equal to the current divided by the impedance of the inductor.[1][2] The voltage's polarity is at every moment the reverse of the input voltage.[1][3]
The term Back electromotive force, or just Back-EMF, is most commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature of the motor and the magnetic field from the motor's field magnets, or windings. From Faraday's law, the voltage is proportional to the magnetic field, length of wire in the armature, and the speed of the motor. This effect is not due to the motor's inductance and is a completely separate effect.
In a motor using a rotating armature in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. This produces a voltage in the coil; the motor is acting like a generator (Faraday's law of induction.) at the same time it is a motor. This voltage opposes the original applied voltage; therefore, it is called "back-electromotive force" (by Lenz's law). With a lower overall voltage across the armature, the current flowing into the motor is reduced.[4] One practical application is to use this phenomenon to indirectly measure motor speed and position since the Back-EMF is proportional to the armature rotational speed.[5]
In motor control and robotics, the term "Back-EMF" often refers most specifically to actually using the voltage generated by a spinning motor to infer the speed of the motor's rotation for use in better controlling the motor in specific ways.[6]
To observe the effect of Back-EMF of a motor, one can perform this simple exercise. With an incandescent light on, cause a large motor such as a drill press, saw, air conditional compressor, or vacuum cleaner to start. The light may dim briefly as the motor starts. When the armature is not turning (called locked rotor) there is no Back-EMF and the motor's current draw is quite high. If the motor's starting current is high enough it will pull the line voltage down enough to notice the dimming of the light
https://www.youtube.com/watch?v=VfvfkXhHw04
I don't need to add any more, this says it all!
Tom
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Inductive components like motor winding resist sudden changes in current. That's because the magnetic field caused by the current needs time to build up or decrease. That means that when current is flowing and this is suddenly cut off, the winding will try to maintain that current, and becomes a power source generating a voltage to be able to do so. It gets its power from the built up magnetic field.
Since the winding is now a power source instead of a consumer the voltage is reversed for the same current flow direction. That also explains how the voltage on a coil can become higher than the power supply: instead of subtracting the voltage over it you add it to the power supply. That's why you need a flyback diode on for instance a relay coil: the diode will allow the back emf to flow back to the power supply without damaging the switching transistor.
...
Tom
That is the essence of the phenomenon.
Simple is best.
Except for the last sentence.
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Many people just think they know the answers!
Inductive components like motor winding resist sudden changes in current. That's because the magnetic field caused by the current needs time to build up or decrease.
The field increases or decreases according to the time integral of applied voltage.That means that when current is flowing and this is suddenly cut off, the winding will try to maintain that current, and becomes a power source generating a voltage to be able to do so. It gets its power from the built up magnetic field.
That is a close enough description for city music. Since the winding is now a power source instead of a consumer the voltage is reversed for the same current flow direction.
Correct, the voltage is whatever it takes to sustain the immediate current.That also explains how the voltage on a coil can become higher than the power supply
That is also correct.: instead of subtracting the voltage over it you add it to the power supply.
Polarity is determined by Lenz' Law. The voltage rises or falls in such a way as to resist a change in current. That's why you need a flyback diode on for instance a relay coil: the diode will allow the back emf to flow back to the power supply without damaging the switching transistor.
A diode recirculates the current through the relay winding.
When a current flows through a conductor it generates a magnetic field around the conductor. with that being said in a solenoid the exact process take place,
No it is the exact same thing. The magnetic fields around each turn on the coil link with the rest of the other fields on other turns to form complete loops around on the out side and the inner core of the coil. These line of flux will determine the polarity and strength of the solenoid. No matter how tight are the turns there will be flux lines that will always remain around each turn, these smaller flux lines will induce a current in the coil when there is an applied voltage(these currents that are induced are known as Eddy currents).
Eddy currents inside conductors give rise to the skin effect. But when these currents are induced they will be in a opposite direction with the applied current
Eddy currents induce voltage that opposes the driving voltage. They are about resisting current changes. and since it is in a counter direction therefore it is known as the back EMF.
Counter-electromotive force
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The counter-electromotive force also known as back electromotive force (abbreviated counter EMF, or CEMF)[1] is the voltage, or electromotive force, that pushes against the current which induces it. CEMF is the voltage drop in an alternating current (AC) circuit caused by magnetic induction (see Faraday's law of induction, electromagnetic induction, Lenz's Law). For example, the voltage drop across an inductor is due to the induced magnetic field inside the coil, and is equal to the current divided by the impedance of the inductor.[1][2] The voltage's polarity is at every moment the reverse of the input voltage.[1][3]
That is correct. BEMF resists changes in the current that any applied voltage attempts to drive.[quot]
The term Back electromotive force, or just Back-EMF, is most commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature of the motor and the magnetic field from the motor's field magnets, or windings. From Faraday's law, the voltage is proportional to the magnetic field, length of wire in the armature, and the speed of the motor. This effect is not due to the motor's inductance and is a completely separate effect.
In a motor using a rotating armature in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. This produces a voltage in the coil; the motor is acting like a generator (Faraday's law of induction.) at the same time it is a motor. This voltage opposes the original applied voltage; therefore, it is called "back-electromotive force" (by Lenz's law). With a lower overall voltage across the armature, the current flowing into the motor is reduced.[4] One practical application is to use this phenomenon to indirectly measure motor speed and position since the Back-EMF is proportional to the armature rotational speed.[5]
In motor control and robotics, the term "Back-EMF" often refers most specifically to actually using the voltage generated by a spinning motor to infer the speed of the motor's rotation for use in better controlling the motor in specific ways.[6][/quote]That is the generator BEMF.
To observe the effect of Back-EMF of a motor, one can perform this simple exercise. With an incandescent light on, cause a large motor such as a drill press, saw, air conditional compressor, or vacuum cleaner to start. The light may dim briefly as the motor starts. When the armature is not turning (called locked rotor) there is no Back-EMF and the motor's current draw is quite high. If the motor's starting current is high enough it will pull the line voltage down enough to notice the dimming of the light
https://www.youtube.com/watch?v=VfvfkXhHw04
I don't need to add any more, this says it all!
Tom
Yes it does.
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Many people just think they know the answers!
Inductive components like motor winding resist sudden changes in current. That's because the magnetic field caused by the current needs time to build up or decrease. That means that when current is flowing and this is suddenly cut off, the winding will try to maintain that current, and becomes a power source generating a voltage to be able to do so. It gets its power from the built up magnetic field.
Since the winding is now a power source instead of a consumer the voltage is reversed for the same current flow direction. That also explains how the voltage on a coil can become higher than the power supply: instead of subtracting the voltage over it you add it to the power supply. That's why you need a flyback diode on for instance a relay coil: the diode will allow the back emf to flow back to the power supply without damaging the switching transistor.
When a current flows through a conductor it generates a magnetic field around the conductor. with that being said in a solenoid the exact process take place, The magnetic fields around each turn on the coil link with the rest of the other fields on other turns to form complete loops around on the out side and the inner core of the coil. These line of flux will determine the polarity and strength of the solenoid. No matter how tight are the turns there will be flux lines that will always remain around each turn, these smaller flux lines will induce a current in the coil when there is an applied voltage(these currents that are induced are known as Eddy currents). But when these currents are induced they will be in a opposite direction with the applied current and since it is in a counter direction therefore it is known as the back EMF.
Counter-electromotive force
From Wikipedia, the free encyclopedia
Jump to: navigation, search
The counter-electromotive force also known as back electromotive force (abbreviated counter EMF, or CEMF)[1] is the voltage, or electromotive force, that pushes against the current which induces it. CEMF is the voltage drop in an alternating current (AC) circuit caused by magnetic induction (see Faraday's law of induction, electromagnetic induction, Lenz's Law). For example, the voltage drop across an inductor is due to the induced magnetic field inside the coil, and is equal to the current divided by the impedance of the inductor.[1][2] The voltage's polarity is at every moment the reverse of the input voltage.[1][3]
The term Back electromotive force, or just Back-EMF, is most commonly used to refer to the voltage that occurs in electric motors where there is relative motion between the armature of the motor and the magnetic field from the motor's field magnets, or windings. From Faraday's law, the voltage is proportional to the magnetic field, length of wire in the armature, and the speed of the motor. This effect is not due to the motor's inductance and is a completely separate effect.
In a motor using a rotating armature in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. This produces a voltage in the coil; the motor is acting like a generator (Faraday's law of induction.) at the same time it is a motor. This voltage opposes the original applied voltage; therefore, it is called "back-electromotive force" (by Lenz's law). With a lower overall voltage across the armature, the current flowing into the motor is reduced.[4] One practical application is to use this phenomenon to indirectly measure motor speed and position since the Back-EMF is proportional to the armature rotational speed.[5]
In motor control and robotics, the term "Back-EMF" often refers most specifically to actually using the voltage generated by a spinning motor to infer the speed of the motor's rotation for use in better controlling the motor in specific ways.[6]
To observe the effect of Back-EMF of a motor, one can perform this simple exercise. With an incandescent light on, cause a large motor such as a drill press, saw, air conditional compressor, or vacuum cleaner to start. The light may dim briefly as the motor starts. When the armature is not turning (called locked rotor) there is no Back-EMF and the motor's current draw is quite high. If the motor's starting current is high enough it will pull the line voltage down enough to notice the dimming of the light
https://www.youtube.com/watch?v=VfvfkXhHw04
I don't need to add any more, this says it all!
Tom
And that is what i have been trying to tell you all along-BEMF is not inductive kickback.
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Generator BEMF is not inductive kickback. But inductive kickback is definitely BEMF. The D1 diode in Tom's set-up conducts BEMF from the motor winding (inductive kickback) to his capacitor, or if the switch is closed back to the motor.
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Hi All,
I think BEMF is what you want it to be, it seems nobody can really explain how it works.
Maybe I should call is back electrons magnet force, or what ever.
"Generator BEMF is not inductive kickback. But inductive kickback is definitely BEMF. The D1 diode in Tom's set-up conducts BEMF from the motor winding (inductive kickback) to his capacitor, or if the switch is closed back to the motor."
When I closed the switch as you seen in the video, the motor slow down due to BEMF,CEMF or what ever.
MIT claims this is BEMF:
https://www.youtube.com/watch?v=aSmMFog10D0 (https://www.youtube.com/watch?v=aSmMFog10D0)
Tom
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Hi All,
I think BEMF is what you want it to be, it seems nobody can really explain how it works.
BEMF is very simple: It is the EMF that results from changing magnetic flux crossing perpendicular to a conductor that acts in opposition to the applied voltage across the length of the conductor.
Maybe I should call is back electrons magnet force, or what ever.
It's a bad idea to invent substitute terminology. Call BEMF what it is.
"Generator BEMF is not inductive kickback. But inductive kickback is definitely BEMF. The D1 diode in Tom's set-up conducts BEMF from the motor winding (inductive kickback) to his capacitor, or if the switch is closed back to the motor."
When I closed the switch as you seen in the video, the motor slow down due to BEMF,CEMF or what ever.If the motor slows down as a result of closing the switch, that means that some amount of energy that powers the motor when the switch is off is being expended in the switch.
MIT claims this is BEMF:
https://www.youtube.com/watch?v=aSmMFog10D0 (https://www.youtube.com/watch?v=aSmMFog10D0)
It is.
Tom
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Tom and everyone,
Perhaps a simple way to distinguish between bemf and inductive kickback (IK) is to ask yourself, "does the effect take place simultaneously, or in sequence?"
bemf is a simultaneous phenomenon, whereas the IK effect takes place in a certain sequence of events. In a motor, the instant you apply power there is very little if any bemf generated, so the input current is high, but once the motor is running at speed, the applied emf, and bemf are in opposition simultaneously. When you are switching inductors, there is a definite sequence of events; i.e. you energize a coil, then remove the applied emf, then the coil tries to continue the current flow and as such reverses its output voltage, most often in a short high voltage spike. But that is determined by the load seen by the IK spike. THAT is IK.
So generally speaking, if you are switching/pulsing inductors, you are generating/collecting IK. If you are powering motors, the bemf effect is taking place, limiting the current into the motor. Without bemf, motors would draw huge amounts of current all the time.
Hope that clears things up a bit.
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In motor control and robotics, the term "Back-EMF" often refers most specifically to actually using the voltage generated by a spinning motor to infer the speed of the motor's rotation for use in better controlling the motor in specific ways.[6]
To observe the effect of Back-EMF of a motor, one can perform this simple exercise. With an incandescent light on, cause a large motor such as a drill press, saw, air conditional compressor, or vacuum cleaner to start. The light may dim briefly as the motor starts. When the armature is not turning (called locked rotor) there is no Back-EMF and the motor's current draw is quite high. If the motor's starting current is high enough it will pull the line voltage down enough to notice the dimming of the light
https://www.youtube.com/watch?v=VfvfkXhHw04 (https://www.youtube.com/watch?v=VfvfkXhHw04)
I don't need to add any more, this says it all!
Tom
Is there any difference between your circuit and this ?
http://www.free-energy-info.com/Chapter6.pdf (http://www.free-energy-info.com/Chapter6.pdf)
Automotive Relay Battery Charger. Page 28
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Tom and everyone,
Perhaps a simple way to distinguish between bemf and inductive kickback (IK) is to ask yourself, "does the effect take place simultaneously, or in sequence?"
bemf is a simultaneous phenomenon, whereas the IK effect takes place in a certain sequence of events. In a motor, the instant you apply power there is very little if any bemf generated, so the input current is high, but once the motor is running at speed, the applied emf, and bemf are in opposition simultaneously. When you are switching inductors, there is a definite sequence of events; i.e. you energize a coil, then remove the applied emf, then the coil tries to continue the current flow and as such reverses its output voltage, most often in a short high voltage spike. But that is determined by the load seen by the IK spike. THAT is IK.
So generally speaking, if you are switching/pulsing inductors, you are generating/collecting IK. If you are powering motors, the bemf effect is taking place, limiting the current into the motor. Without bemf, motors would draw huge amounts of current all the time.
Hope that clears things up a bit.
Thank you poynt.
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Read again all comments. Nobody here proved the direct relation between backEMF and inductive kickback.
I request please prove it with experiment. backEMF is going on when motor is running and inductive kickback show up only when inductor is disconnected.
for example :
If we replace the capacitor collecting inductive spikes with a 1:1 transformer with low resistance windings, what voltage we get on secondary ? Does it depend on the speed of switching off motor coils ? There was stated coil and power supply are in series during inductive spike so what we should expect ? If energy collected in coil is due to backEMF , what is the limit of generated inductive spike voltage ? Obviously not 2x power supply voltage, so what is going on inside ? Transformation due to windings ratio ? or due to current change ratio ? or maybe due to resonance effect ?
Better yet, take just a coil and I'm asking what is the process of converting backEMF into inductive spike and on what it depends ?
If I use coil and set timing and then connect capacitor across coil would that change the output voltage of inductive spike ?
Questions are limitless..... I'm glad you know all answers according to modern theory, that's fine - you don't have suchn mess in head like I have....but....the theory boundary is the limit of clear thinking...
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Read again all comments. Nobody here proved the direct relation between backEMF and inductive kickback.
I request please prove it with experiment. backEMF is going on when motor is running and inductive kickback show up only when inductor is disconnected.
for example :
If we replace the capacitor collecting inductive spikes with a 1:1 transformer with low resistance windings, what voltage we get on secondary ? Does it depend on the speed of switching off motor coils ? There was stated coil and power supply are in series during inductive spike so what we should expect ? If energy collected in coil is due to backEMF , what is the limit of generated inductive spike voltage ? Obviously not 2x power supply voltage, so what is going on inside ? Transformation due to windings ratio ? or due to current change ratio ? or maybe due to resonance effect ?
Better yet, take just a coil and I'm asking what is the process of converting backEMF into inductive spike and on what it depends ?
If I use coil and set timing and then connect capacitor across coil would that change the output voltage of inductive spike ?
Questions are limitless..... I'm glad you know all answers according to modern theory, that's fine - you don't have suchn mess in head like I have....but....the theory boundary is the limit of clear thinking...
Forest, there are literally thousands of academic references on the www for BEMF as it manifests: 1) As generator voltage, and 2) "Inductive kickback". In the former case we have Faraday induction, and in the second self induction. A DC motor can be modeled pretty accurately as a generator in series with an inductor and a resistor.
Inserting a transformer lets us change the voltage reference.
An unclamped, rapidly switched inductor spikes up at a rate limited by the local parasitic capacitance and leakage current until something begins to conduct enough to carry the current. Whatever the voltage reached determines the rate at which the current decays.
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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 ?
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 ? Isn't that the EMF or voltage according to Ohm's law the force generating current across resistance ? 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?
Or in other word : why I didn't saw video of Tesla coil generating lots of amps on secondary ? Impossible ? EMF is just a force, why it is limited to generating voltage spikes during inductive kickback ?
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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.
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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
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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
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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.
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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.
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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.
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;) ;)
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.