At what point does Back EMF occur?

[Shadesz]

Hey guys, sorry if this has already been discussed. I searched but couldn't find anything. I am trying to find out at what point back emf occurs.

Think...
Magnet approaches the coil, coil charges.
Magnet is TDC, what happens?
Magnet leaves coil, what happens?
How far away from what until the Back EMF occurs?

Also,
Does a core change the timing of back EMF?
If so, how?

In essence, how can I determine the rotor magnet placement during a back EMF charge? I am looking for this information so I can better design my generator.

Thanks in advance.

David

[ztec2002]

bemf

bemf starts the second the magmatic field collapses in the coil, the time is the speed of light ,micro seconds.Hope this helps.

cheers ztec2002

[AetherScientist]

hope this helps
How back emf has a dual polarity. - YouTube

[Shadesz]

Hmmm,

Interesting. Seems my thinking was wrong. I guess I was actually looking for the flyback timing. The flyback timing seems hard to give a value to as it is when the magnetic field from the rotor magnet is no longer strong enough to maintain the induced field. Is this correct?

Also, does this video correctly explain a Bedini circuit?
Understanding the Bedini Circuit - YouTube

Thanks again

[citfta]

Close

Hi Shadesz,

That description is very close to being right. His description of how a Bedini SSG works is exactly right. His description of Back emf is not right. What he is describing is EMF. EMF is voltage produced by a magnetic field moving past a coil. Back EMF is used to describe the counter voltage produced in a conventional motor. When a motor comes up to speed the rotating field of the armature generates a counter voltage or Back EMF in the stator windings. This counter voltage opposes the applied voltage and causes the current flowing into the motor to be reduced. This is why an idling electric motor draws very little current. As we apply a load to the motor and slow down the armature the Back EMF is reduced causing the input current to rise which tries to get the motor back up to speed. Back EMF only applies to electric motors. In a generator it is just called EMF. I am not surprised you are confused by the term as it is used incorrectly on this forum almost every day. I worked in industrial maintenance for over 30 years so I am pretty sure my description is correct. You can also look at Peter Lindemann's motor secret thread and he will tell you the same thing.

Carroll

[Shadesz]

Hi Shadesz,

That description is very close to being right. His description of how a Bedini SSG works is exactly right. His description of Back emf is not right. What he is describing is EMF. EMF is voltage produced by a magnetic field moving past a coil. Back EMF is used to describe the counter voltage produced in a conventional motor. When a motor comes up to speed the rotating field of the armature generates a counter voltage or Back EMF in the stator windings. This counter voltage opposes the applied voltage and causes the current flowing into the motor to be reduced. This is why an idling electric motor draws very little current. As we apply a load to the motor and slow down the armature the Back EMF is reduced causing the input current to rise which tries to get the motor back up to speed. Back EMF only applies to electric motors. In a generator it is just called EMF. I am not surprised you are confused by the term as it is used incorrectly on this forum almost every day. I worked in industrial maintenance for over 30 years so I am pretty sure my description is correct. You can also look at Peter Lindemann's motor secret thread and he will tell you the same thing.

Carroll

Thank you that explains a few things.

[boguslaw]

Hi Shadesz,

That description is very close to being right. His description of how a Bedini SSG works is exactly right. His description of Back emf is not right. What he is describing is EMF. EMF is voltage produced by a magnetic field moving past a coil. Back EMF is used to describe the counter voltage produced in a conventional motor. When a motor comes up to speed the rotating field of the armature generates a counter voltage or Back EMF in the stator windings. This counter voltage opposes the applied voltage and causes the current flowing into the motor to be reduced. This is why an idling electric motor draws very little current. As we apply a load to the motor and slow down the armature the Back EMF is reduced causing the input current to rise which tries to get the motor back up to speed. Back EMF only applies to electric motors. In a generator it is just called EMF. I am not surprised you are confused by the term as it is used incorrectly on this forum almost every day. I worked in industrial maintenance for over 30 years so I am pretty sure my description is correct. You can also look at Peter Lindemann's motor secret thread and he will tell you the same thing.

Carroll

Please if you can teach us a bit to open our eyes ! Let's take electric motor for example. Is back EMF the cause of drag when I try to rotate motor by hand when not connected to power ? If I guessed right then that would be the easiest way to test if any experimental motor has implemented method to avoid back EMF.
You said we are using term "back emf" incorrectly almost every day, can you
specify what we do wrong ? I for example have a difficulty to recognize what is flyback spike, forward emf (EMF?)

Can we fantasize a bit ? Imagine a motor with eliminated back emf on stator. How it would behave in idle state and when loaded ? Wouldn't it run faster and faster up to destruction end ?

[Farmhand]

Hi Shadesz,

That description is very close to being right. His description of how a Bedini SSG works is exactly right. His description of Back emf is not right. What he is describing is EMF. EMF is voltage produced by a magnetic field moving past a coil. Back EMF is used to describe the counter voltage produced in a conventional motor. When a motor comes up to speed the rotating field of the armature generates a counter voltage or Back EMF in the stator windings. This counter voltage opposes the applied voltage and causes the current flowing into the motor to be reduced. This is why an idling electric motor draws very little current. As we apply a load to the motor and slow down the armature the Back EMF is reduced causing the input current to rise which tries to get the motor back up to speed. Back EMF only applies to electric motors. In a generator it is just called EMF. I am not surprised you are confused by the term as it is used incorrectly on this forum almost every day. I worked in industrial maintenance for over 30 years so I am pretty sure my description is correct. You can also look at Peter Lindemann's motor secret thread and he will tell you the same thing.

Carroll

Ok this is good, the sharing of solid info. Maybe I can learn something here too. I'm sure we would all like to use the correct terms. But lots of us having only picked up what we know with no formal training, means we can get confused. Especialy when different people use terms differently.

So my question is if BEMF only applies to motors then what do we call the force that prevents a transformer from using full power at idle ?

Because if we collect the field collapse from the primary or use power from the secondary the input increases and if we stop taking power it decreases.

I thought it was the force of reverse EMF which restricts the current to only cover losses so as to keep the flux at maximum. When the flux is reduced by drawing power from the secondary more current flows in the primary to keep flux at maximum or attempt to. Isn't that what happens in a transformer ?

Like it says in this link below.

Transformers Part 1 - Beginners' Guide to Electronics

One thing that obviously confuses many people is the idea of flux density within the transformer core. While this is covered in more detail in Section 2, it is important that this section's information is remembered at every stage of your reading through this article. For any power transformer, the maximum flux density in the core is obtained when the transformer is idle. I will reiterate this, as it is very important ...

For any power transformer, the maximum flux density is obtained when the transformer is idle.

The idea is counter-intuitive, it even verges on not making sense. Be that as it may, it's a fact, and missing it will ruin your understanding of transformers. At idle, the transformer back-EMF almost exactly cancels out the applied voltage. The small current that flows maintains the flux density at the maximum allowed value, and represents iron loss (see Section 2). As current is drawn from the secondary, the flux falls slightly, and allows more primary current to flow to provide the output current.

It is not important that you understand the reasons for this right from the beginning, but it is important that you remember that for any power transformer, the maximum flux density is obtained when the transformer is idle. Please don't forget this

It's no wonder we get confused, because people confuse us.
This guy say's what i see in my transformers is caused by BEMF. And he says it is true for all transformers. As you can see we are being confused we are not always confusing ourselves. But I agree we should use common terms. It would help a lot.

Anyway I respect you're knowledge Carroll, that's why I asked.

Cheers

[Farmhand]

So to quote from that page.

At no load, an ideal transformer draws virtually no current from the mains, since it is simply a large inductance. The whole principle of operation is based on induced magnetic flux, which not only creates a voltage (and current) in the secondary, but the primary as well! It is this characteristic that allows any inductor to function as expected, and the voltage generated in the primary is called a "back EMF" (electromotive force). The magnitude of this voltage is such that it almost equals (and is effectively in the same phase as) the applied EMF.

Although a simple calculation can be made to determine the internally generated voltage, doing so is pointless since it can't be changed. As described in Part 1 of this series, for a sinusoidal waveform, the current through an inductor lags the voltage by 90 degrees. Since the induced current is lagging by 90 degrees, the internally generated voltage is shifted back again by 90° so is in phase with the input voltage. For the sake of simplicity, imagine an inductor or transformer (no load) with an applied voltage of 230V. For the effective back EMF to resist the full applied AC voltage (as it must), the actual magnitude of the induced voltage (back EMF) is just under 230V. The output voltage of a transformer is always in phase with the applied voltage (within a few thousandths of a degree).

For example ... a transformer primary operating at 230V input draws 150mA from the mains at idle and has a DC resistance of 2 ohms. The back EMF must be sufficient to limit the current through the 2 ohm resistance to 150mA, so will be close enough to 229.7V (0.3V at 2 ohms is 150mA). In real transformers there are additional complications (iron loss in particular), but the principle isn't changed much.

If this is all to confusing, don't worry about it. Unless you intend to devote your career to transformer design, the information is actually of little use to you, since you are restrained by the "real world" characteristics of the components you buy - the internals are of little consequence. Even if you do devote your life to the design of transformers, this info is still merely a curiosity for the most part, since there is little you can do about it.

When you apply a load to the output (secondary) winding, a current is drawn by the load, and this is reflected through the transformer to the primary. As a result, the primary must now draw more current from the mains. Somewhat intriguingly perhaps, the more current that is drawn from the secondary, the original 90 degree phase shift becomes less and less as the transformer approaches full power. The power factor of an unloaded transformer is very low, meaning that although there are volts and amps, there is relatively little power. The power factor improves as loading increases, and at full load will be close to unity (the ideal).

Looks like in a transformer the Reverse EMF in in the same phase almost as the EMF. But this would be different in a motor I imagine.

As I imagine it there is BEMF or Lens or whatever from the magnet in generating. And there is reverse EMF generated in drive coils from the applied EMF.

It's fairly easy to see though how important reverse or BEMF is. Without it there would be big current flow through the coil.

Cheers

[citfta]

Back EMF etc.

boguslaw

Back EMF is the voltage that opposes the forward EMF or applied voltage. If there is no applied voltage then we shouldn't refer to any voltage generated as "back EMF". A generator does produce voltage but it should just be called EMF because it is not opposing another voltage. The drag you feel when you try to turn a motor that has an electrical load attached to it is caused by Lenz law. It takes power to move electrons. When you are turning the motor shaft you are applying the power to move the electrons through the load. The heavier the load the more electrons you are trying to move and the more force it takes to turn the shaft. This relationship is called Lenz law. This is all according to conventional electrical theory. In the Mueller generator thread guys are coming up with some interesting ideas for getting around Lenz law.

When power is removed from a coil the collapse of the magnetic field around the coil induces a voltage in the coil that is called the flyback voltage or spike. Most electronic techs consider this to be different from forward EMF or a true generated EMF. This current tries to continue to flow in the same direction as the applied current. The real difference between the flyback pulse and a generated voltage is that the flyback pulse will adjust its voltage to whatever the load will allow. If the load is high resistance then the flyback pulse will increase in voltage to overcome the resistance. A normally generated forward EMF cannot do this.

You are correct in assuming a motor with no back EMF would accelerate until it self destructs. The only thing that would prevent this is that most circuits cannot provide enough current to allow this to happen. Usually a fuse or breaker will blow first. In a large DC motor like 10 hp or greater there is a circuit in them to kill the power to the armature if there is a loss of power to the field. This is because if the field coils lost power then the armature would not be able to generate a back EMF to limit the motor speed and the motor would try to run away just as you thought. Also with no back EMF the armature current would try to go as high as the armature resistance would allow. Since the armature resistance of a large DC motor can be as low as a few tenths of an ohm the current could go as high as several thousand amps. I once was working on a 50 hp motor when it suddenly lost field power. The sudden surge of current into the armature made a very loud bang like a stick of dynamite going off before the breaker had time to trip. People came running from all directions to see what had blown up.

If you don't have Peter Lindemann's Motor Secrets 2 DVD I would recommend you get it. Even if you are not interested in building the Lockridge device it has a lot of good info in it about how conventional motors work although I have tried to give you the basics.

If you have any more questions I will try to answer them if I can.

Carroll

[citfta]

Transformers and back EMF

Farmhand

I don't think I have ever seen anyone else refer to transformer impedance as back EMF. Maybe that is the way they are teaching transformer action now. It was a long time ago when I went to school on all this stuff. His description of transformer flux change and the effect on the current is correct though. In one way transformers and motors are the same. As you increase the load the input current goes up. Thanks for posting that info because I am sure it will help some people to understand transformers better.

Carroll

[Farmhand]

Hi Carroll, Thanks very much for the reply. It always helps to hear answers that make sense. There will always be different "labeling" of things.

I get from that link he is actually talking about the internally generated voltage as BEMF. And unloaded the transformer appears to the supply as just a large inductance. I guess the impedance to actually almost stop the current flow is caused by the internally generated voltage providing a potential to restrict or oppose current flow, or in my simple way of looking at things BEMF.

Impedance could conceivably describe any restrictive force or impediment really. As in impedance could have different causes and still be impedance.

What causes the impedance for it to be different when idle to loaded except voltage ? inductance, reactance, resistance ? ummm Flux ?

I need to find some source of compete explanation for all this. I realise it would be a nightmare to explain to people with no formal training.

I will search and research some more. If I can get a complete picture in my head with some enough clear text, I can understand it if others can.

Thanks

[Farmhand]

OK here is a good source by the look of it.

Inductors

When a Voltage (step function) is applied to an Inductor, a current is made to flow through its conductors creating an expanding Magnetic Field (Flux).

This expanding Field induces a current (Counter E.M.F.) possessing an opposite polarity from that of the applied voltage. The amount of this counter E.M.F. is directly related to the derivative of the applied current, i.e., the faster the rise/fall the greater the current. The effect of this is to impede the rate of expansion of the Magnetic field.
Next, the Magnetic field stops expanding and becomes steady-state. At this point the Inductor is now only a Resistive load to the applied Voltage Source.

Finally, the applied voltage is removed--made to equal Zero (Vapplied = 0 volts); the steady-state Magnetic Field now starts to collapse, which induces an E.M.F into the inductor thus causing a Current to flow (of the same polarity as that of the original applied voltage source). As in the applied case there is a Counter E.M.F. generated by the Current generated from the collapsing Magnetic Field, again impeding the collapse of said Magnetic Field

This tells me there is BEMF with every instance of applied EMF weather from a collapsing field or otherwise.

This is what I have so far in my head.

All EMF in an inductor is opposed by BEMF.

The collapsing field is EMF not BEMF.

And adding from my what i can pick up no BEMF equals buku current flow and problems.

The BEMF is almost in Phase with the EMF.

The current Lags by 90 degree's.

This below is just my thought derived from logic.

And to also add if all EMF in an inductor is opposed by BEMF then this must also be true for generators.

[citfta]

Always good to learn

Farmhand,

While you are looking for info on transformers look for info on magnetic amplifiers. These are a type of transformer that has a winding of many turns of small wire that is used to control the output of the secondary of the transformer. With a control current of only a few milliamps you can control the output current of several hundred amps on a large magamp transformer. If you can get a good explanation of how that works you will have a good understanding of how all transformers work. Another interesting transformer is the ferro-resonant transformer. This transformer can adjust itself so that the output voltage remains the same under changing load conditions. It can also keep the output voltage steady even if the supply voltage drops 10 to 20 volts below the normal input voltage. You can spend several months or I guess even years learning about transformers. I have long ago forgotten the theory behind how these special transformers work so you'll have to do some research for them yourself. I do remember that both of them involve controlling the flux.

Carroll

[Joit]

There is something else going on.
The BEMF for a Bedini Setup occur at the beginning. Usually, when Scopes are not lying.
The first Impuls of Energy goes through the Coils, when it is not loaded, thats why i assume, it has high Voltage.
But you have 2 Streams actually, once, the flow from Energy, the second the Magnetic Field, what is build with the Eddy currents or whatever else is there.
The first Stream goes into the Coil until its saturated, and slows down throught the Magnetic Field, when the Transistor turns off, the Coils collapse total and deplete the Energy back to Plus.
I showed allready in a Video and a strobe light, that the Pulse goes backwards through the Coils to plus, not through the Anode from the Diode, which points to the Plus from the Charge Batterie, but to the Cathode from the Diode at Charge Side.
The BEMF is in this Case weaker as at ie a Motor as Load, and it happens at an other Level, because of the different Potentials there.

When i think, that a DC Motor produce BEMF, then i have to say, this is an Assumption. When Energy flows through a DC Motor, then it repels or attracts with the Stator Magnets, but not enough, that there is a generating Effect in it. The real 'Load' on a Motor comes from pysical Resistance like Bearings and other Loads and more 2 Magnetic Fields what are outside of her optimal Timing, but not from a seperate generated EM Field.