Open discussion for projects on this forum.

[citfta]

As is normal on this forum there are a couple of threads that are about improving electric motors. As is also normal for this forum there seems to be a lot of confusion about what back electromotive force (BEMF) really is. There is at least one thread where the theme seems to be that BEMF is some kind of evil that must be gotten rid of. Very, Very wrong conclusion!

To help you understand what BEMF really is I need to lay some background information. Let's first look at one of the most versatile of all motors. I am talking about the shunt wound DC motor. This is a motor that has field windings that are separate from the armature circuit.

When full voltage is applied to the field windings this motor can develop high torque at very low rpm. I have worked on a 10 HP motor of this type that could move a 20 ft milling table with only 2 volts applied to the armature. And it was turning so slowly you could count the commutator segments as they moved under the brushes! Very high torque at low rpm.

How can it do that? The answer has to do with BEMF. At that low of rpm almost no BEMF is generated. However the voltage applied is also very low. So when the motor starts to turn the BEMF generated is still enough to limit the current to maintain the speed of the armature. This motor requires a cooling fan to keep it from overheating when run like this.

Where does the BEMF come from? As the armature starts moving the magnetic field of the field windings causes a current to be generated that opposes the applied current going to the armature.

As we increase the speed of the motor by applying more voltage to the armature the BEMF also increases to limit the speed of the motor by opposing again the applied voltage. This is not a BAD thing. What would happen if there was no BEMF? I can tell you from personal experience exactly what would happen.

My partner and I were troubleshooting a 25 HP shunt wound DC motor circuit. The motor was running. We accidentally bumped the relay that feed the field current to the field windings. Of course there was an immediate loss of magnetic field in the field windings. Without a magnetic field in the field windings no BEMF was produced. The armature current immediately went up to the max current the armature windings could carry limited only be the resistance of those windings. This sudden surge of current created a very load boom that shook the whole building! My boss came running out of the office to see what had blown up.

BEMF is NOT our enemy. It is the only thing that limits the current through the armature. BEMF is the natural and very efficient governor that helps us control the motor.

As I said earlier the shunt wound DC motor is a very versatile motor. One of the neat things you can do with it is to get it to turn faster than it would normally go at full armature voltage. How do we do that? By reducing the field current. Once the motor is up to full speed we can then reduce the field current by a small amount and get the motor to turn faster. This works because when we reduce the field current we also reduce the BEMF. This allows more current to flow through the armature which makes the motor speed up until the BEMF again balances out the applied armature voltage. The only drawback to this is this also reduces the torque of the motor. This is usually not a problem because we only do this after the motor is at normal full speed and therefore has whatever load it has up to speed also.

To be continued.

[Ufopolitics]

As is normal on this forum there are a couple of threads that are about improving electric motors. As is also normal for this forum there seems to be a lot of confusion about what back electromotive force (BEMF) really is. There is at least one thread where the theme seems to be that BEMF is some kind of evil that must be gotten rid of. Very, Very wrong conclusion!

Cifta,

I do not care how many times you keep posting and referring to my Thread, even "indirectly" like above...I will come back here and reinstate/correct whatever you are trying to express I am doing wrong.

And...if your comment above -by any very small chance- was not meant towards my work...then, disregard/ignore this/my post.

To help you understand what BEMF really is...{...}
To be continued.

We really do not need to engage into one hundred chapter book to know what BEMF is...at least not with me, reason why I deleted all about your very long example.

So, in very simple and plain words by Wikipedia...easily available to all without the need of any "counseling":

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).

So, concluding, BEMF is a NATURAL reversed Voltage generated by the facts above -briefly- described.

Or even simpler...it is a combined/added reaction from Two Main and NATURAL Electrodynamic Principles/Discoveries: Faraday-Lenz Laws.

I do not think we need to bring each one here -in detail- as must of Us know them very well.

Now, the 'reversed' "evil" (like you wrote)...or like I call it "Witch"...is NOT the one You and Me are referring in above examples and citations.

I am referring clearly (starting by my First Video :Asymmetry to Enlightenment in 2012) to the reversed voltage occurring at the brushes plane whenever armature coils are forced to reverse their voltage input by being FORCED within a Continuous Cycle to do so, based on A Symmetrical Design.

And that is not a Natural BEMF generated based on Faraday-Lenz Laws.

That is a Forced reversed voltage that takes place every time a coil in the series group of the whole rotating armature reverses input.

I just wanted to make sure, you understand to which reversed voltage I was, and I am still referring to as "The Witch".

I am showing -In my Thread- how to collect that NATURAL BEMF without generating a FORCED BEMF every time armature spins.

NATURAL BEMF is not our/my enemy either

FORCED BEMF is, our enemy.


Simple.


Ufopolitics

[Turion]

Enemy?

And it is my belief that NEITHER is the enemy if you run the motor between two positives (between a 24 volt potential of two batteries in series and a discharged battery) giving a natural outlet for this energy to the discharged battery, as in the 3BGS. You can SEE it come out using simple meters. Voltage on the output side of the motor is several volts HIGHER than the voltage going into the motor. And you don't need an asymmetrical motor for it to work properly, or at least that is my experience. Off the shelf motors will work. However, I also believe more testing needs to be done. I will have to admit that one of the BEST runs I ever got on the 3BGS was using an asymmetrical motor I wound following UFO's plans. oh, and I have THAT run on video, or at least important PARTS of it.

Dave

[Ufopolitics]

And it is my belief that NEITHER is the enemy if you run the motor between two positives (between a 24 volt potential of two batteries in series and a discharged battery) giving a natural outlet for this energy to the discharged battery, as in the 3BGS. You can SEE it come out using simple meters. Voltage on the output side of the motor is several volts HIGHER than the voltage going into the motor. And you don't need an asymmetrical motor for it to work properly, or at least that is my experience. Off the shelf motors will work. However, I also believe more testing needs to be done. I will have to admit that one of the BEST runs I ever got on the 3BGS was using an asymmetrical motor I wound following UFO's plans.

Dave

Hello Dave,

Well, like you have said, to run an off the shelf motor, avoiding those forced reversed spikes...between your two positives...it works because you are allowing a path for them to go through...without staying in the motor armature.

I did it using another way, on my first Thread...when I was collecting the reversed Cycle from the pulsed coil, filtered through diodes...if you feed motor that way the results are very clear as well as in your set up.


Regards


Ufopolitics

[citfta]

Dufo is a very funny guy.

He got all outraged the other day when I posted something he didn't agree with on another thread. It was not even his thread. So after being very condescending towards me he tells me to go post in my own thread. Well here I am and he doesn't like that either. Tough, little baby, but you can't tell me what to post in my own thread. I have been respectful to not post in your thread so all I can say is if you don't like what I post here then use the ignore button. Or does someone need to teach you how?

Now back to the discussion about real BEMF. Now that we know what it is, what can we use that information for? For those that may have gotten lost in all the babble about "forced BEMF" let me post again what affects BEMF. BEMF is a result of basically 3 things. The strength of the magnetic field, the speed of the armature, and the number of turns of the armature coils.

There is also another factor that comes into play and that is the position of the brushes. If we get the position of the brushes correct we accomplish a couple of things. We want the position of the brushes to be arranged so that there is maximum reaction between the magnetic field of the field coils and the magnetic field of the armature coils. If we can accomplish that the motor will have the most torque for the amount of current drawn by the armature. This is of course because if we have the strongest reaction between fields then we also have the strongest BEMF.

Most industrial shunt wound DC motors have brushes that can be adjusted for what is called the neutral plane. This is that are where you get the most torque for a given armature current and also you get the least amount of sparking on the brushes. This gives longer brush life and a cooler running motor. And we get that by adjusting for the most BEMF or in other words the lowest armature current for a given load.

Can we use the information about BEMF to help us do other things? Suppose we want to rewind a motor to make it use less current. Since we know BEMF is a function of the number of turns in the armature coils we can use a smaller gauge wire and wind more turns and we would have a more efficient motor. But, we would also have a motor that has less torque unless we up the voltage to make up for the loss of current.

If we want a motor that has more torque then we wind the armature with fewer turns of heavier gauge wire and because of fewer turns the BEMF is reduced allowing more current to flow and thus the motor has more torque and more speed. So we gain torque and speed at the expense of using more current. This is basically what I see on this forum a lot. Someone thinks because their motor is faster and stronger they have a better motor. Maybe they do but most of the time all they have done is rewind it to use more current which is fine if that is what they want.

There are some things you can do to improve a motor or make it more suitable for your particular project. Matt Jones has designed a motor for the purpose of having a lot of torque and speed and also for generating some strong BEMF spikes. His modification to a regular scooter motor has accomplished all the above. I have run one of them for quite a while under heavy loads and it has gotten pretty warm but not hot. That is because he designed it to have some off time between pulses. He understood what he wanted to do and designed a motor to do that. His motor uses a little more current than an unmodified motor but it also produces a lot more speed and torque. His motor will easily spin loads the unmodified motor can't get up to speed.

If there is some interest shown about this discussion we can continue with questions and hopefully answers. If I am wasting my time trying to help new people wanting to know about motors then that is OK too. I have wasted a lot of time for more foolish things than trying to help someone learn.

Carroll

[citfta]

For those of you that would like to see first hand the effects of varying the strength of the field coils here are some ideas for you to try.

You will need an old cordless drill or other small cordless 12 to 18 volt tool. If you are like me you probably have a couple lying around that have bad batteries and the batteries cost more than a new one so now you have an old one and you don't know what to do with it.

Take it apart. Hopefully it will be made so the motor assembly will come out in one piece. After you get it out examine the wiring and find the wires that go to the field coils. Some motors may have magnets instead of field coils. If that is the case then it will not be usable for this experiment. You will need to find another one. When you find the field coil wires disconnect them from the rest of the circuit. Now you have a shunt wound DC motor and the fun begins.

You will need a 12 volt battery or maybe a 12 volt battery and a 12 volt power supply. The best thing would be to have two power supplies with adjustable output voltage, but I realize a lot of people won't have those.

If you are using a 12 volt battery you need to figure a way to control the voltage for two separate circuits. What our goal is is to have two sources of power we can control and adjust. One source will be connected to the field coils and the other source to the armature.

One other thing you will need is a way to measure the torque of the motor. You can get really fancy and build a prony brake. But that is not really necessary for just playing around with a small motor. What you can do instead is to use some heavy string and some different weights like some large hex nuts or very large washers. Anything really that you can attach to a string and vary the amount of weight.

We are going to use our simple prony brake like this. With the motor mounted solidly so it can't move and the shaft horizontal and hanging over the edge of a bench or what ever we are going to put our weight that is attached to the string on the floor under the motor shaft. Next wrap the string around the shaft in the same direction the shaft will be turning so that when the shaft turns it will pick up the weight. After making about 3 or 4 turns around the shaft, (The shaft has to be smooth) attach the loose end to something so it will not hang free. At this point you want the string to be loose on the shaft so it will slip when the shaft is turning.

Now you want to connect full power to the field coils and full power to the armature coils. This will give us our base torque reading. If you have a tach that would be nice but is not really necessary. After the motor gets up to speed you want to pull on the end of the string that does not have the weight on it. In other words you are going to tighten the string around the motor shaft so the motor will be picking up the weight. If the motor easily picks up the weight then repeat the test with more weight until you get a good idea how much weight the motor can lift without stalling out. This is our baseline measurement.

Now let's repeat the experiment but keep full field voltage and reduce the armature voltage. If you don't have an easy way to reduce the armature voltage you may be able to reuse the speed control that was part of the cordless tool you took apart. Or you can put an automotive bulb in series with the armature to reduce the voltage. Now see what kind of torque and speed you get. You should see reduced speed but almost the same torque because the field current is still full on.

For the next test do just the opposite. Keep the armature voltage at full voltage and reduce the field current. Be careful here and do not reduce it too much or you could easily have a run away motor or damage the armature because of too much current. If you can keep it at around 8 volts instead of 12 you should still be able to see a difference. When you apply power to the armature you should see that it now turns faster than the baseline speed. When you test the torque you will see that the torque has fallen off.

These are some simple ways anyone can play around with motors and learn more about how they work. Enjoy

Carroll

[Matthew Jones]

"dufo", lol I love it. From Carroll too the most loving kind person I can think of. LOL

I got tears in my eye's I'm laughing so hard. LOL

Matt

[citfta]

Hey Matt,

I think I know why he doesn't want me trying to teach people about motors. If they learn how they really work then they will know he is full of cow poop. Did you get your big present you were looking for today? (A little birdie told me about it).

Carroll

PS: I also got some good news today too. Dave can tell you about it.

[mbrownn]

Keep the armature voltage at full voltage and reduce the field current. Be careful here and do not reduce it too much or you could easily have a run away motor or damage the armature because of too much current. If you can keep it at around 8 volts instead of 12 you should still be able to see a difference. When you apply power to the armature you should see that it now turns faster than the baseline speed. When you test the torque you will see that the torque has fallen off.

These are some simple ways anyone can play around with motors and learn more about how they work. Enjoy

Carroll

This sounds counter intuitive, but makes sense.

With reduced field current, it is expected to have reduced torque under maximum load conditions. Under no load conditions less BEMF will be generated in the armature than in the previous experiment, so more effective voltage is across the armature than would be the case with fully energised field coils. This means we have more current in the armature, and higher armature torque under very low/no load conditions hence more speed. Correct me if I am wrong.

[shylo]

Hi Carroll, Now This is where I get lost, If the armature is at 12 volts ,let's say the winding is 10 ohms then the current should be 1.2 amps right?
So if I reduce voltage to 8 volts the same 10 ohms winding , then shouldn't the new current be .8 amps?
So how does reducing voltage increase the amperage?
I have an old generator out of a pre 60's car ,it has 2 field winding and 2 armature windings , I took it apart and it has 2 brushes but 1 is grounded right to the body. So the 3 pins in are F G and unmarked. I assume F is field G is generator unmarked is common ground?
Thanks artv

For the next test do just the opposite. Keep the armature voltage at full voltage and reduce the field current. Be careful here and do not reduce it too much or you could easily have a run away motor or damage the armature because of too much current. If you can keep it at around 8 volts instead of 12 you should still be able to see a difference. When you apply power to the armature you should see that it now turns faster than the baseline speed. When you test the torque you will see that the torque has fallen off.

These are some simple ways anyone can play around with motors and learn more about how they work. Enjoy

Carroll[/QUOTE]

[citfta]

You have it exactly right. And of course then the higher speed will generate enough BEMF to balance out the input current. And if you reduce the field current too much the motor will self destruct before it can reach a speed high enough for the BEMF to balance out the input current. That is why all industrial DC motor controllers have a safety circuit built in that will immediately shut off the armature voltage if the field current drops below a certain level.

Carroll

[shylo]

Sorry I think I misunderstood, So your saying that reduce the voltage and this reduces the amps? Because the resistance of the winding your feeding stays the same?
artv

[citfta]

Hi Shylo,

I am not sure I understand what you are asking, but yes if you reduce the voltage going to the armature then the amperage will drop and the motor will turn at a slower speed.

If your reduce the voltage going to the field then the current through the field will drop because as you said the resistance stays the same. However because of the weaker field the current through the armature will go up until the armature reaches a higher speed that will generate enough BEMF to balance the higher current going through the armature.

If that didn't answer your question then please clarify your question and I will try again to answer it.

Carroll

[shylo]

I think I see what your saying. Because the field windings produce a weaker magnetic field due to less current, the armature sees less opposition therefore it spins faster and creates a larger bemf?
Do the field windings create an opposite field when the supply is disconnected?
Sorry I'm not very good at expressing my thoughts.
Not sure if you saw my post ,since we posted at the same time.
Back 3 or 4 about the old generator?
Thanks artv

[citfta]

Hi Shylo,

I did miss your post about the generator. My post under that one was to mbrownn.

To answer your question about the armature current what you said would be correct if the armature is locked so that it can't move. Then the current would be a direct function of the voltage and resistance just like you calculated. And that is called the locked rotor current rating. This rating is used to calculate the protection device (fuse or breaker) that is needed to keep from burning up the motor.

However as soon as the rotor starts to turn because the rotor windings are moving in the field created by the field coils then the BEMF is generated which reduces the current going through the armature windings. As the armature turns faster and faster the BEMF increases until it balances out the applied armature voltage minus the power needed to power the load and overcome friction and other losses.

So if we reduce the field current then the BEMF being generated is less for the same speed so the armature current will go up until the faster speed is reached. At locked rotor the current is the same whether there is any field current or not. However the motor will have no torque and would not be able to turn the load and would quickly overheat if the protection device did not shut off the current to the motor. Although as I said industrial motor controls will not allow voltage to be applied if there is no field current. So there is a form of double protection there.

Thanks for your questions. Few free to ask for more help if anything is not clear.

Carroll