How electric motors consume energy?

[Charles123]

What exactly makes the motor consume the electrical energy? I s heat dissipation higher that in a normal coil? Is it counter-electromotive force? What makes the voltage drop?
Thank you
regards

 

[pgardn]

What exactly makes the motor consume the electrical energy? I s heat dissipation higher that in a normal coil? Is it counter-electromotive force? What makes the voltage drop?
Thank you
regards

Basically you have a force produced on wire because you have charge moving through wire in a B field. This occurs over a distance. So work is done. You get kinetic energy out of it. Of course some heat.

Thats really the basics. Sorry if you wanted something deeper.

Here is a nice simple illustration: http://www.walter-fendt.de/ph14e/electricmotor.htm

 

[Charles123]

Thank you for your answer!
I understand how they work, how electrical energy produces mechanical energy. My question is how is the electrical energy consumed/dissipated?
Heat, but more than if the motor wasn`t there? If so, why?
For instance in an incandescent light bulb it's easy to see that it’s the structure of the filament itself that produces the resistance, and its due to that resistance that heat and the desired light is produced. In the electrical motor one uses the magnetic field generated by the current passing in the electromagnets to interact with the current passing in the wires wrapped around the armature making the armature move (generation of mechanical energy). But by what mechanisms does the voltage drop?
Regards"

 

[pgardn]

Thank you for your answer!
I understand how they work, how electrical energy produces mechanical energy. My question is how is the electrical energy consumed/dissipated?
Heat, but more than if the motor wasn`t there? If so, why?
For instance in an incandescent light bulb it's easy to see that it’s the structure of the filament itself that produces the resistance, and its due to that resistance that heat and the desired light is produced. In the electrical motor one uses the magnetic field generated by the current passing in the electromagnets to interact with the current passing in the wires wrapped around the armature making the armature move (generation of mechanical energy). But by what mechanisms does the voltage drop?
Regards"

The electrical energy is converted to Kinetic energy and heat.

The little electric motors actually just use permanent magnets so this will make it a bit simpler maybe. The motor can be thought of as a resistor if you like. Except its not used to "control" current. The voltage drops occurs because you are indeed using Joules of energy per Coulomb of charge. The Joules of energy go into turning the motor and heat.

The charge loses its energy and it is "regained" at the battery in a simple DC setup. We could get into the E-fields produced that drive the charge in the wire and such but I don't think that is necessary here.

 

[pgardn]

Thank you for your answer!
I understand how they work, how electrical energy produces mechanical energy. My question is how is the electrical energy consumed/dissipated?
Heat, but more than if the motor wasn`t there? If so, why?
For instance in an incandescent light bulb it's easy to see that it’s the structure of the filament itself that produces the resistance, and its due to that resistance that heat and the desired light is produced. In the electrical motor one uses the magnetic field generated by the current passing in the electromagnets to interact with the current passing in the wires wrapped around the armature making the armature move (generation of mechanical energy). But by what mechanisms does the voltage drop?
Regards"

Oh I think I see what you are thinking:

Say you add a resistor in series with a lightbulb. You notice that the light is dimmer. You have reduced the amount of charge that flows through the wire because you added a resistor. The resistor does give off a minor amount of heat. The light bulb has part of the circuit (the filament) that instead of turning the electrical energy into just heat, it also has properties that actually give off energy in the visible spectrum.

Now if you just have a battery with just a resistor in a closed circuit and the resistance of the resistor is high you get very disappointed. You see nothing. And you can barely feel the resistor heat up at all. This is because the resistance is high so you don't carry as much charge thru the wire. If there is not as much charge flowing, then you can't carry as much energy assuming the battery has not been changed.

I think its really helpful to think about the units that describe current, voltage and power to understand this in a more complete way. The units for resistance just lead to confusion.

 

[NascentOxygen]

If the rotor was locked and not turning, then all of the electrical energy would be converted into heat in the motor´s resistances.

 

[Charles123]

First of all thank you for your patience and time trying to explain this to me!
"The voltage drops occurs because you are indeed using Joules of energy per Coulomb of charge. The Joules of energy go into turning the motor and heat." - I understand this, conceptually I have no questions about it. My question is what happens in order for it to be the case?
What I don’t understand is by witch mechanisms you dissipate electrical energy in running current trough the motor wires. See what I mean?

 

[Charles123]

Is it about counter-electromotive force? And why is the het dissipation higher than if there was no motor? Same motive?

 

[pgardn]

Charles I just wrote you a big explanation of electric fields set up in wires by batteries and related it to a gravitational field and a rock and explained work and all and this darn site kicked me off as I guess I took too long.

And I am very tire now...

Maybe tomorrow.

 

[Charles123]

That`s always unfortunate…
Hope you find the patience to re-write it tomorrow!
regards

 

[CWatters]

What exactly makes the motor consume the electrical energy? I s heat dissipation higher that in a normal coil? Is it counter-electromotive force? What makes the voltage drop?
Thank you
regards

In an ideal motor all the electrical energy is turned into mechanical energy and there are no losses. Real world motors have losses for a number of reasons depending on their type and construction.. Here are some examples..

* Losses in the windings. The windings are typically copper and copper has resistance.
* Various parts of the motor might be made from iron and there can be eddy current losses
* Bearings - friction losses.
* Brushes - electrical resistance and friction.
* Rotors - air drag

Consider a DC Permanant magnet motor. They typically accelerate until the back emf matches the supply voltage (or comes close). The back emf depends on the strength of the magnets and the number of turns. So the stronger the magnets the fewer turns are required. That might mean shorter, thicker and fatter wire can be used. That means copper losses are lower. So stronger magnets can help reduce copper losses and improve overall efficiency.

Some brushless DC Permanant magnet motors are >95% efficient.

 

[Charles123]

CWatters, thank you for your answer.
"In an ideal motor all the electrical energy is turned into mechanical energy and there are no losses.", I understand this conceptually, but how does it happen? How is the electrical energy transformed? What makes the voltage drop?
Regards

 

[pgardn]

CWatters, thank you for your answer.
"In an ideal motor all the electrical energy is turned into mechanical energy and there are no losses.", I understand this conceptually, but how does it happen? How is the electrical energy transformed? What makes the voltage drop?
Regards

Charles "you son of a gun..."

I started thinking about this a bit more and you may have asked something beyond my capabilities and I would now apologize for seeing what you where trying to ask. See the two setups I have below that put me in quandry (that maybe you do or do not understand; MY QUANDRY)

Case I: Battery connected to an electric motor(with permanent magnets) and you hold the electric motor not allowing it to turn. So you get the wire heating up and you read a voltage drop across the wire. The motor acts a resistor. Assume no internal resistance in the Battery. Done with experiment and now to compare it to the next.

ok...

Case II: Battery connected to an electric motor as above but we let in turn. We now have two sources of Pot. energy in my mind: The battery and the B-field from the permanent magnets. You do indeed get a back emf. So the voltage drop you read across the motor should not be the same as above, it should be less. Not only that the current is of course less. Not only that, but I think this may actually get into magnetic vector fields which is way out of my league. This has a lot more to it than a first thought. I would have to read up on this a lot more.

Sorry for making this seem simple at first because it appears not to be.

You cooked me. I must leave you to someone else to correct me or make additions or both. I have never read the phrase "electric motors consume energy" and I took this to be a big misunderstanding on your part. But in the end, you have confused me.

 

[Charles123]

pgardn, thank you for giving this extra input to the subject. I believe we are on the same page now!
I "always" assumed I understood the behavior and mechanics of an electric motor, but yesterday I realized that I was missing a big point - what happens to the electric energy as it goes in and out of the motor? Something makes that energy "disappear" as it is transformed in mechanical motion/energy, otherwise you could have as much motors connected in series with no energy losses and they would all perform the same way, clearly this does not happen. it would be a miracle making infinite energy from a finite energy source. So what happens in the wires that make the electrical energy "disappear" as mechanical energy is "created"?
"Case I: Battery connected to an electric motor(with permanent magnets) and you hold the electric motor not allowing it to turn. So you get the wire heating up and you read a voltage drop across the wire. The motor acts a resistor." - Could not agree more, but why is it so? What happens in the wire?
"Case II: Battery connected to an electric motor as above but we let in turn. We now have two sources of Pot. energy in my mind: The battery and the B-field from the permanent magnets. You do indeed get a back emf. So the voltage drop you read across the motor should not be the same as above, it should be less. Not only that the current is of course less." - Absolutly right, but again, why? Is It only back emf?
Best regards

 

[pgardn]

The basic for Case I:

The electrical energy, as we call it, is supplied basically because there is a difference between two diff. metals ability to hold electrons. This allows a sort of gradient that allows electrons to go from one metal to another transferred often through a fluid (acid in cars) or some paste. Batteries get heavily into electrochemistry a bit much so I leave that. Now that sets up an electric field in the wire in which electrons flow from the - to the +. But we use conventional current (+ to -) primarily due to historical reasons. It works either way. Now electrons will, in any metal (not attached to a battery) be randomly drifting about in that metal (actually in a lattice), but when you apply this Electric field (you need the battery for this) the electrons drift in an overall direction in the electric field creating current. When the electrons enter an area of higher resistance the type of material usually only allows certain amount of energy in the electrons, lower than in a more conductive wire or the battery itself. The diff. in energy states allowed (this gets quantum) releases energy as heat. The diff. in the ability to hold electrons in a particular lattice results in electrons must have a certain range of energy. If you change going from one lattice to another you must lose energy. (Or gain energy but this would be like electrons walking uphill thus requiring an outside agent to do this work.) The energy is "lost" as heat.

Those are basics. Sorry if the above is not deep enough. We could go on about exactly how gravity works and get just as confused. There comes a point when mechanistically our brains only go so far with analogies and we have to rely on math that fits experimental evidence. We just can't see the mechanism.

Case II... got a bit complex for me. I would have to go back to read more about B-field stuff, especially in permanent magnets.

I think the bolded above is very important.

 

[Charles123]

I agree that the bolded is a god point, but, and thinking about case I, what happens is that you are not able to produce work, so all energy must be dissipated as heat. Simple thermodynamics tells us that. The entering in an area of higher resistance is easy to see in the example that I gave of the filament in a incandescent light bulb, but not so much in the motor. Because the reason that, and still thinking about case I, all energy is dissipated as heat, is because the rotor is not able to move. So why is that this inability to move the rotor causes a drop in electrical energy?

 

[pgardn]

Because the motor is no longer a motor.

Its just essentially a long piece of wire. Long thin wire that attaches to the terminals of a battery so that current flows. Work IS still done because there is an electric field in the wire. The electric field does work on the electrons. They are forced over a distance of Efield within the wire.

Now if you go back to the applet I showed you, current would flow as long as the black insulating strip did not touch the terminals (actually the brushes that attach to the terminals of the battery.

 

[pgardn]

And for Case II:

More confusion that I just found.

https://www.physicsforums.com/showthread.php?t=621018

I only read parts. You might want to view this, or not.

 

[Lsos]

Charles123 the simple answer to your question, per my understanding, is indeed "back emf".

Electrons flow through the wire, creating a magnetic field which pushes the wire/ motor. If the motor turns as a result of this and there's any load on it, then that load will push back on the motor, which will push back on the magnetic field, which will push back on the electrons.

 

[pgardn]

Charles123 the simple answer to your question, per my understanding, is indeed "back emf".

Electrons flow through the wire, creating a magnetic field which pushes the wire/ motor. If the motor turns as a result of this and there's any load on it, then that load will push back on the motor, which will push back on the magnetic field, which will push back on the electrons.

Yes but that is only part of his question, case II.

If a motor does not turn, there is no back emf.

My final explanation from Case II was however incomplete and possibly wrong. And it remains that way. The ideas of what is doing work on charge and in which direction is still muddled in Case II imo. Maybe you could clear that up for both of us. The other thread https://www.physicsforums.com/showthread.php?t=621018 appears to be a mess with more details; last time I read it. Thanks.

 

[Charles123]

pgardn:
"Because the motor is no longer a motor.

Its just essentially a long piece of wire. Long thin wire that attaches to the terminals of a battery so that current flows." - So the fact that it is now a resistor has only to do with the characteristics of the wires?
"Work IS still done because there is an electric field in the wire. The electric field does work on the electrons. They are forced over a distance of Efield within the wire." - you are of course right, I meant no mechanical work making the rotor turn.
Lsos, thank you for your answer! So back emf is the answer then…

 

[truesearch]

What do you mean by 'back emf is the answer'.
Do you know how to calculate back emf?
what is your background knowledge, your profile gives no clues, what textbooks on this topiuc have you referred to?...were they of no help?

 

[pgardn]

pgardn:
"Because the motor is no longer a motor.

Its just essentially a long piece of wire. Long thin wire that attaches to the terminals of a battery so that current flows." - So the fact that it is now a resistor has only to do with the characteristics of the wires?
"Work IS still done because there is an electric field in the wire. The electric field does work on the electrons. They are forced over a distance of Efield within the wire." - you are of course right, I meant no mechanical work making the rotor turn.
Lsos, thank you for your answer! So back emf is the answer then…

Yep. If we ignore internal resistance in the battery to make it simple.

Back emf...

Charles you actually asked a question that opened up a much larger can of worms.
For me anyway... back emf does not answer my problem with what you presented. And thanks for asking, because I still don't have it straight. And after reading about it, I think its a bit beyond what I was thinking.

 

[Charles123]

pgardn, but, and thinking about your case I, if you could build a motor with wires that would offer the same resistance as the wires leading to the motor, by not allowing the motor to turn electrical energy would have to be dissipated. What would be the explanation in that case?
truesearch, I don’t understand your question. Back emf (http://en.wikipedia.org/wiki/Counter-electromotive_force) is part of the functioning of an electrical motor. Lsos wrote that back emf is responsible for the voltage drop across the motor. I am just wondering if that is the definite explanation… Please feel free to give your inputs.

 

[pgardn]

You can play with back emf with those little hand generators as well. You can hook two of them together, one acts as the motor, and the other supplies the voltage. I got that. Back emf is an just a name with a clear mechanism and rules. I explained what I thought it would do to the current and the voltage drops in an earlier post but...

I don't understand which fields are doing work and at what points in time in the motor/battery setup. And that is the essence of understanding the energy situations in your very first post that I did not realize, for me. Work. I personally get myself to understand most of this stuff with fields and a reference object and what is doing work on that object through a series of events. I cannot manage that with this problem. Maybe you understand what fields are doing work on electrons, or the electrons within the lattice, during the different phases of a motor turning? If so, please give it to me.

As for case I. I was already assuming the wires had the same resistance. Therefore there are two diff ranges of energy allowed for the electrons. 1. in the battery 2. In the wire. No motor necessary.

The energy ranges differ for electrons in the two places mentioned. The range allowed in the battery is higher than in the wire therefore we have a conservation of energy problem. My solution, heat given off in the wire. Take note that I do not even want to get into the guts of a battery for this case, I just know it supplies a potential difference. Case I is not a problem for me. You don't have to have a motor. Its just a battery and a single wire. Thats how I am considering it.

 

[Lsos]

What do you mean by 'back emf is the answer'.
Do you know how to calculate back emf?
what is your background knowledge, your profile gives no clues, what textbooks on this topiuc have you referred to?...were they of no help?

I'm just a guy off the street with some layman knowledge. I never said my answer is the final answer, and I clearly indicated that this is simply "my understanding". I then went on to offer an explanation that gives insight into why I believe this is the answer. I expect OP won't take my word as gospel, but will wither use it to direct his own research or will seek confirmation from more qualified sources.

 

[truesearch]

I would not consider Wikipedia to be the equivalent of a standard textbook.
Back emf is part of the functioning of an electric motor

Tells you almost nothing.
You need to check Faradays laws of electro magnetic induction to make a start.

 

[CWatters]

CWatters, thank you for your answer.
"In an ideal motor all the electrical energy is turned into mechanical energy and there are no losses.", I understand this conceptually, but how does it happen? How is the electrical energy transformed? What makes the voltage drop?
Regards

Not sure I understand your question...

Electric motors work by exploiting the magnet rule that like poles repel and oposites poles attract.

The motors back emf only equals the supply voltage when the motor has no load. If the motor is loaded (slowed down) the back emf falls and current rises. The voltage drop as you put it appears across the coil and brush resistance. In an ideal motor with zero losses there is not voltage drop and it allways runs flat out regardless of the load.

Back to the real world motor...Consider what happens when you stall the motor. The only thing limiting the current is the coil and brush resistance. All the voltage drop appears across these.

 

[Lsos]

I am not fully understanding the confusion about case I (and indeed case II), but I will offer my (again, layman's with limited formal training) simplified understanding about it.

In the case of a motor not being allowed to turn, I basically disregard the motor as it is doing no work, and as pgardn pointed out it has now essentially become a long piece of wire. A long piece of wire with low resistance. Essentially you have a closed circuit with no load: a short circuit. I never checked this with an ohmeter to confirm, but I imagine that as with any other short circuit the current would immediately shoot through the roof. If there was a fuse, it would blow. If not, the high current would cause high heat and probably burn and destroy something. Likely the motor.

If the wires in the motor were superconducting, for instance, then the motor would behave as if it was simply not there. There would still be forces inside of it, but since no work is being done it would have no long-term resistive effect on the circuit (it would probably act as an inductor for some time). Ultimately, all the load on the circuit would be coming from the wires leading in and out of the motor. It would be extremely low and it would still be a short circuit, and most likely something will still burn.

 

[sophiecentaur]

There is no essential difference between the electrical energy being transferred to thermal in the resistance and into 'bulk' kinetic as the motor moves. The thermal energy that turns up in the hot resistor is only another manifestation of kinetic energy - it's just randomised. That may not have helped you because it now makes both transfers equally difficult to appreciate.
It is easy to confuse 'understanding' with 'familiarity' and you seem to be able to accept some sort of 'friction' explanation to explain resistive heating because there is a nice mechanical analogy. But it's not a particularly good, true explanation of what goes on in a conductor.

 

[Charles123]

pgardn:
"I don't understand which fields are doing work and at what points in time in the motor/battery setup. And that is the essence of understanding the energy situations in your very first post that I did not realize, for me. Work. I personally get myself to understand most of this stuff with fields and a reference object and what is doing work on that object through a series of events. I cannot manage that with this problem. Maybe you understand what fields are doing work on electrons, or the electrons within the lattice, during the different phases of a motor turning?" - I am sorry, but I don’t follow.
"As for case I. I was already assuming the wires had the same resistance. Therefore there are two diff ranges of energy allowed for the electrons. 1. in the battery 2. In the wire. No motor necessary." - I don`t understand either. The motor has to have some effect, doesn't it? If we are holding the motor still, we are applying a force equal to the one applied in the rotor trough electromagnetism. Therefore mechanical work is not being done, so all energy has to be dissipated as heat. Am I wrong?
Lsos:
You just gave your input, an important one to the discussion, thank you for that!
In regards to your second post, see my second comment above…
CWatters:
"In an ideal motor with zero losses there is not voltage drop and it allways runs flat out regardless of the load." - If this was accurate then power going in the motor would equal power going out, so you would not be transforming electrical energy in mechanical energy, but creating mechanical energy maintaining all the electrical one. You could have as much motors in series as you wanted…
"Back to the real world motor...Consider what happens when you stall the motor. The only thing limiting the current is the coil and brush resistance. All the voltage drop appears across these." - again, If we are holding the motor still, we are applying a force equal to the one applied in the rotor trough electromagnetism. Therefore mechanical work is not being done, so all energy has to be dissipated as heat. There has to be an extra effect besides the normal resistance of the wires.
sophiecentaur:
" There is no essential difference between the electrical energy being transferred to thermal in the resistance and into 'bulk' kinetic as the motor moves. The thermal energy that turns up in the hot resistor is only another manifestation of kinetic energy - it's just randomised." -Agree.
Can you give some input to how we "loose" the electrical energy in the motor. Power in must be higher that power going out the motor, regardless of the fact that we let the rotor turn or not. By what mechanisn(s)?
Thank you all for your inputs!

 

[truesearch]

Power in needs to be greater than power out because there are energy losses.
The useful power out from an electric motor is mechanical kinetic energy. The armature wires get hot because of their resistance and hi is a form of kinetic energy but it would be misleading to include this in the total KE output .
There are several ways by which energy is wasted in an electric motor
1) heat produced by current flowing through the resistance of the armature
2) eddy current in the iron core
3) hysteris losses in the iron
4) resistance at the commutator contacts
5) windage and bearing friction...larger motors have a cooling fan
6) distortion of the applied magnetic field due to the armature field
All of these need to be taken into account to understand what happens to energy in electric motors.

 

[Lsos]

truesearch, you shot down my theory that back emf is how the kinetic energy output of an electric motor manifests itself as a load on the generator/ battery. I guess that since that time I (and I imagine other readers) was hoping that you would offer a better explanation. In the above, you explain how energy is wasted in an electric motor. But how is it NOT wasted?

Let me reiterate my understanding of what's happening:
Electrons flow through the wire, creating a magnetic field which turns the motor. If there is a load on the motor, then that load will push back on the motor, which will push back on the magnetic field, which will push back on the electrons flowing through the wire...and you lose electrical energy.

Is there much more to it than this? Is what I'm describing above not "back emf" but something more...perhaps Lenz's Law?

 

[russ_watters]

I'm confused here because it looks to me like the question has been thoroughly answered from multiple directions. But for clarity, Charles, there are two categories of energy dissipation in a motor:
1. Conversion to mechanical.
2. Losses.

Your question is about #1, right?

Then through Lenz's law and back EMF, do you get that the motion of the electrons creates an EMF that literally pushes back against the motion of the electrons? That's voltage (drop). So what else is there that you don't get?

 

[pgardn]

pgardn:
"I don't understand which fields are doing work and at what points in time in the motor/battery setup. And that is the essence of understanding the energy situations in your very first post that I did not realize, for me. Work. I personally get myself to understand most of this stuff with fields and a reference object and what is doing work on that object through a series of events. I cannot manage that with this problem. Maybe you understand what fields are doing work on electrons, or the electrons within the lattice, during the different phases of a motor turning?" - I am sorry, but I don’t follow.
"As for case I. I was already assuming the wires had the same resistance. Therefore there are two diff ranges of energy allowed for the electrons. 1. in the battery 2. In the wire. No motor necessary." - I don`t understand either. The motor has to have some effect, doesn't it? If we are holding the motor still, we are applying a force equal to the one applied in the rotor trough electromagnetism. Therefore mechanical work is not being done, so all energy has to be dissipated as heat. Am I wrong?
Lsos:
You just gave your input, an important one to the discussion, thank you for that!
In regards to your second post, see my second comment above…
CWatters:
"In an ideal motor with zero losses there is not voltage drop and it allways runs flat out regardless of the load." - If this was accurate then power going in the motor would equal power going out, so you would not be transforming electrical energy in mechanical energy, but creating mechanical energy maintaining all the electrical one. You could have as much motors in series as you wanted…
"Back to the real world motor...Consider what happens when you stall the motor. The only thing limiting the current is the coil and brush resistance. All the voltage drop appears across these." - again, If we are holding the motor still, we are applying a force equal to the one applied in the rotor trough electromagnetism. Therefore mechanical work is not being done, so all energy has to be dissipated as heat. There has to be an extra effect besides the normal resistance of the wires.
sophiecentaur:
" There is no essential difference between the electrical energy being transferred to thermal in the resistance and into 'bulk' kinetic as the motor moves. The thermal energy that turns up in the hot resistor is only another manifestation of kinetic energy - it's just randomised." -Agree.
Can you give some input to how we "loose" the electrical energy in the motor. Power in must be higher that power going out the motor, regardless of the fact that we let the rotor turn or not. By what mechanisn(s)?
Thank you all for your inputs!

charles at this point I don't know what you are looking for. Truesearch went through a multitude of engineering points about what happens at every single step if you DONT consider a the held motor a single wire. That would require going back to the original applet and looking at all the details that really exist. Of course in case I when you start to loop the wire, the brushes, etc... this is detailed engineering. What one would do to make sure a motor worked as efficiently as possible... This is patently not what I was trying to accomplish. And not what I thought you were after.

To get to the meat of the matter which is what I think you are after... for me, you have gotten to the point of asking the equivalent of: by what mechanism does gravity work? I can't answer by what mechanism masses attract each other. The world about us has told us that they do. You can model fields, you can model distorted space, at some point you reach a set of observations and math that are just not mechanistic anymore and we are left with math that describes the phenomena accurately but the mechanistic modeling only goes so far, they are flawed. So force carrier particles is a better model to explain why masses attract each other. The particles, we will call them gravitons... I am cooked by this time...