# How electric motors consume energy?

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 text books 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.

I would not consider Wikipedia to be the equivalent of a standard text book.
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
Homework Helper
Gold Member
"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.

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
Gold Member
2020 Award
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.

pgardn:
"I dont 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 dont 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!

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.

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?

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russ_watters
Mentor
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.

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?

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pgardn:
"I dont 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 dont 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 dont 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 cant 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...

truesearch, thank you for this answer.
I agree with all that you said, but all those things are due to a real motor not being an ideal one. But in an ideal one Power out still has to be lower that power in. Why?
regards

russ_watters
Mentor
In an ideal motor, power out is EQUAL to power in.

If you include ALL forms of energy then power out always equals power in.
Usually we are interested in USEFUL power out, the rest is called 'losses'
Electric motors can be very efficient, less than 10% of the energy supplied by the battery is wasted.

I am sorry I put the question the wrong way. Of course in an ideal motor power out equals power in, but power in is electric power and power out is mechanical power. So electrical energy is fully transformed to mechanical work, rotor rotation. So electrical power out, in an ideal motor is zero. Why? That was what I meant before.

Electric motors are machines that convert electrical energy to mechanical energy, they do not produce electrical energy.
The only other thing that I can think of that may shed some light for you is a battery connected to a bulb.... all of the electrical energy from the battery is converted to heat and light by the bulb, the bulb does not produce any electrical energy !!!
Hope this is some help.
(I feel that it is back emf that is causing you problems.... but I am not certain.)

russ_watters
Mentor
So electrical power out, in an ideal motor is zero. Why? That was what I meant before.
This represents a common but basic misunderstanding of electricity. You appear to believe that power flows through wires regardless of load, thus any device that doesn't consume all of that power lets some pass through. That is not correct. An electrical energy consuming device has no electrical energy output, only input.

All machines are devices that convert an input of energy into an output of energy.
Transformers convert electrical energy input to a different electrical energy output.
Levers convert mechanical energy into mechanical energy.
motors do not produce electrical energy as an output.
etc

Sure, but what causes the voltage drop? (in an ideal motor)

Sorry !!! don't understand what you mean !!

russ_watters
Mentor
Sure, but what causes the voltage drop? (in an ideal motor)

CWatters
Homework Helper
Gold Member
Ok try this...

If you put a permanant magnet close to an electro magnet that's off nothing happens. When you turn the electromagnet on a repelling or attracting force is created between them. This force causes the permanant magnet to move.

Work = force x distance

The permanant magnet does negative work on the winding...or the winding does work on the permanant magnet which ever way you prefer to look at it.

When th permanant magnet has moved away from the first winding the commutator switches off the first winding and applies power to another winding just as as the permanant magnet arrives in the right position. So the permanant magnet does negative work on each winding in turn until it gets back to the first one.

Energy is therefore convertde from electrical to mechanical form.

"What about the explanations you already got did you not understand?" - The only explanation given that would happen in an ideal motor is back emf, that if you read the first posts was actually suggested by me, while asking if it was it, all the others are loses related to the functioning of a real motor. What I mean is that it has to be a voltage drop in the motor, or as I said before you could have all the motors you wanted in series, and they would all perform the same way.
CWatters, I understand how electrical energy, using electromagnetism produces mechanical work. That is not my question, I you read the first posts you will see that I am not questioning how an electrical motor works. I am asking how is that the electrical energy going in the motor, and use to produce mechanical work (and heat in a real motor), is no longer available in the wires going out of the motor.
Thank you all
Regards

russ_watters
Mentor
Then back emf really is the answer you are looking for. The wording of your post implies you don't like that answer though. Is there something about back emf you don't like/get?

I have no problem with the answer, actually it was my initial intuitive answer, but I wanted to be sure it was the only factor (in an ideal motor). But in the example given of one holding the motor so the rotor doesnt turn, what happens there? There is no back emf, but there has to be a voltage drop, how does it happen?

russ_watters
Mentor
But in the example given of one holding the motor so the rotor doesnt turn, what happens there? There is no back emf, but there has to be a voltage drop, how does it happen?
That's potentially dangerous. The lack of back emf means much higher current until the source voltage is dragged down or other losses increase to dissipate the energy as heat...potentially burning out the motor.