Talk:Counter-electromotive force

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Current divided by impedance is.... voltage?

"For example, the voltage drop across an inductor is due to the induced magnetic field inside the coil, and is equal to the current divided by the impedance of the inductor." I do not have direct access to the supposed sources of this statement. Whatever the case, the units do not match up, so I would suggest we remove it. 128.210.106.73 (talk) 12:55, 28 April 2015 (UTC)[reply]

I guess I'm watching ths article...got an email. No, 106.73, that line is not correct.

The article leaves a lot to be desired and is done from the motor perspective. The lead paragraph isn't quite correct and it is trying to cover too much ground by going to motors right a way. I'm an EE, have experience in this area and teach this stuff. It is very difficult for me to read it and it shouldn't be, especially for me. Therefore, I know it is difficult for a beginner. It is common for someone who understands a subject to want to include all the "Important" things to be "complete", but doing that too soon only confuses the student and makes understanding the fundamentals difficult. It is also not just in AC circuits, but DC as well; and both AC and DC motors.

Back EMF has two fundamental "definitions". One for just inductors and the other for motors (which also have the inductance form of back emf). The third now common use of BEMF to determine motor speed can be considered a third name for it, though it is a way to USE the "motor-BEMF" rather than the effect itself. In robotics the term "Back-EMF" actually means "Back-EMF sensing".

In my experience I see a slight preference like this: Counter-EMF is used for inductors and Back-EMF is used for motors.

It seems to me that this distinction should be clearly made in separated sections, starting with the inductor, then motor, then speed sensing.

In general, Back EMF refers to the phenomenon that opposes the original action and occurs in things that have current and significant magnetic fields. However, in an inductor it operates to keep the current constant (unchanging), but in the motor sense it operates to reduce the current.

1) I don't care for the word "push". I prefer "oppose".

Inductance: There are a couple of ways to look at 'inductive' back EMF.

2a) One characteristic is that V = L * di/dt. This says that the voltage is proportional to the *change* in current (times inductance). BEMF is most commonly talked about when you try to stop the current and the BEMF causes the classical voltage spike. This has also been called "inductive kick" This voltage is caused by the current decreasing. Fundamentally, the change in current causes a change in field and this change then induces the voltage back into the coil that opposes the change of current. This occurs when the voltage across the inductance is 'free to change" or is unconstrained by external circuitry. It is also the primary effect used in engine spark coils to create the spark. ... However, you can also put a voltage source across the inductance and the voltage is then determined by the source. In this case, when you change the voltage, the CEMF slows, or delays the change in current. This is responsibale for the Inductive time constant with a time constant given by the formula t = L/R. It is also responsible for the limiting of AC current called "Inductive Reactance".

2b) Because that is rather complex, we not only can, but should start simpler. By simplifying the introduction definition to just the end result, we can say:

"The Back-EMF is a voltage that is in a direction which acts to oppose the change in current."

Or, economizing on words it a bit more:

"Back BEMF is a voltage that opposes a change in current through an inductor."

You can then go on to describe the changing magnetic field's role in causing this to happen with a reference to Farady, Fleming, or Lenz Law.

The motor BEMF explanation needs be simplified. The comment that it is different than the inductive BEMF should introduce the section, not be buried in it. Talking about Faraday's Law and the length of wire is unnecessary detail. That should be left to the section ON Faraday's Law.

I recommend starting something like this:

"The counter-electromotive force (abbreviated counter EMF, or CEMF),[1] also known as the back electromotive force, is a voltage, or electromotive force, that opposes the action that caused it. There are three areas this term is used: Inductance, motors and motor speed measurement.

In an inductor, the counter EMF opposes any change in current. This EMF, or voltage, can become very high if the current is forced [or caused] to change quickly. This is the characteristic that causes the reactive opposition to AC current flow and initiates the inductive kick of a spark coil, or relay coil.

In a motor, the BEMF causes the motor current to decrease as the rotor speed increases. A motor is also a generator. The faster the rotor turns, the greater the generated voltage. Any DC permanant magnet motor can generate a DC voltage simply by turning it and connected to another motor can become a speed sensor. When run as a motor, the generated voltage is opposite to the applied voltage. A motor can be modeled as a generator in series with a resistor. As the speed and BEMF increases, the voltage drop across the resistor (which is the difference between the applied voltage and the BEMF) decreases and therefore, so does the current. Because the rotor speed is zero when voltage is first applied, the BEMF is also zero and the current is highest. This state is also called "lucked rotor" and can cause the dimming of lights due to the high starting current when a large motor starts. If a motor is unable to turn, it will draw this high current continuously, thus the term "locked rotor". AC motors have a rating to indicate the locked rotor current.

In robotics, the BEMF of a motor can be used to sense the speed of rotation because the BEMF is directly proportional to the rotation speed. Because there will be a voltage applied to the motor to run it, this voltage must be removed and any inductive CEMF effects allowed to die out before measuring the generated voltage.

............................................... Links to Faraday, time constant, reactance, etc and other appropriate terms can be very sparingly sprinkled in the appropriate places. Cheers, -- Steve -- (talk) 23:35, 28 April 2015 (UTC)[reply]

Very poor example of back emf

Isn't it possible for a better example of back EMF? The one given of an electric motor in a car is terrible. It asks the reader to note the change in the idle speed of the (gasoline) engine when using the car window actuator. Many cars won't respond this way as the idle speed is controlled independently of alternator loading (e.g. Prius) or the current drain is too low for the alternator to even notice (e.g. trucks). Nasukaren (talk) 21:34, 4 July 2011 (UTC)[reply]

See my talk above and the generator explanation as well as the last indented paragraph above for the "dimming of lights" example. -- Steve -- (talk) 23:38, 28 April 2015 (UTC)[reply]

Got an email about bot changes to refernces and see the lead pargraph is still poor. I went ahead and made corrections. -- Steve -- (talk) 19:43, 13 August 2017 (UTC)[reply]

AGREED

I'm not even sure that statement is correct. Terrible example. — Preceding unsigned comment added by 138.162.0.46 (talk) 18:17, 5 July 2011 (UTC)[reply]

I agree. A better example would be the startup of a motor in a refridge, an air conditioner compressor, table saw, or perhaps even a vacuum cleaner. The larger motors will produce a noticable brief dimming of lights in a home due to the low back EMF of the stalled rotor.

I stopped by here beasuse I was curious about the wiki's explanation due to a comment from a friend. I had never heard CEMF used in conjuction with inductors, just motors. -- Steve -- (talk) 01:34, 8 June 2014 (UTC)[reply]

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The comment(s) below were originally left at Talk:Counter-electromotive force/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

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Counter-electromotive force (back emf) occurs not only in motors but in all components that are inductive - all inductors and even wiring which has inductance (although in the latter the effect is usually very small).

What about in a variable speed motor, there is a device which loock like a transformer, located in the bottom of the blower motor. Thecnically is called line shocker but also is calle filter of bad frequencie and prevent a feeback of the bad frequenciegoes to the board of the motor. I am very confused because, I'm trying to figured out what is a bad frequencie, somebody told me the feedback of bad frequencie could affect your appliances such us tv, radio, etc etc etc. Somebody could helpme whit this?

While this is not necessarily the place for this type of discussion, I'll provide an answer. I can not be fully sure of your question because of your terminology (line shocker is a foreign term to me). Variable speed motors typically use Pulse Width Modulation (PWM) to control the current in the motor. This controls the torque and therefore the speed. To have a very high efficency, this system uses high speed pulses that have very high frequency components that can interfere with radio and TV.

I will assume that you are referring to the "Line Filter" used to prevent the high frequency pulses of the motor drive from traveling on the power lines (mains). This filter has nothing to do with the back EMF of the motor. You should Google: "Pulse Width Modulation", "Variable Speed Motor Drive", "Line Filter" and "Radio Frequency Interference". Regards, -- Steve -- (talk) 01:53, 8 June 2014 (UTC)[reply]

Last edited at 01:53, 8 June 2014 (UTC). Substituted at 12:21, 29 April 2016 (UTC)

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