What is the difference between eddy current and induced current?

[brainyman89]

what is the difference between eddy current and induced current? and how could eddy current cause energy loss? does the induced current cause also energy loss?
what is the difference between back emf and induced emf?

all these things are confusing me.

thanks in advance

 

[Andrew Mason]

what is the difference between eddy current and induced current?

An eddy current is a current that is induced in the iron core (iron being a conductor as well as having a high permeability). The current flows back and forth in the iron core as the alternating current in the windings changes directions.

and how could eddy current cause energy loss?

Eddy currents do no useful work. They cause the core to heat up. So the energy in those induced eddy currents is lost as heat.

does the induced current cause also energy loss?

The induced current is useful energy. It can be used to run a motor, computer etc. The eddy currents cause random motion of atoms in the iron core so we can't get at that energy as easily in order to do useful work with it.

what is the difference between back emf and induced emf?

Induced emf usually refers to an emf induced in a separate conducting circuit (ie. a coil that is separate from the coil to which a voltage source is applied). However, when a current builds up in a conducting coil, the increasing magnetic field associated with that build up of current induces an opposing emf in that same coil (as well as the separate coil). This is an emf that is opposite in direction (polarity) to the applied emf.

AM

 

[brainyman89]

who causes the eddy current, self induction or back emf

 

[Andrew Mason]

who causes the eddy current, self induction or back emf

Self induction and back emf are created in the primary coil (the one to which AC power is connected). The eddy current is not in that coil (ie. not self induction or back emf). The eddy current is not in the secondary coil (ie. induction). It is in the core.

The eddy current is caused because the alternating current in the coil creates an electric field within the area enclosed by the coil. This emf is determined by Faraday's law:

emf = \oint E\cdot ds = -\frac{d\phi}{dt} = -\oint B\cdot dA

The line integral of the electric field around a closed loop path is equal to (-) the rate of change of the magnetic flux through the area enclosed by the path.

So, for a coil of wire carrying an alternating current there will be an electric field created all along any closed path inside the area enclosed by the coil. The magnitude of the emf created along any path is determined by the time rate of change of the flux through the area enclosed by that path.

If there is just air in that enclosed area, there are no eddy currents: the induced emf cannot move the electrons because they are stuck to the atoms in the air. However, if there is an iron core in that space, the electric field causes electrons in the iron core to move (since iron is a conductor, the electrons are free to move). These electron motions are the eddy currents.

AM

 

[brainyman89]

"an electric motor that is already spinning consumes less power than one that is starting or changing direction" my question is why?

isn't the variation of magnetic flux when the motor is already spinning is greater than that when the motor is starting, since the angular speed is larger when the motor is already rotating?

is back emf constant or variable?

if the metallic core causes eddy current that leads to energy loss, then why we don't use coils with empty core?

 

[brainyman89]

what causes Inrush current drawn by an electrical device when first turned on in case we are supplying the device AC current?

is the phenomenon of self induction exists in motors??

are all these "back emf, self induction, eddy current" exist in a an electrical motor?

is the energy consumed by back emf equals the energy transformed from electrical state to mechanical state?

 

[brainyman89]

i have read that coils with iron core have variable inductance according to the variation of current, is there any formula that relate the variable L with I?

C = Q/v is there any proof for this formula?

thanks in advance

 

[Andrew Mason]

"an electric motor that is already spinning consumes less power than one that is starting or changing direction" my question is why?

That is not necessarily true. It depends on the load on the motor. If there is no load on the motor it takes very little energy to keep the motor going. If the motor is accelerating, its energy is increasing so it consumes energy.

isn't the variation of magnetic flux when the motor is already spinning is greater than that when the motor is starting, since the angular speed is larger when the motor is already rotating?

It depends on the load on the motor. It does not take energy to keep a frictionless rotor spinning. It takes energy to increase its rate of rotation.

is back emf constant or variable?

It varies, depending on the rate of change of current in the motor armature and field windings.

if the metallic core causes eddy current that leads to energy loss, then why we don't use coils with empty core?

Because the iron core intensifies the magnetic field. If you removed the iron core, you would have much less magnetic force between the field magnets and the armature.

AM

 

[brainyman89]

what causes Inrush current drawn by an electrical device when first turned on in case we are supplying the device AC current?

is the phenomenon of self induction exists in motors??

are all these "back emf, self induction, eddy current" exist in a an electrical motor?

is the energy consumed by back emf equals the energy transformed from electrical state to mechanical state?

i have read that coils with iron core have variable inductance according to the variation of current, is there any formula that relate the variable L with I?

C = Q/v is there any proof for this formula?

thanks in advance

 

[Andrew Mason]

what causes Inrush current drawn by an electrical device when first turned on in case we are supplying the device AC current?

The is very little resistance in a copper coil. What limits current is the inductive reactance of the motor. As the motor increases spin, the inductive reactance increases (reactance is proportional to the time rate of change of flux through the armature coils, which is proportional to the number of rotations/second of the armature coils).

is the phenomenon of self induction exists in motors??

Yes. All motors experience inductive reactance, which is a self-induction phenomenon.

are all these "back emf, self induction, eddy current" exist in a an electrical motor?

Yes.

is the energy consumed by back emf equals the energy transformed from electrical state to mechanical state?

Energy is not consumed by a back emf. The back emf reduces consumption of energy by reducing the current flow.

i have read that coils with iron core have variable inductance according to the variation of current, is there any formula that relate the variable L with I?

Yes. V = L dI/dt

C = Q/v is there any proof for this formula?

That is a definition of capacitance. So it is true by definition

AM

 

[brainyman89]

then how could we calculate the energy transformed from electrical state to mechanical state in an electrical motor??

 

[Andrew Mason]

then how could we calculate the energy transformed from electrical state to mechanical state in an electrical motor??

By measuring it. You could try to calculate it using the moment of inertia of the rotor, the I^2R heat losses, speed, area of coils etc, but that is very complicated. No one does it. We just measure the output energy of the motor and compare it to the input energy. The efficiency is around 70-90%, depending on the type and size of motor.

AM

 

[brainyman89]

i can't understand how an eddy current brake works, may u illustrates it to me or give me any links that explains it simply with animated images. actually i understand what eddy current is in a motor, but how could it be used as a brake?

thanks for helping me

 

[Andrew Mason]

i can't understand how an eddy current brake works, may u illustrates it to me or give me any links that explains it simply with animated images. actually i understand what eddy current is in a motor, but how could it be used as a brake?

thanks for helping me

Have a look at http://www.railwaygazette.com/news/single-view/view/eddy-current-braking-a-long-road-to-success.html" . Essentially, it works the same way as a generator except that the electric current that is generated by moving a conducting loop in a magnetic field is used to heat a metal disc rather than charge a battery or run something. The braking force is magnetic. The advantage is that you do not wear out mechanical brakes. A huge saving on maintenance.The same principle is used for regenerative braking in hybrid cars such as the Prius, except that the energy generated in braking is used to recharge the battery rather than generate waste heat.

AM

 

[brainyman89]

what causes inrush current when an electrical device is turned on?

 

[Andrew Mason]

what causes inrush current when an electrical device is turned on?

See my post #10, first paragraph. What causes the inrush of current is the application of a voltage. Impedance (a combination of resistance and inductive reactance) limits current: V = I/Z. So the better question is: why is the impedance much lower when the motor is starting up?

AM

 

[brainyman89]

"If the motor was frictionless and superconducting, it would use no power. This is because the back emf opposes the imposed voltage."

in this case, will the motor keep on spinning? will the current keep on traversing the armatures? if yes, how this would happen without consuming any energy i.e with no power?

can you tell me what this calculation give us : back emf*current=?

 

[Andrew Mason]

"If the motor was frictionless and superconducting, it would use no power. This is because the back emf opposes the imposed voltage."

I don't know where this quote is from. It is not really correct. A frictionless and superconducting motor would still consume energy. It would consume energy in starting up and in doing work (a motor is after all a device for converting electrical energy into mechanical energy by doing work).

in this case, will the motor keep on spinning? will the current keep on traversing the armatures? if yes, how this would happen without consuming any energy i.e with no power?

It does not require any work to keep a frictionless motor armature spinning at any speed once you have gotten it up to that speed.

AM

 

[brainyman89]

"It does not require any work to keep a frictionless motor armature spinning at any speed once you have gotten it up to that speed. "

in case the armatures are in magnetic field, will this statement be also true?

 

[brainyman89]

when we say the back emf opposes the imposed voltage, can we regard the back emf somehow as resistance.

can we through the back emf calculate the mechanical energy produced?

can you tell me what this calculation give us : back emf*current=?

 

[brainyman89]

what is the back emf constant?? does it characterizes a motor? does it differ from back emf??
why isn't constant and not variable?

thanks for answering my question

 

[Andrew Mason]

in case the armatures are in magnetic field, will this statement be also true?

It depends on how the direction of the magnetic field changes with the turning armature. Generally, to keep a frictionless armature going at no load you would turn off the magnetic field coils and let it spin freely.

when we say the back emf opposes the imposed voltage, can we regard the back emf somehow as resistance.

No. Back emf is part of the inductive reactance not resistance. Resistance is like friction: it causes energy loss. Inductive reactances opposes or sustains current depending on the way the current is changing. The inductor stores energy in the magnetic field as the current builds up and releases energy from the magnetic field to sustain the existing current as the current is decreasing.

can we through the back emf calculate the mechanical energy produced?
can you tell me what this calculation give us : back emf*current=?

The back emf = LdI/dt. So back emf x I = LIdI/dt

what is the back emf constant?? does it characterizes a motor? does it differ from back emf??
why isn't constant and not variable?

thanks for answering my question

You need to study electro-magnetic theory and study the answers I have given you before you ask any more questions here.

AM

 

[brainyman89]

this is what is written in my book:
E= Ir+E' where E' is the back emf, r and E' are constants
P(mechanical)=E'*I
an electrical motor functions only if the generator connected across its terminals delivers a positive potential difference that exceeds its back emf.So my question is, why are they regarding the back emf as constant quantity that characterizes each motor though the current across the motor is not always constant?

 

[brainyman89]

"in case we connected a pure inductor(coil) to AC generator, during a part of the cycle, energy is actually transferred from the inductor (load) back into the voltage source. we get a net power dissipation of zero."

how could energy be transferred to a generator??
in this case, a current is traversing the circuit without dissipating energy, will the current traversing the coil in this situation has a very large intensity as the case of a short circuit?

 

[brainyman89]

is there any proof why the average power equals cosine the phase angle (Φ) between voltage and current(case of sinusoidal voltage)?

 

[brainyman89]

does the back emf have sinusoidal form?

 

[brainyman89]

P(mechanical)=E'*I where E' is the back emf
is this formula of mechanical power converted by the motor right??

 

[brainyman89]

1-this is what is written in my book:
E= Ir+E' where E' is the back emf, r and E' are constants
P(mechanical)=E'*I
an electrical motor functions only if the generator connected across its terminals delivers a positive potential difference that exceeds its back emf.
So my question is, why are they regarding the back emf as constant quantity that characterizes each motor though the current across the motor is not always constant?

2-"in case we connected a pure inductor(coil) to AC generator, during a part of the cycle, energy is actually transferred from the inductor (load) back into the voltage source. we get a net power dissipation of zero."
how could energy be transferred to a generator??
in this case, a current is traversing the circuit without dissipating energy, will the current traversing the coil in this situation has a very large intensity as the case of a short circuit?

3-is there any proof why the average power equals cosine the phase angle (Φ) between voltage and current(case of sinusoidal voltage)?

4-does the back emf have sinusoidal form?

5-P(mechanical)=E'*I where E' is the back emf
is this formula of mechanical power converted by the motor right??

6-does the electrical energy converted to mechanical energy by the motor increase when the motor is loaded? will the intensity of the traversing current increase?

 

[brainyman89]

??

 

[brainyman89]

in case a motor is loaded, will the mechanical power converted by the motor be smaller though the intensity of current has increased??

since P(mechanical)=E'*I where E' is the back emf, E' decreases when the motor is loaded?