# What is the difference between eddy current and induced current?

1. Mar 15, 2011

### 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.

2. Mar 15, 2011

### Andrew Mason

Re: electricity

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.

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.

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.
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

3. Mar 15, 2011

### brainyman89

Re: electricity

who causes the eddy current, self induction or back emf

4. Mar 15, 2011

### Andrew Mason

Re: electricity

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

5. Mar 16, 2011

### brainyman89

Re: electricity

"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?

6. Mar 16, 2011

### brainyman89

Re: electricity

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?

7. Mar 16, 2011

### brainyman89

Re: electricity

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???

8. Mar 16, 2011

### Andrew Mason

Re: electricity

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.

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

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

9. Mar 17, 2011

### brainyman89

Re: electricity

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???

10. Mar 17, 2011

### Andrew Mason

Re: electricity

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).

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

Yes.

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

Yes. $V = L dI/dt$

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

AM

11. Mar 17, 2011

### brainyman89

Re: electricity

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

12. Mar 17, 2011

### Andrew Mason

Re: electricity

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

13. Mar 17, 2011

### brainyman89

Re: electricity

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

14. Mar 17, 2011

### Andrew Mason

Re: electricity

Have a look at http://www.railwaygazette.com/news/single-view/view/eddy-current-braking-a-long-road-to-success.html" [Broken]. 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

Last edited by a moderator: May 5, 2017
15. Mar 18, 2011

### brainyman89

Re: electricity

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

16. Mar 18, 2011

### Andrew Mason

Re: electricity

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

17. Mar 19, 2011

### brainyman89

Re: electricity

"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=???

18. Mar 19, 2011

### Andrew Mason

Re: electricity

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).

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

19. Mar 19, 2011

### brainyman89

Re: electricity

"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?

20. Mar 19, 2011

### brainyman89

Re: electricity

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=???