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#1

# How and why eddy currents are formed in generators?

In generators why do we have eddy currents formed if there is an unbalance in the currents. Can any one explain the theory behind it with some maths. I mean why is these currents proportional to the square of the unbalanced currents.
what are others conditions where we can see eddy currents in the rotor?
08-30-2013 09:12 PM
Top #2
eddy current is produced where there is an alternating field and a mass of conducting material, i mean having reasonable surface area or volume. this is the localised circulating current within the above said mass of conducting material due the emf (eddy emf) induced within the material having reasonably mass or volume. practical example of this mass of material is core of a stator, core of a transformer, etc. this mass (which has considerably length and breath) may be considered as an infinite nos. of conductors. as they are short circuited itself, even a small amount of induced emf causes huge current in the material. as the current is circulating, it is called eddy current and the emf is eddy emf. main current in the main ckt. has no influence in this eddy current
08-30-2013 09:13 PM
Top #3
If there is unbalance in the currents you have a field that is counterrotaing the rotor: That is the rotor sees a field with frequency of 100 Hz @ 50 Hz grid (or 120 Hz @ 60Hz). The mathematics are in the decomposition to I1, I2 and I0 currents Since rotor in principally designed to have DC field that is constant flux in normal case you will have eddy currents induced everywhere in the rotor, foremost in the damper windings since they have the lowest conductivity.

Also during grid faults you will see eddy currents in the rotor again in the damper windings : that is what they are desinged for: To damopen the oscillations with forces due to eddy currents in the damper windings

Striclty speaking you will see edddy currents also in steady state situation with balanced currents: the stator slotting will cause variability in the air gap flux and there will be eddy currents in the rotor surface, esüacially in massive rotors.
08-30-2013 09:13 PM
Top #4
Eddy Currents, are a problem with a System.
İf a correct installation as been put in place. Then there is no İssue.
People have a Habit, of telling a Engineer that, ( Nobody as Touched anything )
İ have travelled around the World Helping People with Electrically Problems, for. 30 Years.
Propulsion Systems, Main Switchboard, Engine Control Systems, in Difficult Curcumstances, but nobody as ever Touched Anything.
Trust me, they have had Everybody and their Dog, before they Pick UP the Phone.
Ships Need To Move.
08-30-2013 09:15 PM
Top #5
Hello Mr. Santosh
If alternate current and material (like core of generator) are only requirements for eddy currents then there should be eddy currents in normal power flow condition also. But we will observe eddy currents only in unbalanced condition.

Smith.
Are you saying there won't be any eddy currents even if we have unbalance if we install it correctly?

Harri
Thanks for the concept. but what can be the percent of eddy currents if there is no unbalance?
08-30-2013 09:15 PM
Top #6
Unbalancing in the currents has nothing to do with the induction of eddy current. Eddy voltage and hence the eddy current is induced only when a considerably amount of mass of ferromagnetic material experiences a quick variation of magnetic field. and the amount of eddy voltage and hence eddy current is depend on rate of variation of magnetic field (frequency), strength of the field and the configuration of the ferromagnetic material.
08-30-2013 09:16 PM
Top #7
Is it possible people are confusing magnetic induced eddy currents with out of balance circulating electrical currents.
08-30-2013 09:16 PM
Top #8
Hello Satish,

There are several sources of eddy currents in the generator: Even if you eliminate the stator current unbalance, there is still eddy current on the rotor caused by stator slotting. Also in the stator there are eddy currents due to radial and axial magnetic field in the stator conductors
08-30-2013 09:17 PM
Top #9
Eddy Current has nothing to do with current or voltage unbalance. It is an error confuses unbalance current with eddy-currents. Eddy-current is a physical phenomenon of current "loops" induced in conductive parts due to a changing magnetic field and occurs even if you have only one phase. Because it is an induced current by nature, there are losses associated with eddy current and heat generated associated with such losses.
A common way to reduce eddy current in electrical machines is use laminations (typically 0.35 mm, e. g. M15) and/or low conductive materials. For high speed machines some low-loss material and smaller laminations are used (Sura 7, Sura 8, HF10…)
However, eddy current is not always a "bad-thing" in electricity. There are several applications where eddy-currents are used to produce heat, like induction ovens, or sensing applications like eddy-current sensors.
08-30-2013 09:17 PM
Top #10
The core of a generator armature is made from soft iron, which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. These currents that are induced in the generator armature core are called EDDY CURRENTS. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss.
08-30-2013 09:18 PM
Top #11
Hi all,
I think eddy current is a by-product of electro-magentism. as we know it is very difficlt to insulate the magenetic field, in any electromagnetic circuit, leakge fluxes are unavoidable. these leakage fluxes prduce the 'eddy emf' as explained by Harri, when find a ferro-magnetic path tend to circulating currents called eddy currents. Thus it is not proportional to the load current directly, it depends on the leakage fluxes, and rate of change of this leakage flux, which is dependent on frequency of main flux
we can't avoid them totally but can minimize them by desining the magentic circuit properly, slecting correct flux density, using the low relative permeability material for magentic cores etc.
As some one already mentioned, if we cant avoid the eddy currents they are been utilized for good cause in the applications like eddy current drives, eddy current couplings, clutches etc.