Relay Coil Limitation

What is the definition of "Eddy Current Loss" ?

since Eddy Current is induced current circulating within a mass of metal, the "loss" part should be self-explanatory.

I would take a slightly diffent stance to that of Stasis. The best know application of eddy currents is in the magnetic speedometer common in cars. Here a magnet spins within an aluminium cap .... the cap attempts to revolve 'dragged' by eddy currents restrained only by a spring. The angular rotation is displayed by a needle against a scale.

In a relay I would assume the eddy current losses to be the difference between the actual current required to operate the relay and the lower theoretic current. The difference being lost in the frame and associated metalwork

When there is relative motion between a conductor and a magnetic field placed perpendicularly to it, there will be an EMF produced in the conductor.

A solid piece of metal that is in close proximity to a moving magnetic field will develop internal currents that merely circulate within the interior of the metal. Any current flow in a conductor will create heat on the magnitude of I^2*R. The resistance of the current loop/path will vary with the type of metal and other factors.

Typically, this term is applied to AC motor cores. Both the rotor core and the stator core will be exposed to (relatively) moving magnetic fields. To reduce the amount of loss, the cores are usually constructed of thin sheets of a ferro-magnetic material (typically carbon or silicon steel) that are electrically insulated from one-another. As these sheets, called "laminations," are rotated, the bulk of their mass is moved PARALLEL to the magnetic field, reducing eddy current creation, and thus the amount of current flow, and thus the amount of heat loss from the circulating (eddy) currents.

Donsdaman

Laminations are also used in transformers to reduce both eddy current and hysteresis loss -- are they not??

Been a while since my theory in schoool but my recollection is you get both eddy current and hysterisis in AC and DC motors and transfformers (AC only of course).

Dan Bentler

Dan Bentler

Yes, laminations are also used in transformers, as you say. I am sorry if it seemed I said otherwise. I just gave one example other than the original poster's "relay coil" reference. Anything that is conductive and moved through a magnetic field will have eddy current losses. You just have more interactions in a motor, so it's a good illustration of the principle.

DC motors, as well, are moving magnetic fields and stationary magnetic fields with laminations and motor housing, and bearings, and many other conductive metals. They're all along for the ride. Laminations on the armature, field, stator, rotor, core... wherever... are just a way to lessen the losses caused by the induced circulating currents.

Hysteresis loss isn't really eddy current loss, though they're both related to the moving magnetic fields. Hysteresis losses come from the energy it takes to magnetize and the reverse magnetize a ferro-magnetic material exposed to an alternating field. It is a loss due to the magnetic conversions. Eddy current losses come from the current produced by the magnetics caused by the AC interactions (induction).

Also, you'll still get eddy current losses in non-ferro-magnetic metals (such as copper) that are conductors.

I think you'd get hysteresis losses in something like magnetite without getting appreciable eddy current losses, if any. I know magnetite is a poor conductor, but I don't really know how poor.

I don't know if laminating the metal will reduce hysteresis losses. There is probably some small effect, but I don't know how significant it might be.

Now, one back for you, ltuMichJoe:

Reading through the preceding posts, what effect do eddy currents and hysteresis have on a relay armature pulled in by a DC current? Assume any random off-the-shelf 24Vdc coil and power supply, infinite current source, zero ripple.

Used to hear about Eddy Current Losses very rarely. Copper losses/Eddy Current losses (spelling may be incorrect), were always under the title 'losses'.

What is an Eddy Current? I've no idea. In very very northern Hydro Electric Generating systems, Eddy Currents are a factor for long power transmission. So much so that AC is rectified to DC. 44KV transmission lines have too much eddy current loss on AC systems over thousands of miles. Usually northern Quebec Hydro systems send over 400,000 Volts DC to southern systems.

An eddy current is just an opposing force. It is induced into any metallic (fereous) object close by. It goes much deeper. Over long transmissions, reactance comes into play.

So, the metal that is in your motor will produce and eddy current from primary mover. It will oppose primary mover's force. It only happens in AC systems. There is a relapse time factor involved. It is much like a spring.

Donsdaman

This is a good review for me.
ASSUMING
the DC is applied with the same polarity to the coil.
Then there is no change in magnetic field in operation - magnetic field
changes of course when pulling in or dropping out.
THEREFOR during operation (pulled in) there should be no eddy current or hysterisis loss.
I wold think since DC the domains in the magnetic material (iron core) should stay mostly in same orientation as that when last energized. Therefor hysterisis loss during pull in should be minimal.
I would guess since there is some relative movement in pull in there would be some eddy current loss but I don't have the EE design training to even start the calculations.

Dan Bentler

Teechnically you will always have eddy currents when a magnetic field intersects a conductor, this in turn causes losses created bu "joule" heating. Joule heating refers to the increase in temperature of a conductor as a result of resistancee to an electrical current flowing through it. This is minimized by using lamination or core materials with low conductivity.

As for relays, in general when energized the coil windings appear as a "short", causing an "inrush of current, until the current flow "induces" an opposing EMF (voltage).

Holding voltage should be just what it states, voltage with be at rated level for the relay with sufficient current to maintain the magnetic field.

When a relay is de-energized there is a collapsing field that can "spike" a high voltage back into the system.

Always Wondered

While we`re on this subject, which is very enteresting i`ve always wondered what job the little shading coil in the lamination on an AC coil is for and why it`s not needed in the
DC contactors lamination?? For some reason if the shading coil ever breaks the contactor hums badly. Anyone have any ideas on this?

:site:

Because AC alternates between positive and negative the magnetic field also varies in strength (or pull) which causes chatter or hum. The current created in the shading coil lags the applied current so the magnetic field is maintained.

Thomas Sullens said:

While we`re on this subject, which is very enteresting i`ve always wondered what job the little shading coil in the lamination on an AC coil is for and why it`s not needed in the
DC contactors lamination?? For some reason if the shading coil ever breaks the contactor hums badly. Anyone have any ideas on this?

Click to expand...

The shading poles are there to prevent the armature dropping out during the AC zero-crossing. The collapsing field induces current in the shading coil which produces enough of a magnetic field to hold the armature in during the zero-crossing.

The shading poles are there to prevent the armature dropping out during the AC zero-crossing. The collapsing field induces current in the shading coil which produces enough of a magnetic field to hold the armature in during the zero-crossing.

Click to expand...

Absolutely correct Gerry. I remember (just- a lot of years ago) going to a job in the middle of the night (of course). 3AM to be precise. The shading pole turned out to have a crack in it. Took 2 hours to find it. Pulled the contactor apart and cleaned it several times. No spares of course. Amazing what a twisted piece of earth wire, a soldering iron and a screwdriver and hammer can do at 5AM in the morning. Was still working 5 years later when the plant closed.

Thanks for the info

i understand now! Thanks Guys And Thanks PHIL for


this :site: When you can explain it to where i can understand it that`s pretty good