physical explanation of induction disk relay 3

[electricpete]

The torque on an induction disk relay (with two windings phsycially separated by 90 degrees I think) is given by T=I1*I2*sin(theta) where theta is time angle between the currents.

Can anyone explain by physical reasoning such as right hand rule how that torque is produced. My understanding is the two currents create a rotating field which induces current in the rotor... which interacts to form a torque, similar (but not the same) to induction motor.

I can understand the induction motor where torque can be derived from right hand rule considering rotor current flowing axially. But there is no axial current flow in induction disk relay.

I have one explanation in mind, but it is lacking in physical intuition. Does anyone have any ideas on ways to understand where the torque comes from?

 

[electricpete]

Don't let the long question discourage you.

Please take a stab at the physical explanation of induction disk relay

thx

 

[Bung]

In an induction disk relay, I1 and I2 are the same, so torque is proportional to I^2. It is essentially just the same as a watt-hour meter, except that a shading coil is used to provide the out-of phase flux and hence out of phase eddy currents in the aluminium (diamganetic) rotor. From there, the B and I and the RH motor rule give a rotation. At least, that is my understanding of the pretty much stock standard stuff from the text book!

As for a real physical picture, it has been on my list of "I wonder how it actually does it..." for some time. I look at all these nice vector diagrams showing fluxes and currents and things, but it is hard to translate it into a picture. So I'll be keen to read the answer, too!

Bung

 

[tgott]

I'm sure that you've probably read many text book explanations, but I can only offer another. GE's Art & Science of Protective Relaying manual has a section titled "The Production of Actuating Force"

http://www.geindustrial.com/industrialsystems/pm/notes/artsci/art02.pdf

(page 10 of 22).

 

[electricpete]

Thanks to both of you for very helpful responses.

In the GE link, the forces are shown pointing in directly opposite directions (from center of one flux coil to center of the other). (There may be some time phase difference not shown which should not affect the direction.) I have a hard time seeing how that would produce torque (there is no moment arm).

 

[peterb]

Read a bit further down the page, Electricpete. The two forces are not equal in magnitude (although the drawing looks that way) and Eq. 3 is derived from the summation of the two.
If I can offer an extract from the GEC PRAG -
"The energizing quantity generates a flux across the magnet gap, in which is situated an aluminum disc. The area of the pole faces is subdivided into subsidiary poles, one of which is surrounded by a solid copper loop. The induced current circulating in this loop causes a phase displacement between the flux emerging from the shaded pole and that in an adjacent pole. The effect is to produce a laterally moving field which in sweeping across the relay disc produces a dragging force on the latter because of the currents induced in the disc."

 

[electricpete]

peterb- I agree that they're not equal in magnitude, but my point was they are co-linear (along the exact same axis). To produce a torque they would have to be acting along two different axes, perhaps parallel to each other, but separated by some distance d between the axes.

(I understand that the shading coil is one of many ways to produce two out-of-phase field components, depending on the relay.)

 

[peterb]

Electricpete -
I would agree with you if the axis of the disc were centered between the two fluxes shown in the diagram. However, the poles are located on the periphery of the disc and so any[\b) resultant radial force will result in rotation of the disc.
Looking back on your original post, I agree that the shading pole is only one subset of the general group of induction disc relays, although it is definitely the most prevalent application of the principle.

 

[peterb]

Oops. Should have read -

Electricpete -
I would agree with you if the axis of the disc were centered between the two fluxes shown in the diagram. However, the poles are located on the periphery of the disc and so any resultant radial force will result in rotation of the disc.
Looking back on your original post, I agree that the shading pole is only one subset of the general group of induction disc relays, although it is definitely the most prevalent application of the principle.

 

[electricpete]

I'm still missing the point, but I like you introducing the term radial. Let's talk in terms of the axial, radial, and tangential directions.

The disk can spin about an axis which defines the directions. The flux goes through the disk in an axial direction.

The current produced by the flux flows tangentially with respect to that flux. Also, at the outer extreme of the disk, the only direction current can flow is tangentially with respect to the disk axis (radial component must approach zero as we approach outside of disk since there is no path).

So F=IxB will be radial (as you said). Radial force about does not produce toruqe. (tangential force produces torque).

I'm thinking that for the equation to work, there has to be some radial component of the current. If the two coils were physically 180 apart with respect to disk axis, then I can't imagine any radial component. If the two coils were 90 degrees I can imagine some radial component. That part is starting to make some more sense because I remember reading the two coils would be 90 degrees apart.

For shading coil I know the physical angle is small. I'd have to give some thought to the time angle.

 

[peterb]

OK, back to the GE text. Say the axis of the disk is at the top of the page - the resultant force will indeed be tangential, giving the required rotational component.

I think that the key concept here is the phase angle difference between the two fluxes - this will give a vector sum of the forces that has a tangential component, resulting in disk rotation. BTW, the shading pole is on the same core as the main pole, there is no physical angular displacement between the two. Are you thinking of an induction cup relay, where the energizing quantities are applied to separate poles, 90 degrees apart physically?

 

[electricpete]

peterb - I am with you about putting the axis at top of drawing. That might be equivalent to haveing those two flux-producing coils at 90 degrees or 60 degrees or 120 degrees apart with respect to the disk axis (not 180 degrees which was my initial perception of the drawing... apparently incorrect).

It makes a lot more sense now. Thanks.

I am not too familiar with the exact principle of the shaded pole. From what I know it's a shorted coil around one edge or corner of the core. The flux resulting from this shaded coil must occur at a different location than the main flux. (Otherwise no torque.)

 

[electricpete]

looking over the ge writeup some more... It looks like any requirement for 90 degree physical spacing between coils must have been a figment of my imagination. The max force does occur for 90 degree electrical (time) spacing.

I would think that the relay works better with lower physical spacing approaching zero degrees.
(since force becomes more and more tangential as physical spacing decreases).

I think I can see where I came up with the imaginary requirement for 90 degree physical spacing between coils.

A rotating magnetic field would be established by two coils which are both 90 degrees apart in space and in time. I read the 90 degree in time maximum force part. I thought that a rotating magnetic field was the goal and would also require 90 degree physical separation to obtain. But ge's explanation doesn't seem to involve a magnetic field that is rotating around the axis of the disk (unless it pops out from analysis of the phases... I don't think so).

 

[sertec]

Having been there before, I often wonder if something is found in a "lab experiment" and then we spend an enormous amount of time trying to describe it. I wonder if a great deal with regards to induction discs was empirically derived? I don't think there are any engineers left that actually "engineered" induction disc relays.

 

[peterb]

Sertec -
Speaking for a (slightly) older generation of engineers, I need to point out that there was life before microprocessors. Check some of the papers written in the earlier days of the last century to get a feel for what was accomplished with slide rules (anybody else remember them?).

While some ideas may very well have come out of the primordial soup of somebody's lab without deliberate intent, I find it hard to imagine a couple of guys filling in time on a slow day in the lab by saying -
"Hey, what do you think would happen if we wound some wire around this chunk of steel and passed some current through it? Just for laughs, let's put a loop of copper around part of the pole and then put a sheet of aluminum in the air gap"

Although the technology is on the decline, there are still tens of thousands of induction disc and induction cup relay elements, as well as energy meters, in service worldwide. The elegance of some of the electromechanical designs is evidence of intelligent life out there.

 

[sertec]

Peterb; I too went thru school with a slide rule, and microprocessers still baffle me...I can design a tube ckt still...I think. My point was that often times, in the lab or just fooling around, you "discover" something and then imagine a practical application for it. The "why" may not be apparent but none the less, the event still happens. We taken advantage of magnetism for centuries, but are still not able to fully define it.

Thats all I'm saying.

PS: I still don't think there's an active electro-mechanical design engineer working in the USA.