physical explanation of induction disk relay
physical explanation of induction disk relay
(OP)
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?
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?






RE: physical explanation of induction disk relay
Please take a stab at the physical explanation of induction disk relay
thx
RE: physical explanation of induction disk relay
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
RE: physical explanation of induction disk relay
http://www.geindustrial.com/industrialsystems/pm/notes/artsci/art02.pdf (page 10 of 22).
RE: physical explanation of induction disk relay
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).
RE: physical explanation of induction disk relay
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."
RE: physical explanation of induction disk relay
(I understand that the shading coil is one of many ways to produce two out-of-phase field components, depending on the relay.)
RE: physical explanation of induction disk relay
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.
RE: physical explanation of induction disk relay
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.
RE: physical explanation of induction disk relay
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.
RE: physical explanation of induction disk relay
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?
RE: physical explanation of induction disk relay
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.)
RE: physical explanation of induction disk relay
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).
RE: physical explanation of induction disk relay
RE: physical explanation of induction disk relay
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.
RE: physical explanation of induction disk relay
Thats all I'm saying.
PS: I still don't think there's an active electro-mechanical design engineer working in the USA.