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J.D.

In Reply to: RE: That's a strange way to go.. posted by jsm on August 01, 2007 at 12:38:08

Eddy current brake advantages include :
* Adjustability of damping system.
You can't change the 'effective inertial mass' of the platter -- especially not on-the-fly, but you can-- in effect--- adjust the top end of that inertial mass via the eddy-brake. (Though obviously doesn't increase flywheel effect).
* Motor drive runs at mid-capacity.
With the eddy-brake engaged somewhere between full and off, the motor is only being compelled to run somewhere in the mid-range of it's capacity. Not at the extremes (assuming max capacity would be with brake full-on).
* Pitch control.
Eddy-current brake allows adjustability of platter speed, magnetically, smoothly and without physical or electrical connection to the motor, further allowing full-current and full-torque operation of motor.
Sorry if this isn't a real physics-class explanation. I may even have some aspect slightly off... But it is the general gist of the Eddy Brake.
Others will be able to supply the physics part of things if needed.
J.

Edits: 08/01/07

Follow Ups:

Ummm, well no. - Mark Kelly 16:30:18 08/01/07 (1)
Sorry J.D. but your physics is a bit off here.

First, an eddy brake does not add inertia to the system, it increases the system's viscous loss. In a linear analogy the first is like increasing the mass of you car, the second is like increasing the wind resistance. The mass will slow you down when going uphill but speed you up when going downhill, the increased resistance will always slow you down but be more effective the faster you are going*

Your second point is reasonable but I don't see that it makes much difference.

Your third point is not correct, the eddy brake does not affect the speed because the motor is synchronous.

*Actually there's a gloss here, aerodynamic resistance is proportional to the square of the velocity where viscous loss is proportional to velocity. I just couldn't think of a linear analogy that didn't complicate the argument.

Mark Kelly
- RE: Ummm, well no. - J.D. 14:30:16 08/02/07 (0)
  I did say that it "..obviously doesn't increase flywheel effect.." and maybe I should have reiterated that it wouldn't add inertia -- but I did mean that.
  I did say "You can't change the 'effective inertial mass' of the platter".
  As regards the brake "not affecting the speed because the motor is synchronous" ..
  ...well, original poster Jsm wasn't specifying, either, as far as I could tell, other than saying, as he does, "the driving force".
  My mistake would be thinking of the Induction motor as the drive force, as in the Garrard. Wherein the brake does in fact adjust speed.
  (( And --now that I think of it ---maybe this is the source of the lack of agreement here, with many considering the standard implementation of eddy-current brakes (garrards & thorens, others) as the "drive force" .... And others presuming ac synchro, as in the teres drive. ))
  Also, I'd hazard to say, there's a difference between what would ever be considered allowable in a physics textbook, and what stands in for that, at least as regards the rest of the non-physics speaking universe.
  If I say, for example, that .... :
  "an eddy-current brake is like sticking your finger on the platter wall as you play a record. Its going to slow down, for sure, but in spite of that, it's going to smooth out."
  
  ... well ... it might not scan or parse properly for a physics analysis, I don't know. But it has the distinct advantage, nonetheless, of being ... well, true .
  J.D.
  
  Edits: 08/02/07
Some comments. - jsm 15:11:08 08/01/07 (1)
Your second two points have nothing to do with variable stylus drag. It is beyond me how simply adding a drag force to a table damps it unless, as I said in my first post, it goes both ways. The force decreases when the table slows down and increases when the table speeds up. That would have the desired affect, but the Teres write-up didn't describe its actions in terms of a variable drag force. The whole thing is silly anyway, if one considers the driving forces and stylus drag forces of well-designed TTs. Many other things will have a much bigger effect on TT speed that the stylus drag variability.

Joe
- RE: comments. - J.D. 15:19:06 08/01/07 (0)
  Damping that is not physically or electrically making connection to the motor drive --- as with the eddy current brake--- certainly reduces or smooths out the Cogging side effects of the motor.
  
  It would appear to have a beneficial aspect re Stylus Drag --(which after all, like cogging, is a temporary but noticeable detriment to speed stability)--- but I'll let you & CB wrangle out the math on that one since it's not my category.
  
  J.D.

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