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Natural Science Forum / Physics / General Physics / December 2007A mechanically driven homopolar amplifier
Consider a doughnut shaped sheet metal object, split around the periphery so that it can be separated into two halves. The two halves are assembled so that they make contact along the cut edges, re-forming the doughnut shape. One half is now driven to rotate relative to the other half, while maintaining electrical contact along both the inner and outer matching edges. If a magnet is now placed in the central hole, a toroidal current will flow in the assembly due to the voltage induced in the moving part. This current will produce a magnetic field around the assembly, similar to that developed in a Rogowski coil, or in a wound toroidal core. If the magnet is replaced by a solenoid energised by an alternating voltage, the magnetic field in the device will also alternate. If the assembly passes through a toroidal winding a voltage would be induced in it to drive a current in an external load that would be proportional to the strength of the solenoid field and the rate of rotation of the moving half assembly, so that the device would act as a mechanically driven amplifier. All the best Ian Macmillan > Consider a doughnut shaped sheet metal object, split around the periphery > so that it can be separated into two halves. [quoted text clipped - 21 lines] > > Ian Macmillan Watt exactly is being amplified? > If the assembly passes through a toroidal winding a voltage would be > induced in it to drive a current in an external load that would be > proportional to the strength of the solenoid field and the rate of > rotation of the moving half assembly, so that the device would act as a > mechanically driven amplifier. Excuse me Ian, but what is the point of all this low-efficiency apparatus? Any generator (Faraday or conventional) can become an "amplifier" if you modulate the field. There was a popular device back in the 1940s called an "amplidyne" (if I recall correctly) that worked on this principle. In fact, I used to control the field current in the university cyclotron using an amplidyne which fed the field of a large DC generator which actually supplied the heavy current. Very fine control was possible. The large problem with the system however, was that due to phase shifts and lags in the mechanics of it, the whole thing was rather marginally stable. A nice solid state high current transistor array would have done a much better job. > > If the assembly passes through a toroidal winding a voltage would be > > induced in it to drive a current in an external load that would be [quoted text clipped - 13 lines] > of it, the whole thing was rather marginally stable. A nice solid > state high current transistor array would have done a much better job. Well, of course, its just a curiosity. Pretty obviously its an impracticable contraption that might however be of interest to those with an interest in such things... By the way, it potentially amplifies the signal from the solenoid All the best Ian Macmillan > > If the assembly passes through a toroidal winding a voltage would be > > induced in it to drive a current in an external load that would be [quoted text clipped - 13 lines] > of it, the whole thing was rather marginally stable. A nice solid > state high current transistor array would have done a much better job. Amplidyne eh? I junked an ancient 100 HP Ward Leonard system only last year. A carbon filament lamp was dangled out of the control panel, presumably central to system stability... All the best Ian Macmillan >> > If the assembly passes through a toroidal winding a voltage would be >> > induced in it to drive a current in an external load that would be [quoted text clipped - 5 lines] >> apparatus? >Amplidyne eh? Ian, Thanks for posting this. I am always interested in non-electronic amplifiers. The familiar examples are: the capstan amplifier ( http://homepage.mac.com/a.eppendahl/work/torque-amp.html ) and the liquid container with drain hole which acts as an amplifier of velocity of rotation http://groups.google.com/group/sci.mech.fluids/msg/ece9873741ce11f2 "Fluid motion is governed by the equation (partial with respect to)(P( )/dr) r(2*pi()*u*l*r^3*(dw/dr))= 0 This leads to the solution omega, w = k/r^2 + C, which under given boundary conditions leads to the conclusion that omega_d, the angular velocity of water at the edge of the hole is on the order of 10^4 greater than at the margin of the pan. Hopefully others will supply examples. > Ian, > Thanks for posting this. [quoted text clipped - 8 lines] > > Hopefully others will supply examples. John, heres another one for you... A simple mechanical servo amplifier: Consider a plate sprocket mounted on a threaded section of a shaft. A similar sprocket with a bearing, is mounted each side of the first sprocket, separated from it by an oiled felt (or Teflon) friction disc. The shaft has provision to control the clearance between the sprockets and friction discs, to contain lateral thrust, and is mounted on suitable bearings. The input is by turning the shaft. The output is from the central sprocket. The outer sprockets are driven in contra-rotation such as to tend to drive the threaded centre sprocket away from them if the shaft is at rest. At rest, little or no torque is delivered to the centre sprocket which remains at rest. As soon as the input shaft is turned, the thread moves the centre sprocket towards the driven sprocket that rotates in the same direction. If the output over-runs, the thread backs it off so that the output is synchronised to the input. Oiled felt or Teflon friction discs are generally used in low torque applications to minimise hunting, there being little difference between the starting and running friction. A bidirectional drive is described, but a single direction drive with a fixed brake disc is an alternative. A common present day implementation of this is in hand winches with automatic brakes. You might also be interested in this: http://www.research.ibm.com/journal/rd/011/ibmrd0101H.pdf All the best Ian Macmillan |
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