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The Electrolytic Variable Resistance Test Load THE ELECTROLYTIC VARIABLE RESISTANCE TEST LOAD/SWITCH EP-RR 4 FOR THE CANBERRA HOMOPOLAR GENERATOR R. A. MARSHALL First Published: May, 1964 Re-issued: April, 1967 Department of Engineering Physics Research School of Physical Sciences THE AUSTRALIAN NATIONAL UNIVERSITY HANCOCK ra’ A.C.T., Australia. TJ163.A87 EP-RR4. f T J16 3* 1924126 . A8 7 EP-RR4 A.N.U. LIBRARY i This book was published by ANU Press between 1965–1991. This republication is part of the digitisation project being carried out by Scholarly Information Services/Library and ANU Press. This project aims to make past scholarly works published by The Australian National University available to a global audience under its open-access policy. THE ELECTROLYTIC VARIABLE RESISTANCE TEST LOAD/SWITCH FOR THE CANBERRA HOMOPOLAR GENERATOR by R. A. MARSHALL First Published, May, 1964 Re-issued, April, 1967 Publication EP-RR 4 Department of Engineering Physics Research School of Physical Sciences THE AUSTRALIAN NATIONAL UNIVERSITY C anberra, A.C.T. A ustralia - 6 FEB 1968 CONTENTS page 1. Introduction 1 2. Description of the Test Load 1 3. Use of Load as a Current Control Resistor and Switch 7 4. Characteristics of the Test Load 7 5. Operating Characteristic of Quarter Homopolar Generator 15 6. Theory for Load Conductance - Time Program 16 7. Test Load Conductance - Time Program 17 8. Cam Co-ordinates and Classical Pulse Calculations 18 9. The F ast Pulse 21 10. The Emergency Brake Pulse 23 11. The Bus Bar Test Cam 24 12. Cam Manufacture 24 13. References 27 ii 1. Introduction When the Department's homopolar generator was nearing completion in its Mark I form the design and construction of a device for testing it was begun. The re­ quirements for this were as follows: a. To be able to carry the 1. 6 million amps produced by the homopolar generator. b. To be able to make and break the circuit with a voltage of 240 volts in it. This is the maximum voltage that is generated in one rotor disc. c. To be able to have its resistance continuously variable in a pre­ programmed manner so that pulses can be shaped as desired. d. To be able to absorb all the energy from one rotor at full speed. This is 290 megajoules at 900 revolutions per minute of the rotor. Details of the nature of the tests can be found in references 1 and 2. 2. Description of the Test Load The test load consists of a series of sheet steel electrodes 1/8 in. thick interleaved so that they have alternate polarity, Figure 1, which are hung from a gantry type frame as shown in Figures 2 and 3. r -r 7v\\\\ , 'v\\ \\i t-J IO? 1 * ! I i _rr\ v, ■_ 'A y r r *r<fo —t\~\ \ \ v a 3 s ^ Ad ' - t- C O Y x * JYW-Ö EL s r ; o* 3 : .'o r c\ j L\ ^ YY s r 5 t v" U- r;— (■— Figure 1. Detail of Interleaved Electrode Plates Description of the Test Load 2 The top ends of the tongues of these electrodes are dimpled and clamped to the alumi­ nium conductors which connect to the homopolar generator. Below these is an 800 gallon capacity tank which is supported in a tubular stem rising in its centre on a hydraulic ram, which enables the tank to be raised and lowered. This causes the caustic soda electrolyte to rise to a greater or lesser height between the electrode plates, giving the desired electrical resistance in the generator’s discharge circuit. This resistance can be made as low as 4 micro-ohms. Vj O O Ct t w ö y? ö o • o / c o n tro l val'/l Figure 2. Front Elevation of Test Load - Semi-schematic Description of the Test Load 3 Figure 3. End Elevation of Test Load - Semi-schematic Description of the Test Load 4 The ram is actuated by high pressure oil fed from an accumulator via a valve which is controlled by a control cam and the electrolyte tank's vertical position, (see Figures 4 and 5) The accumulator is used to enable the desired speed of move­ ment of the tank to be obtained without using an excessively large pump. This speed is such that the plates can be immersed in about a second. HYDRAUL 1C ACCUMULATOR &AZ BOTTLE AUb RAM tf / [f 9 Figure 4. Hydraulic Circuit for Test Load - Schematic When the tank is being lowered on the servo system after a pulse, it is possible for the valve to shut right off under some circumstances. This tends to occur at the end of the downstroke. To prevent this from causing large destructive pressures in the system, a pressure relief valve is included in the oil line to the ram. When this hydraulic servo system was first operated, it behaved in an unstable manner. This was cured by decreasing the gain of the servo by modifying the valve spool to make the characteristics of the valve linear. Initially these had pronounced S shape with mest of the controlling being done over a small portion of the centre of its travel. Other points of interest concerning the design of test load are as follows: Caustic electrolyte was chosen rather than acid, even though its resistivity is higher, because this allowed steel, Description of the Test Load 5 which is both stronger and cheaper, to be used for the electrode plates. There are two main forces acting on the plates when current is being carried. Because the current-carrying tabs of the plates are separated rather than interleaved, this gives rise to a force of several tons at full current pulling the two groups of plates together. This is resisted by supports between the top clamps and by continuing some of the plate spacer tie rods across between the two stacks. The other force is one tending to pull the positive and negative plates into contact with each other if their spacing is not perfect; a case of unstable equilibrium. When the load was first built an attempt was made to support each polarity of plate by means of two angled sets of struts that were kept out of the electrolyte. However these failed during a test of the generator1 allowing the plates to touch and put a short circuit on the machine during which the current rose to 1. 9 million amps. Following this, the bracing was done with horizon­ tal insulated struts which get wet during a pulse. These can be seen in Figure 6 and proved satisfactory. CAM INPUT Feeo back chain CONTROL VALUE LEVEN Anchor point CONT/pOL VALUE Figure 5. Test Load Tank Height Feedback Control System - Diagrammatic Description of the Test Load 6 Figure 6. Photograph of Test Load, Showing Electrode Plates, Insulated Struts and Electrolyte Tank. The electrode plates are arranged in the two stacks to allow room for the tank ram to rise up between them. The devices shown around the plate tabs in Figure 2 are pick-up coils to enable rate of change of current, and hence current, to be mea­ sured during pulses (this is one of several ways employed to measure the current). Between the load and the generator, the bus bars which are interleaved to minimise inductance and forces, pass tnrough a commoning unit which, as the name implies, connects all bars of each polarity. The purpose of this is to prevent any unsymmetri- cal short circuit which may occur in the load from getting back to the generator in its unsymmetrical form. This could cause serious forces on the rotor bearings and con­ ductors. Nearly all of the energy delivered to the test load from the generator appears as heat in the electrolyte and to remove this the caustic soda solution is pumped out of the tank through a heat exchanger into a hold-up tank. It is then pumped back into the load tank, all this being done between pulses. So that successive pulses may be as similar as possible, it is necessary to ensure when the electrolyte is pumped back that the surface is at the correct level. This is done by pumping an excess quantity back, which subsequently flows out over a weir slit. The weir collector can be seen on the tank in Figure 6. Characteristics of the Test Load o o o os 11 0*4S l-o 14 0-96 l-s 36 (•44- 20 4S l' 91 2-S CO 2-39 ZO 1Z 237 325 IS III iso 84'2 IIS ZlS 9 o-G 1 3-59 400 2)7-5 3-52 475 104-7 4-06 4 s o 112'4 4-30 47S" 121-0 4-53 S'00 l}OS 4-76 SIS 1411 4*99 SSO 155-8 S'-2 2 S b 9 u c 6 188 S'CC 7 260 G-55 8 332 7*40 9 4o4 8-27 lO 476 9 /3 n S48 10-00 (Z 620 10-17 U 651 lb 74 14 764 |2-6( Fig. 7: Test load plate shape and relationship between depth of immersion (L), area IS (3-48 836 of immersion (A), tank movement (H). IG 9o$ I4-35 L, H in inches. A in square inches. n 930 IS22 IS 1 052 1609 *9 112 4 (6-36 20 1(56 n-*3 21 /26$ 18-70 22 1140 19-56 23 1412 20' 43 24 14*4- 21-30 L A H 7 3.
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