Southern Electric Group 4Cor unit 3142 Preservation Report 7

4 COR Preservation Report Winter 2000

There is no doubt that the most exciting developments to have taken place recently are with the electrical system. In the last article we had reached the point where the lighting had been rewired on the motor coach 11187 and the lights had been run on the emergency batteries which are housed in the guards compartment. We were planning as the next stage to run up the motor generator using a three phase rectifier to provide about 550 volts. I am pleased to be able to tell you that this has now been installed with many thanks due to Bob Baines for wiring it up for us. The rectifier is very similar to the set up which the EPB preservation group is using to charge the batteries on the MLV on the East Kent railway to haul their 2 EPB.

This rectified supply is obviously considerably lower than the normal juice rail but still adequate to drive the motor generator (MG), which then generates 70 volts to run lighting and auxiliaries. A considerable amount of work has been done in cleaning and repainting the MG prior to running it, all the work being done in situ because removing such a heavy item would be very difficult. A lot of fingers were crossed when the power was first applied but in the best Southern Electric traditions it sprang into life without any trouble. The lights were connected and the note of the MG dropped noticeably, but the lights came on powered from the juice for the first time for over 30 years. The lights were considerably brighter than when running on batteries and a measurement soon confirmed that instead of the expected 70 volts the MG was generating more like 90. Nice bright lights are all very well, but the bulbs were not going to last very long at that voltage.

Our next problem then was to try and sort out how to get the voltage down to the correct level. The device used for this is called a carbon pile regulator. Figure 1 shows how this operates with the MG. Inside the MG is an armature which is the rotating coil which produces the output, and also a stationary field coil. In order for the armature to generate the voltage it needs a magnetic field which is provided by the field coil. The output voltage is proportional to the field strength from this coil, and it is the job of the carbon pile regulator which is situated behind the drivers compartment to control this field.

The carbon pile regulator is basically a resistor in series with the generator field coil. It is formed of carbon granules, and its resistance is dependent on the pressure applied across the granules. The output voltage from the generator is used to drive an electro magnet inside the regulator which controls the pressure across the granules. Levers driven by the electro magnet apply a force on the granules in such a way that if the voltage rises and the electro magnet exerts more force the pressure across the granules falls, the resistance therefore increases and the voltage falls back again.

Careful adjustment is required to set the output, I am indebted to a friend of mine who served an electrical apprenticeship in Eastleigh works in the 1960's, for digging out a description of the regulator and how to adjust it. Armed with this we set about the adjustment but could not get closer than about 73 volts. This was probably near enough at the time, but does create a problem with battery charging as I will describe later. As luck would have it, soon after this we managed to get hold of a modern (EPB standard) electronic regulator which does the same job much more accurately, this was rescued from withdrawn stock. Once this was fitted our 70 volts became almost exact.

This leads on to our next problem i.e. battery charging. In the pre-electronic 1930’s a fairly elaborate method was employed to overcome the basic problem with any generator charging a battery, that when the generator runs down the batteries will try to drive it as a motor which would soon exhaust the batteries. Figure 2 shows a comparison of the COR and EPB methods of solving this problem. In the COR system the batteries are configured as two separate 35 volt sections, a complicated arrangement of switches is required to connect these in parallel for charging, or in series to drive the emergency lighting. This switching is achieved by the parallel contactor, like the carbon pile regulator this is situated in the equipment case behind the driver.

The EPB arrangement is much simpler, a blocking diode allows current to flow from the MG to charge the batteries and run the auxiliaries and lighting. This means that the battery is directly across the MG output continuously, with no protection resistor. The MG must therefore remain precisely at 70 volts to prevent under or over charging. This precision is not possible with the old carbon pile regulator.

Of course blocking diodes were not available to the COR designers, but again we have managed to acquire an ex EPB diode and have rewired the circuitry to the EPB system.

We can now run up the MG to charge batteries and have lighting without having to change the wiring over for either lights on or for charging.

The next item of electrical equipment to receive attention was the compressor. The compressor was removed soon after withdrawal. It had been kept however and has since been cleaned up and repainted. This January we at last managed to haul it into place using a block and tackle, luckily it has a lifting eye and we hauled it up using the method shown in Figure 3. When the compressor was hauled near to the correct position, the irons which support it were slid into position and bolted onto the under frame. This was not as easy as it sounds because of the limited amount of room available, but once the irons were in, the compressor was levered into position and was bolted to the irons.

The compressor is powered from the line voltage but the power to it is switched by a contactor, which is a relay operated by the 70 volt line. The contactor that we have installed is one which was rescued from the Brighton belle motor car at Stuarts lane. A certain amount of rewiring was required, but the compressor could not be connected into the reservoir because a small section of pipe is missing. However it should still we felt be possible to run the compressor up. I was working at the front end of the carriage when all of a sudden I heard a noise like a steam engine running. Sure enough the compressor just like the MG had sprung into life after 30 years of idleness. It quickly became apparent that there was very little air being produced from the compressor, we believe that this is because the valves inside will not close until there is some back pressure. It is quite common in fact for stock currently in service to take a while for the compressor to start pumping up after it has been idle for a while because the valves tend to stick open. We have squirted some WD40 into the inlet therefore and hope that it will start to work OK when it is connected up. The compressor is a twin cylinder design, we are not sure if it works as a two stage pump, i.e. one cylinder feeds into the other, this has the advantage that each cylinder only has roughly half as much pressure across it. Possibly some member may know a bit more about their design.

All of this effort on the electrical side does not mean that the body has been neglected. I mentioned in the last article that there was particularly bad split in the main bearer under the guards door on the blind side. Figure 4 shows the extent of the problem. Not only was the main bearer in need of repair but the steel plate across the top of the bogie which rests on the sole bar was also in need of patching. This was in fact the most substantial repair that we have undertaken. The main bearer is about 9 inches wide and about 5 inches had to be cut away over a length of 2 feet or so. Once this had been cut away, the top of the transverse plate could be seen, it was certainly rusted quite badly. The first stage with this was to cut a strip about 2 inches wide off the edge. A replacement strip was then easily cut but the welding required was rather more substantial than my welder could manage. Fortunately St Leonards Railway Engineering have more powerful ones and very kindly welded it into place for us. This plate had to have a hole through the middle to allow one of the fixing bolts to pass through, this being one of the bolts which attach the main bearer and hence the coach body to the sole bar. The welding complete, the plate was thoroughly painted before the wood was repaired. Previous repairs had only required a strip of wood less than 3 inches wide, but this one required two 3 inch square pieces to be fitted one outside the other. The outer one was slotted to take the guards door poSt Also it had to be cut back to allow the recess for the door.

This then was our last major repair to the bottom frame work, and with its completion we were able to replace the two panels which form the side of the guards compartment, one above the other. These panels had been waiting to be replaced for about a year, the bottom one was completely new, but the top had been repaired. This was because it is larger than the standard 2 by 1 metre panels which are the largest that can be obtained easily. Thus by the end of November we had all the major body sections back in place, all still painted in red oxide apart from the front which we have painted in yellow, I am sure you will agree that this is quite a significant milestone.

Work has also been continuing steadily on the windows, we are replacing these at the rate of roughly one a month. All seven have been replaced on the blind side and two on the drivers side only five more to go to complete the passenger saloons.