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Lister Start-O-Matic

Background to the DC Start-O-Matic system

Start-O-Matic engine-house

Start-O-Matic engine-house

This 'lighting plant' consists of a 2.5-kilowatt diesel-engined generating set, a 110-volt storage battery and an automatic control panel. It was built in 1947 and installed in a remote Dartmoor location where for decades it was probably the only source of electrical power, sufficient to run the lighting and general appliance load of an average house. The principle of the "floating battery" system used by the plant is quite straightforward; When the electrical load was light, in this case up to about five lamps or 500 watts consumption, the battery would meet the demand for a long period and there would be no need for the generating set to run. Heavier loads would be also be supported by the battery for long enough to get the set started. Once running, any reserve generator capacity would be used to recharge the battery. Provided the consumption followed a regular domestic pattern of varying heavy and light loads, the generator would spend enough time running to maintain the average state of charge of the battery near to maximum. What really makes this system practical is that the generator starts automatically when the load exceeds the threshold, regulates its output to keep the battery charging steadily and finally shuts down when not required. Apart from a monthly "equalising charge" to top the battery right up to full capacity and routine maintenance as required on any generating plant, the system would look after itself. Start-O-Matic plant was available with ratings up to 20 kilowatts for large premises, with the option of interlocked control of multiple generating sets. If two sets of unequal size were installed, the control gear would start whichever set (or both) was appropriate for the load.

DC vs. AC

Plant schematic

Plant schematic

It is worth noting that this system applies specifically to DC supply, which was obsolescent in 1947 when the plant was commissioned. At that time an increasing number of electrical appliances were made for AC only, so there was a definite disadvantage to using DC floating battery plant on its own. Because AC cannot be stored as such, an AC plant must work in one of two ways; use a battery for starting purposes only and generate the AC directly whenever required, or generate and store DC ready for conversion to AC on demand.

AC automatic-start systems were made by Lister also under the Start-O-Matic name, in which the generator runs all the time load is detected. Although battery and converter energy losses are avoided and the upkeep and cost of a large battery eliminated, the direct generating approach still loses on efficiency overall if small loads are often used, as an engine-driven generator becomes very wasteful at light load. The set is also subjected to longer running times as no power whatsoever is available without it running; the rate of wear is naturally increased. Another disadvantage of this method is the delay after switching on an appliance or light before the generator is ready to power it. When using an appliance such as an electric drill, for example, unless a light or other load is left on, the set will stop every time the trigger is released for more than a few seconds.

The alternative solution was to leave lighting and other uncritical loads on DC but run a converter from the DC supply to provide AC for those loads that required it. This configuration also reduces efficiency due to losses in the converter but allows both DC and AC to be used in moderation without the generator running. The AC converter set could be hand-started or equipped with a demand-start circuit like that of the generator, although this equipment was not supplied by Lister as part of the Start-O-Matic package.


Operational sequence

The "automation" is controlled by six relays, three of which are fitted with oil dashpot time-delays enabling them to control the duration of the several steps in the starting sequence.

Limitations and eccentricities of the floating battery Start-O-Matic system

Switchboard drawing

Switchboard drawing

Under automatic operation the load is always connected to the battery. The battery voltage when charging must be higher than when discharging, thus the voltage delivered to the load must vary. In practice, it is not possible to charge the battery to 100% capacity without raising the voltage so high that the load might be damaged, typically 35-40% in excess of normal system voltage. In order to prevent the battery losing capacity through sulphation, which affects lead-acid batteries that are not kept fully charged, and to avoid the cells of the battery getting "out of step" in their charge status, it is necessary to start the set manually from time to time, and adjust the dynamo output to the full charging voltage of the battery. During the equalising charge it is important to disconnect any load that may be on the system from the charging voltage. A switch is provided to allow it to be connected to a tapping in the battery that delivers the proper system voltage during charging, enabling the supply to remain alive even when the full dynamo voltage would be dangerously high. Large floating battery systems, such as those in telephone exchanges, were equipped with a complex system of counter-emf-cell switching to limit the load voltage, but such extra expense is not warranted in a plant of this size. The control gear on this plant does not start the engine in response to the battery charge state being low. If just one light were left on continuously, the battery would eventually run down because there would be no peak consumption periods to signal the need to start the engine. In this case, the automatic gear would not be able to turn the engine over and the plant would lock out. The remedy would be to start the engine by hand-cranking and then let it run through a full equalising charge to restore the battery to normal.

Voltage regulation

A DC generator can be made to give an output voltage that remains constant despite variations in load current, by means of a series field winding that boosts the field as the load increases, thereby compensating for the voltage drop. Such a machine is termed 'level-compounded', to distinguish it from one with a deliberate fall or rise in voltage with change in load. Although a constant voltage output would be desirable in a charging set for a floating battery, the compound generator is unsuitable for one important reason. When, as often happens during starting and stopping, the output voltage of the machine falls below that of the battery, its armature current reverses as the generator runs as a motor. If the armature current is heavy, the series field winding can reverse the residual magnetism in the iron frame of the machine, causing it to build up with reversed polarity at the next start. This would prevent it charging the battery and possibly cause damage or blown fuses. It is therefore preferable to use a simple shunt-field machine equipped with an alternative means of stabilising the voltage, to assist with maintaining a steady charging rate even while the load varies. Lister used a device they called the magnetic governor; a solenoid actuator mechanically linked to the centrifugal governor and wired in series with the load circuit. As electrical consumption increases, the engine speeds up in proportion to the anticipated voltage droop, overcoming the latter by increasing the dynamo EMF. This is an open-loop system (i.e. it does not sample its own output and attempt to correct it) therefore accuracy of the voltage is critically affected by any adjustments. When the plant was tested at the factory, a line was scored on the field rheostat bezel showing the correct setting for normal float operation. Lister strongly forbade any user adjustment to the governor whatsoever. Note that the series field provided on the Start-O-Matic is connected with opposite polarity to that of a cumulative compound generator, and is cumulative when motoring; it cannot reverse the machine residual field.

CS 5/1 Start-O-Matic No. 6413

Acquisition

Start-O-Matic plant

Start-O-Matic plant

We obtained the plant in remarkably complete, unrestored condition. The pipework and cables had been cut in-situ prior to removing everything intact ready for collection. The accessories were all present and original, including the fuel tank and pipe, cooling tank and even the three-way water cock bearing the Lister name. The switchboards are reasonably sound although there is some damage and corrosion to various parts of the automatic starting gear. With the plant was a motor-generator DC-AC converter of 800 watts rating, which was probably installed to supply AC-only appliances some time after the system was commissioned. An additional contact had been added to the automatic starting gear; it is likely that this was used to energise the converter when the generator was running. Also included were a number of 110 volt DC fractional-horsepower motors that probably drove various appliances and plant around the site.

Preparation

Lister 5/1 diesel engine

Lister 5/1 diesel engine

The first job was to clean the crankcase, which was full of sludge and emulsion. It may not have received proper maintenance attention during the last phase of its working life, during which the lubricating oil had become diluted with fuel and contaminated with water. The sump was then filled with non-dispersant monograde oil, the preferred formulation in an engine with unfiltered oil circulation. At this time the amount of wear on the main bearings, crank and con-rod was assessed; about twice the official Lister recommended maximum but acceptable for test purposes without overhaul. The exposed moving parts and valvegear were lubricated, as was the cylinder bore, prior to turning the engine over for the first time. After exercising the compression changeover valve to clean its seats the engine was found to have excellent compression. The cooling water passages in the block and head were found to be somewhat scaled but not so badly as to require treatment before testing. Lister engines of the period are fitted with washable fabric-tube fuel filters; this filter element required a thorough scrub to remove the solid layer of gum that had formed on it but was found intact and refitted. In internal inspection of the dynamo revealed carbon build-up on the commutator where the undercut between the segments had not been cleaned. This was remedied, the brushes were freed in their holders and the machine bearings were greased. A mouse nest was removed from the terminal box but there was no sign of damage to the insulation, which was tested and found excellent. The switchboard is divided between two panels; the general switchgear and meters on the larger and the automatic starting equipment on the smaller. It was apparent that a fair amount of repair work would be required to make the automatic gear operable, so the general switchboard was tidied up and connected to the generator in such a way as to allow manual control of the electric starting. The exhaust silencer was attached; cooling and fuel supplies were temporarily connected up using the original tanks and the injection pump was primed. A 110-volt battery was assembled from wet lead-acids and wired to the panel along with the dynamo. Watch video: Rigging up the Start-O-Matic

Main switchboard

Main switchboard

Automatic controls

Automatic controls

Automatic control wiring

Automatic control wiring

First startup

The main switch was closed and the plant motored swiftly but failed to start, due to some air remaining in the injector line. After a couple of attempts the last bubble was bled out. On closing the compression change-over valve, which had been left loose to reduce cranking load on the machine during bleeding, the engine fired up immediately and swung the ammeter over to charge. Everything worked well; the revolving tappets revolved steadily, the engine governed smoothly, there was no sign of lubricating oil being burnt. After the engine had warmed up, full load was put on for long enough to warm the dynamo windings and prove correct operation; sparkless commutation was observed and the engine continued to perform well under load, just a trace of fuel-smoke being noticeable. It was then shut down pending the completion of the switchboard repairs. Watch video: Start-O-Matic begins to generate

Here come the clones!

Such was the success of the Lister CS over five decades that many copies of it are still being manufactured in the 21st. century. India is the source of most of these, made as more-or-less direct copies of the original design with various parts improved or modified for ease of manufacture and higher output. In some cases, clone parts will interchange directly with original lister parts, others are too different in detail. These low-cost engines and packaged generating sets are finding favour in off-grid and CHP set-ups, although it is widely reported that few of them match the genuine Lister for reliability. In the third world particularly, the word 'Lister' is used as a generic term for a low-speed diesel, and they say imitation is the most sincere form of flattery!

Further reading

  • E. Molloy (ed). Complete Electrical Engineering. London: Newnes
  • Handbooks Nos. 110, 160, 169 & 204. Dursley: R.A. Lister & Co.
  • Edgington, David. The Lister CS story. Westbury: Stationary Engine Books, 2006
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