Measuring the MET

Keith Wilson - Electrical engineer

The London Underground system carries more than 1.3 billion passengers every year but, even in this environmentally aware age, how many of them give a thought to the amount of energy their train is using? Not many is the probable answer, but way back in 1926, energy consumption was a major concern for the directors of the Metropolitan Railway, a large part of which is now the present-day Metropolitan Line.

What they wanted, according to a paper published at the time, was accurate information about how much energy each train used during its daily journeys. The only way to get that information was to make measurements on the train, but that wasn’t easy. A special testing car couldn’t be attached to the trains as they were already the full length of the platforms and also the extra load of the testing car would have affected the results. Metropolitan trains in those days had luggage compartments, but these were too small to house the necessary instrumentation.

The only available solution, therefore, was to install the instruments in one of the passenger compartments. Even then, no off-the-shelf instruments were going to be satisfactory. The Metropolitan Railway wanted a permanent record of, among other things, the voltage available to the traction motors – which nominally operated at 600 V DC – and the current drawn by them. In 1926, this meant making pen recordings on a moving paper chart.

The chart recorder, however, had to be able to operate reliably in spite of the high levels of vibration experienced on a moving train. It also had to be able to record continuously throughout the whole of daily duty of the train – typically more than 12 hours. Using a slow chart speed wasn’t an option because of the level of detail needed.

The company that took on the challenge of developing the instrumentation to suit this demanding and unusual application was Evershed & Vignoles, the originator of the Megger insulation tester and one of the major constituents of what is now the Megger group.

The measuring installation in one the Metropolitan Railway passenger coaches is shown in the picture. Essentially it is made up of a recording voltmeter, a recording ammeter, a speed recorder, a brake application recorder and a stop-and-start recorder.

Also provided were a voltmeter, an ammeter and three direct-reading kilowatt-hour meters. These were used only to check the results obtained from the chart recorders – the actual energy consumed by the train was computed by integrating the curves on the charts.

Evershed and Vignoles solved the vibration problem by fitting the instruments with oil dashpots that were specially shaped to prevent the spillage of oil if the train accelerated or decelerated suddenly. And the need for long periods of continuous recording was solved by making provision for a new chart to be joined to an old one without stopping the instrument.

The train to which the instruments were fitted was powered by four British Westinghouse motors, each rated at 200 horsepower. It had a total weight of 198 tons and provided seating accommodation for 322 passengers. The instrumentation was found to be capable of delivering accurate and reliable results. It’s reasonable to assume that the train made many test runs but information is available for only one of these.

This was a journey from Harrow to Northwick Park during which the instruments recorded that the traction motor to which they were connected consumed 35.8 kWh. Since the train had four similar traction motors, the total consumption is recorded as 143.2 kWh, four times the recorded figure. It would be interesting to know how this compares with the energy consumption of today’s trains on the Metropolitan Line.

As an interesting aside, the paper notes that “a bonus is given to drivers depending on the current which is saved over and above a predetermined allowance, and it is found that care is taken by the drivers to effect economy in current consumption.” Clearly the Metropolitan Railway was keen to minimise energy consumption as long ago as 1926, although it’s almost certain that this was for financial reasons rather than to reduce the railway’s carbon footprint!

With modern electronic technology, it would be easy to equip a train with reliable and accurate measuring and recording equipment but 60 years ago, but the situation 60 years ago was very different. Nevertheless, Evershed & Vignoles took on the challenge and devised an eminently practical solution. And the development of innovative practical solutions is a tradition that the Megger Group continues to this day.