Justin Smith
Established Member
On the subject of maximum power v continuous power the following has been written by an Railway performance Society member with driving experience (Class 309 specific info is further down) :
I drove various types of electric and diesel traction on the main line in BR days (post 1996) and there were instructions with diesel and electric locomotives about continuous and one hour ratings.
With diesel-electric locos, there was a gauge in the driving cab which took a reading of the current from the main generator in amps. On some locos, like a class 45 or 47 it was divided into green and yellow sectors. On an EE loco, (class 37/40) the original display had a scale from 0-2700, and a red mark to indicate where an overload protection device would cut power from the main generator if the needle was allowed to exceed this figure.
With the class 47, the highest point of the yellow sector was approximately 4,200 amps; this was the continuous rating. On all but the heaviest trains/steepest gradients, with the power controller at maximum, once the train being hauled was on the move, this position (4260 amps) on the ammeter was the maximum that the needle would settle at. If the power unit was producing less than full power due to a defect, the maximum would be lower than this. It may also be lower if the (electric train supply) was drawing power via the ETS generator. The one hour rating comes becomes relevant if the combination of load and gradient are placing more load on the electrical system, which causes the amps to increase above the continuous rating of 4260. There were relatively few locations with passenger trains where this occurred, other than pulling away from a stand, because there were maximum loads specified by engineers and operating staff to avoid this. However, I am told that the heavy overnight trains on the Highland Main Line were an example of where running in the yellow segment of the ammeter on full power would occur. It is something I never experienced, so cannot confirm this. Freight train could be more challenging to the locomotives as they were often significantly heavier, and if the ammeter needle stayed in the yellow for significant periods of time, the driver's instructions were to reduce the power until the ammeter needle remained in the green sector. This prevented excessive heat in the electrical components developing.
With tapchanger controlled AC locos, the driver applied power by notching up the transformer through the power controller. The needles of the ammeter rose with every notch but fell back as the speed of the train increased, creating back EMF. To counteract this, the driver continued notching up, until there was no more power to add from the transformer. The ammeter had green, yellow and red sectors, and the yellow sector indicated that the power being taken was equivalent to the one hour rating of the electrical equipment. The instruction for the driver was not to use the yellow sector for more that 5 minutes in every hour. Again, in most locations and with the planned train loads for passenger trains, the time spent in the yellow sector would not approach the 5 minute limit.
With EMUs, there was no ammeter to refer to, and the design of the electrical system would be such that the one hour ratings were normally only achieved briefly during acceleration. Should one unit have to assist another that had failed, then there would be instructions on how to limit the power, usually by specifying a lower position on the power controller than full.
Re the class 309; I drove the units in the last years of their time in service. They did not cruise at 100 mph like modern trains do, reaching 90+ only when gradients allowed them to do so. I wrote a short article for Milepost illustrating this, which will be somewhere in the archive.
I think the best way of phrasing the power capability of first generation EMUs is to say that they could exceed their continuous rating during acceleration, and that this would only be over short periods as it would move out of the speed range where the continuous rating was being exceeded as it continued to accelerate.
In all instances, there was no electrical circuit that stopped the one hour rating being exceeded. The overload circuit on diesel and electric locos operated when the amps exceeded a certain value, which was too high for the electrical circuits to maintain without damage, generally heat related.
Automatic current control became available as technology changed, such as using a thyristor based system, on electric locos such as class 90/91s, being developed using 87101.
So, nothing technical, just my best shot at a practical explanation of how it applied to operating methods.
