Electrical restoration: Class 50 control units

Electronic Control Units

One of the major talking points when RRRG first acquired 50029 and 50030 was the availability, or otherwise, of the various electrical control units. The Class 50 has a reputation for being electrically complex – some would say needlessly so.  Pre-refurbishment they were even worse as EE (with assistance from BR) had tried to produce a jack-of-all-trades locomotive.  Many of the features were either rarely used or prone to faults, and most were removed by BR during refurbishment.  The electronic control units are mostly unique to the 50 although some are common to other late-build English Electric locomotives.  Unsurprisingly, all the control units had been removed from 29 and 30 before we bought them and so we had to obtain replacements.  We also felt that it would be worth trying to update the electronics to modern specifications where possible, because the 1960s designs either use completely obsolete components for which no modern equivalents exist, or incorporate design faults, or both.

This is a list of the various control units in a Class 50 and how we have obtained them or can obtain them if we still need them.

CU1 – The main control unit CU1 is the central “brains” and obtains information from a number of different sources, namely from the cab controls, engine load condition (from the load limiting potentiometer) and from the traction currents.  This information is passed to the CU3 and in turn the engine governor.  We have one complete, tested unit, one complete and untested unit and the parts to make up another.

CU2 – the field divert unit, the electrical equivalent of the gearbox of the locomotive.  It controls the three stages of field weakening which occur at approximately 25, 40 and 60 mph depending on trailing load and ruling gradient.  We were missing this item from both our locomotives and the former owners of 50021 kindly loaned us an example which we took to an electronics specialist company who “cloned” it and built us two copies to modern standards.  Ironically, shortly after this, we were able to purchase an original Class 50 unit, though this has yet to be tested, however the original EE unit is known to be very reliable in service.

CU3 and CU6 – also known by its EE designation KV10. The CU3 sets the field current from the main generator as a summation of all the input signals and the CU6 similarly controls the ETH generator output. The main body of the unit is identical but is turned into a CU3 or CU6 by adding a CU3A or CU6A daughter card. Originally we had no KV10s but we adopted a policy of buying them whenever they came on the market in the intervening years and we built up quite a collection. However the KV10 is a classic case of obsolete and temperamental electronics and a respected locomotive electronics expert brought a rebuilt version to market some years ago with completely modernised components. We felt that ordering two each of the rebuilt CU3 and CU6 units for 50029 and 50030 was money well spent. The owner of 50008 Thunderer has also chosen to fit these modernised KV10s and is gaining operational experience with them which will prove invaluable to RRRG.  The new design incorporates the electronics of the CU3A or CU6A into the main unit and eliminates the need for a daughter card.

CU4 – coolant control unit.  This unit is designed to monitor the temperature of the cooling water in the two coolant circuits and open/close valves to route the flow of water correctly.  It also sends signals to the CU5.  We have none but the Class 50 Alliance has developed a modernised unit which will be made available to us in due course.  It would have been possible to recreate the original CU4, which was built for EE by Hawker Siddeley, but the new design employs proven marine technology which would be acceptable to Network Rail (an important consideration for the C50A) and, together with a C50A-planned CU5 rebuild would eliminate "bugs" in the system which can cause the engine to be over-cooled in some circumstances.

CU5 – radiator fan control unit, it receives the signals from the temperature probes via the CU4 and controls the speed of the radiator fan accordingly.  We had none but again we were able to borrow one from another Class 50 owner.  Adrian Spencer of Cotswold Mainline Diesel Group, another noted locomotive electronics expert, was again able to copy this circuit and build a copy using modern electronic components.  We have also taken the opportunity to have a batch of the temperature probes remanufactured, in partnership with other Class 50 owners.

CU6 – Regulates the output of the ETH generator. This is basically the same as the CU3, but adjusts the ETH generator field strength to suit varying load demands according the number of coaches requiring electrical heating.

CU7 – ETH field detector unit.  Basically it sees if there is a feed from the coaches which allows the ETH generator to electrically heat them.  It also detects if an ETH fault occurs in a coach, and if one does, in conjunction with Heating Over Volts Relay, it switches off the ETH feed to the coaches and returns the power unit to idling.  It also detects voltage from two 50s in multiple and prevents “doubling” of the ETH supply if this is the case.  This is a fairly simple circuit which I was able to rebuild and install in 50030.  I also obtained the parts to build one for 50029 in due course.  It was a CU7/HOVR fault which caused 50050 Fearless to shut down during the "Fifty Terminator" railtour of March 1994 as for some reason the engine was shut down instead of being returned to idle.

The CU8 designation was never used, for reasons lost in the mists of time, so isn't relevant to our restoration project.

The CU9 is the automatic voltage regulator, or AVR, and basically maintains the output from the auxiliary generator at 110v DC so the compressors, exhausters and motor blowers do not overspeed.  There are two types: electronic (CU9A) and carbon pile (CU9B), both types are compatible and there is one per locomotive.  The majority of Class 50s were fitted with the original ‘Carbon Pile’ unit, and ten 50s were fitted with an electronic version, but because the electronic types additionally controlled battery charging and also contained gas-sensing equipment, their complexity apparently made them unreliable and were replaced with carbon pile units, which are slower reacting but more reliable.  We have an overhauled CU9B for use in 50030 plus two untested CU9Bs.  We may decide to commission new build versions as this unit can also be improved upon with modern electronics, effectively producing an updated CU9A.  We are currently unaware of anyone who could test our CU9Bs and this is also something we need to consider.

The CU10 is the speed control unit.  It's also known as the ‘AEI’ unit after its manufacturer, the Birmingham company Associated Electrical Industries.  It is a complex device that receives signals from the axle driven transmitter unit, and sends signals to the cab speedometers.  It can be adjusted to take into account wheel-wear.  Six card slots but not all these are used on a 50.  One CU10 per locomotive.  We have two units plus some of the control cards, but as the Class 50 Alliance have designed a modern version, we will probably obtain one in due course.

CU11 was the slow speed control unit which was removed at refurbishment.

The CU12 is the lighting delay timer.  It turns off unnecessary lighting 15 minutes after engine shutdown, such as internal lighting and marker lights, should they be left on and the main battery isolation switch be left in the ‘ON’ position. One per locomotive.  It's questionable whether this unit is really necessary for preserved line use, as it was really designed for times when a 50 might finish its BR duties and then be disposed of on depot by a driver with other things on his mind, so that he might forget to turn the lights off and the batteries would be drained and the locomotive subsequently failed the next morning.  This unit is actually still present in 50029 so we could use this as a template to make one for 50030, if we feel we need to.

Things Still To Do

We still have to tackle the difficult, and probably expensive, issue of the severed traction and ETH cabling under the cubicle.  I have been advised not to through-crimp traction cabling, so it will need replacing with new cable at some point.  We may have to use the services of a specialist contractor for their replacement.

The brake frame needs rewiring once the exhauster underneath it has been changed for an overhauled one.  The Driver’s Safety Device (DSD) relay box needs new cabling laying in as the original cables have long gone, likewise for the DSD speed switch in the cupboard underneath the hotplate in No.2 cab.  The comprises several pieces of equipment, including relays plus hand and foot operated switches, which automatically shuts off power and initiates a full brake application after 5-7 seconds if the driver becomes incapacitated and removes his foot from the pedal in the cab.

All the cab desks need the cable crimps cleaning, the lampholders behind where the instrument panels go will need either cleaning or replacing, we still have to source or make the switch panels that go in the ceiling above the driver, and the master controllers need freeing off much more than they are now.  The traction motors will need removing for overhaul, and the traction cable connection boxes under the locomotive will need refurbishing.  The overhauled exhausters and compressors will need reconnecting once fitted.

The engine governor and the load limiting potentiometer, or vane motor, need overhauling.  The electrical rebuild of 50030 has been challenging yet very satisfying.  Anyone fancy helping with 50029?