When Renown Repulse Restoration Group (RRRG) acquired 50029 Renown and 50030 Repulse the electronic control systems had been plundered for spares. We are fortunate to have on the project a number of skilled locomotive electronics experts. Let’s look at an overview of class 50 electronics, how they operate to control the locomotive and how RRRG worked to reinstate them.
The state of play in 2002
Our pre-tender engineering assessment of 50029 and 50030 had observed that, perhaps unsurprisingly, after some ten years of open storage much work was required to reinstate various missing items. The control system of a class 50 is quite complicated, partly because as we saw in the first article in this series, BR ordered the ‘top spec’ model with lots of electronic toys.
Matters are not helped by the fact that, of all railway equipment, electronics is probably the area where technology has advanced the most since the class 50s were designed in the mid-1960s. In the last article we looked at a few of the electrical machines in a class 50 locomotive but these are the output end of a complex control system. Much of the electronic control system of a class 50 is concentrated in a single electrical cubicle positioned towards one end of the locomotive. This cubicle contains relays, contactors and resistors in addition to the major control units that operate the locomotive.
Class 50 electronic control units: CU1
The class 50 electronic control system was very sophisticated by mid-1960s standards. It centres around a number of control units, designated CUxx.
CU1 is effectively the central brain of the system. It receives signals from other control circuits in the locomotive, namely from the cab controls, engine load condition and traction currents. It then passes these to the CU3. The CU1 can be thought of as an analogue computer and this explains some of the difficulties of rebuilding the electronic systems since nobody produces analogue computers anymore! There is one CU1 per locomotive and we have two, plus parts of others. Although they are untested we have contacts who can perform this service.
CU2: controlling the back voltage
CU2 is the field divert unit. Field diversion in a diesel-electric locomotive is analogous to changing gears in a car. As the train speed increases, a back voltage develops owing to the internal resistance of the traction motors and this prevents any increase in electrical current to the motors and thus any further acceleration. The CU2 controls three field weakening contactors to insert or remove resistances into the main generator circuit, overcoming the back voltage. Field weakening in a class 50 occurs at roughly 25, 40 and 60mph although the exact speeds depend on the gradient, the weight of the train being hauled and how aggressively the locomotive is being driven. One per locomotive, we had none but exceptionally generous financial support from a member which allowed us to commission replica units using a loaned original as a template.
CU3: controlling the field current
CU3 controls the field current from the main generator as a summation of the signals from the CU1. Commonly known by its English Electric part number KV10, this forms part of the ‘drive by wire’ system of a class 50 whereby the driver’s power controller instructs the CU3 to set the current output from the main generator rather than directly controlling engine fuel or other parameters. The CU3 is essentially the same as the CU6 but has different controller cards, respectively CU3A and CU6A, and determines if a KV10 acts as a CU3 or CU6. The original EE spec KV10 has an interesting feature known as ‘KV10 lock-on’ whereby the unit malfunctions and commands maximum traction current from the main generator even with the cab power controller at its minimum position. Each locomotive has one CU3 and one CU6 KV10 with associated input cards. We have managed to acquire a number of KV10s and control cards over the years. We became aware of an engineer offering a rebuild and modernisation service (several of the key components in a KV10 are obsolete) and we provided several donor units to allow modernised units to be built.
CU4 and CU5: cooling and regulating temperature
CU4 and CU5 are two units working closely together. The CU4 (one per locomotive) is a coolant control unit which processes signals from two temperature probes and passes them to the CU5. It also controls various actuators to route the flow of cooling water. We have none, but fellow preservationists the Class 50 Alliance have produced a replacement design based on modern industrial and marine technology which we will have the opportunity to purchase. The original EE version uses obsolete components and together with the original CU5 is known to over-cool the engine in some circumstances. The redesigned version addresses both these problems. The CU5 in each locomotive controls the radiator fan speed and again we had none, but as well as the Class 50 Alliance looking to produce a modern version, another friend of RRRG with electronics experience manufactured a modernised replacement for us.
CU6 and CU 7: heat generator control
CU6 works in the same way as the CU3 but controls the electric train heat (ETH) generator and adjusts its output according to varying load demands. These variations include, for example, the number of coaches requiring electrical power and their types – air conditioned coaches draw more power than those with simple resistance electric heaters and catering vehicles require even more electrical power.
CU7: This is a simple circuit which detects a voltage from two or more class 50s being driven together at the head of one train, and prevents doubling of ETH power from them. It also acts to terminate the ETH supply in the event of a fault such as a short circuit in one of the passenger coaches. There is one per locomotive and RRRG’s main electrical guru reverse-engineered the original EE design to construct a replica.
CU9 and the rest
CU9 (CU8 was never used) is an automatic voltage regulator. There are two types – CU9A is electronic, CU9B is carbon pile and they are both compatible. There is one per locomotive with most class 50s in BR service having CU9Bs which are slower acting but more reliable. We have three (untested) CU9B units.
CU10 is a complex device that receives signals from axle-driven transmitters and sends signals to the cab speedometers. It can be adjusted to compensate for wheel tyre wear and it accepts up to six control cards but not all of these are used on a class 50. We have two units plus a number of control cards, with the Class 50 Alliance also looking at building modern versions of this units and its control cards.
CU11 was the slow speed control unit that was removed at refurbishment so is no longer applicable.
CU12 is a lighting delay timer which turns off unnecessary lighting 15 minutes after engine shutdown, if interior and other lights are left on and the battery isolation switch left in the ‘on’ position. The CU12 is still present on 50029 and could be used as a template for 50030 if necessary but it isn’t really needed for heritage railway operation of class 50s. The CU12 was removed from many class 50s in their later BR careers as it was unreliable. We decided that after over 30 years of disuse the one we have probably wasn’t going to be any better so we decided not to bother overhauling it and disconnected the wiring for it in the electrical cubicle.
All the above gives an overview of the control units that are needed to make a class 50 operate and how we approached the task. There was much more work required to complete the electrical cubicle as we shall see next time.
