Electrical checks and service on a VAMBAC tram system

One of the more fascinating jobs recently - checking through & adjusting the electrical controls on a 1952 Blackpool tram!

This is a very unusual vehicle with what is now a unique and very sophisticated control system: "Variable Automatic Multinotch Braking and Acceleration Control", aka VAMBAC, the trams built by Charles Roberts & Co in 1952 and the VAMBAC built by Crompton Parkinson and Allen West.

The VAMBAC system in this tram was displayed as working exhibit at the London south bank "Festival of Britain in" 1951, before being incorporated in the tram body.

A typical tram from this era controller has around six steps from zero to full power, directly switched by the drivers control lever. In contrast the VAMBAC motorised rotary switch unit has around 40 steps, so much smoother acceleration and control.

In addition, it is not directly controlled from just the drivers accelerator lever - it is a feedback system that compares the power requested from the drivers control unit to the motor current and continuously adjusts the motorised switch to achieve the desired power.

All the continuously changing speed switch contacts are moved magnetically, both in the motorised switch and in the drivers controllers!

Overall a superbly designed and very well built system - but unfortunately not well understood at the time, and during the 1950s the trams using these has multiple problems, largely due to ill conceived modifications such as removing the 24V battery system that ran the low voltage side of the control system to save weight - meaning the controls reset and shut down at each overhead line section break, shutting off acceleration - or braking! - until the driver re-engaged the system.

Due to problems such as that, virtually all the VAMBAC systems were removed and traditional controllers fitted instead. This is now the only surviving VAMBAC tram at Blackpool, and the owners are hoping to get it back in service before too long.

Links with more information on this tram and the original Coronation fleet:

00:00:00 VAMBAC operating
00:00:09 The commemorative plaque
00:00:14 The VAMBAC main unit
00:02:54 A drivers controller, which runs the VAMBAC motor

[tekszgd]

The VAMBAC system is actually quite simple, firstly forget about the earlier resistance and controller setups. I have spent many hour working on this system, I still find it fascinating. The reliability is dubious, namely down to vast amount of moving contacts and auxiliary ones on the main contractors. Very good video and explained nicely.

[Thunderace]

Thanks for posting this - a very practical & concise explanation of how the system operates, & interesting to see the system as a whole rather than just the VAMBAC unit in the roof chamber. Your explanation of the current measurement system within each of the drivers controllers I can now tie with the explanation John Markham from Crich gave to me some years ago about the measurement of the accelerating current being matched to the drivers controller position. His one criticism of the VAMBAC system was the use of "moving iron" measurement as opposed to "moving coil" system used in PCC cars, because of patent rights. It would be interesting to see a comparable video explaining PCC technology in operation. Thanks again - very interesting.

[TheHoveHeretic]

Superb explanation, thanks. Great design on paper, but I've a much clearer idea of why the Balloons survived to have some class members rebuilt for full, commercial 21st century operation, whilst the Spivs are little more than an interesting historic digression.

I'd imagine that, in our solid state age, there are entire tramway systems operating with less moving mechanical parts than contained in a single VAMBAC installation.

[malcolmrichards]

I love electro-mechanical gear like this. Much more interesting than solid-state electronic control. I can't help thinking, though, that the extra smoothness compared to the the traditional drum controller didn't warrant the complication. It must have been a nightmare to maintain in service.

[kramreprah]

Great control system for its day and it beets me why they had to tamper with its operation regarding the 24v system. This explains ( I have read it somewhere) why the drivers had said the braking used to occasionally cut out and was dangerous. Similar issues' with the illuminated trams that are on LED. I was speaking with a BPT operator and he was explaining issues regarding the Trawler tram and its battery's going flat and loosing the programs when it was parked up and the overhead line isolated. I could not understand why they did not have a charging circuit in place in the shed that could be used to float the batteries. This VAMBAC system would have been great if a little more education was given when installed. I take it it is 550vdc split in series over two motors to keep the current down, I am looking forward to seeing this tram back on the line shortly. Was you the guy who had to repair it after it had been on fire? I would imagine those resistors would get pretty hot if running at low speed for a long time. Ps great video and very educational.

[malcolmrichards]

Does the Vambac allow series and parallel switching, like a drum controller, or is it all series?