VAMBAC behaviour

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d9016

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Can anyone confirm how VAMBAC units operated in relation to the in cab control lever? Was there an increase, hold, and decrease position similar to early AC electric locomotives or was the only way to decrease the power level to move the handle to the off/coast position then apply power again like with the English Electric 4 series/4 parallel controllers on Railcoaches, Baloons and their derivatives?
 
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Fireless

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I don't know about the particularities of the VAMBAC equipment so I'm going from the assumption that it's just the british version of the accelerator used in the PCC cars.

The very basic idea is that the driver selects how fast he wants to accelerate or brake and the system does the rest similar to today's trams with but with 1930s electromechanics instead of solid-state technology.
Putting the lever into acceleration position will make always make the control gear notch up until it reaches the "final" (full power on the PCC, might be different with VAMBAC) notch with the only difference being the speed it does it at.
Putting it into the braking position will notch up the brake points in a similar fashion.
Putting it in neutral will let the control gear run back towards "neutral" (the PCC will notch up the braking points until it senses current on the dynamic brakes which makes its response to braking faster but also means that it can't actually coast, which might be different with VAMBAC).

Whilst there is no similar video of VAMBAC equipment in action (anyone with access to a suitable tram take note), there are quite a few videos of showing the accelerator of PCC cars in action online.
This particular ones shows the driver's eye view, driver's feet view from the right (front pedal accelerator, middle brake, rear deadman) and the accelerator (with a convenient current meter for the traction motors) all in sync on an american heritage tramway and perfectly illustrates the operation of such control gear.
 

d9016

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That looks very similar, I'm starting to notice a lot of similarities between PCC's and Coronation cars the more I look into them. If you give a PCC car full power to accelerate to line speed, then let the pedal back it to half way. Does it return to half power like a thyristor or transistor based controller or do you need to let off completely to release it then press it in half way in the same way you would on an EMU with camshaft control equipment?
 

Taunton

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I understood that the Vambac was developed after much study of the PCC, and was engineered as close to that as could be done without infringing the patents of the Transit Research Corporation, who had developed the PCC (it was owned by the major US transit operators). Tatra in Czechoslovakia also took the PCC concept more closely for several generations, sold across socialist Eastern Europe; at the time they were thus beyond the reach of any legal action from the USA.
 

Fireless

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If you give a PCC car full power to accelerate to line speed, then let the pedal back it to half way. Does it return to half power like a thyristor or transistor based controller or do you need to let off completely to release it then press it in half way in the same way you would on an EMU with camshaft control equipment?
Neither.
The accelerator will notch up until it has reached full power when the pedal is pressed long enough. Pressing the pedal only halfway will let it notch up (i.e. accelerate slower) but it will still notch up until full power.
Completely releasing the pedal will only change the position from where it will start notching up again to full power.
There is no way to drive it at half power in the traditional sense (as in "top series" on a tradtional tram controller) as the controls were dumbed down to "press accelerator pedal to accelerate, the further the stronger", "press brake pedal to brake, the further the stronger" and "put your feet up to let it roll".
It is possible to drive it at roughly half power by repeatedly pressing and letting go of the accelerator pedal with the right timing though.


Tatra in Czechoslovakia also took the PCC concept more closely for several generations, sold across socialist Eastern Europe; at the time they were thus beyond the reach of any legal action from the USA.
Tatra (Ringhoffer before nationalsation in 1946, restructured into Vagonka Tatra Smíchov in 1950 and into ČKD Tatra in 1963) didn't have to change that much as they actually became a licensee for PCC cars in 1948, just like the belgian La Brugeoise (later BN) on the other side of the iron curtain.
They cancelled the colaboration with the USA for political reasons the same year which delayed production for a couple of years when compared to the belgian counterpart though.
 

Lemmy282

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I've only driven one VAMBAC tram for a short distance and quite sometime ago, it was Leeds 602, but from what I remember the lever was pushed forward through a shunting notch where the speed was limited to about walking pace, then as said before further forward increased the speed of acceleration. Putting it back to the neutral position seemed to maintain that speed, and pulling back started the braking effect.
The system was not reliable though, the Blackpool Coronations would run beautifully for ages then refuse to stop! Waving the lever back and forward would usually solve that!
The actual equipment in 602 was above the centre entrance, and sounded like one of those old knitting machines when in operation.

Nigel L
 

d9016

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I don't mean from half throttle to neutral. I meant if the power is on full but you start easing off but not all the way off, does the controller start reducing the power output or do you need to set it to coast before pushing it forward again to the reduced level?
 

Lemmy282

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I never got it to full power so didn't have that problem. As the controller varied the rate of acceleration, not speed, I would assume that you would have to go back to neutral and coast until you achieved the speed you required.
Normal operation of a standard controller operates this way, you never go from a high power notch to a lower one, known as 'back notching' as it can damage the controller.

Nigel L
 

D365

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Please can we have the abbreviation defined?
 

Gag Halfrunt

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Variable Automatic Multinotch Braking and Acceleration Control
 

d9016

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I never got it to full power so didn't have that problem. As the controller varied the rate of acceleration, not speed, I would assume that you would have to go back to neutral and coast until you achieved the speed you required.
Normal operation of a standard controller operates this way, you never go from a high power notch to a lower one, known as 'back notching' as it can damage the controller.

Nigel L
In what way would it cause damage? I know it is standard practice to go straight to off but I don't know why. Would it be due to a resulting voltage generated by the motors coasting going back in to and overheating the control resistors rather than the braking resistors which were designed for that purpose?
 

Lemmy282

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Yes, when you back notch you introduce more resistance into the circuit, the motors act as generators and you end up with high voltages going back through both the resistor grid and the controller, potentially burning the finger contacts in the controller, and the grid.
 

pieguyrob

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I did my work experience at Blackpool transport tramshed in the mid 90's. The older trams had a huge resistor bank below their controller. Which was basically a system of different cuts of metal that rotated in some other metal, that offfered the resistance.

Variable, Automatic, Multinotch, Braking, and, Acceleration Control.

VAMBAC.
 
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Lemmy282

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The resistor banks don't have any moving parts, and are usually underneath the tram, or under the stairs. I think the bits you are referring to are the moving contacts in the controller which the driver operates. Basically the power handle is on a long spindle which has many copper 'fingers' arranged to make contact with fixed contacts in the controller box. Each power notch joins the appropriate contacts together, moving to the next notch breaks some contacts and makes others. All these are connected by a mass of cables to the resistance box and thence to the motors. It is up to the driver to use the controller to manage the electrical power used as efficiently and smoothly as possible. The VAMBAC system was to try and automatically use the power in the most efficient way, but it wasn't a success.

Nigel L
 
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