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Electric Propulsion

An Introduction to Electric Propulsion

Electric propulsion has the beauty of being instantly available for running, unlike steam that requires maybe half an hour of loco preparation, not to mention their general oiliness and the necessity for a gentle cooling down after running. It’s rather akin to the huge benefits of availability and running cost reduction realised by British Rail when diesel replaced steam. Back to our models, there are more advantages still! One is that both “Steam Outline” and “Traction” can be electrically powered and furthermore it’s easy to have a “seat of the pants” driving experience by using onboard power with radio control. This article assumes that most will adopt this very convenient and readily achievable mode of driving, rather than using track power with the associated cleaning difficulties on an outdoor line.

Refinements can be added; such as a sound system dedicated to the noises made by the prototype and for the really dedicated it is even possible to produce smoke or steam. Such refinements add significantly to the cost, (but as a generalisation), an electrically powered loco will be cheaper to buy or make than the equivalent with steam power.

Radio control units are discussed elsewhere, but can often be bought as a ‘plug and play’ assembly; complete with pre-wired switch, receiver, charging plug and fuse. The only wiring required is to the Electronic Speed Controller (ESC) and the drive motor.

Some of the basics for electrically powering your model are discussed in the sections below.

“Steam Outline”

Most models can be electrically powered in Gauge ‘3’. Even the smallest tank engine can have on-board power and radio control. Typically for the most challenging, like an 0-4-0 tank, the motor will be in the firebox driving the rear axle. There will be a small battery pack in the boiler or tank space with the receiver and ESC in the bunker (pic). Because of the restrictions on physical size for these items, the battery pack may only be 6 Volts. Thus giving only a modest performance, but of course that may be commensurate with the real thing. Such a model can still give hours of pleasure working a fiddle yard or operating a country branch line service with one or two coaches.

Larger tank engines or those with tenders, give far more scope for battery packs of higher voltage for extra equipment such as a sound system and speaker. Whilst a tender may be an attractive place for a large battery pack, optimally batteries should be positioned above the driven axles with their weight helpfully adding to tractive effort. With regard to tenders, it is not unknown for power units and batteries to be contained within and power the tender, with the loco ‘freewheeling’ in front. Of the common reasons for doing this, one is that steam powered locos can be readily converted to electric. Another that tender axles often have external frames this giving more room between the wheels for motors and gearboxes. One such is photographed below (Pic).

Battery packs for these restricted spaces are normally made up to the shape required by specialist suppliers, (eg Strikalite). These use NiMH type AA cells with solder tags and a shrunk sleeve to hold the pack together. Typical voltages used are 9.6V and 14.4V. ”C” type cells should be specified in general to give a longer running period. A charger specifically for that pack should also be sourced, preferably from the same supplier. Alternative battery types of high power-density; such as LiPo or Lion cells, are not recommended because of their reduced tolerance to miss-handling and the higher risk of spontaneous explosion.

For motors and the gearbox, those on a tight budget and with engineering skills might press surplus motors, with other gears, worms, pulleys and belts into service. But most will adopt a commercial and self-contained motor-gearbox unit supplied for the purpose. Before choosing a gearbox, some consideration should be given to your expectations for loco performance. Whilst a worm drive is very suitable for slow moving and low powered applications like shunters, the generally more expensive spur gear type will have greater efficiency and be better suited for faster running. Gear ratio is another variable, the optimum will depend on; the required loco speed, the motor torque, rotational speed, voltage of the power pack and the wheel diameter. Advice should be sought from the supplier, but those going their own way can consult the articles in the “Gauge ‘3’ Wiki” (link needed).

Commercial supplies of the narrow motor-gearbox units needed for inside framed steam locos could be the worm drive unit from Walsall Model Industries (pic), or the spur and skew gear unit marketed by Slater’s Plastikard (pic). For tender drives the scope can often be widened to include bogie motors sold for “Traction”.

“Traction” Electric, Diesel and “Other” powered Locos

These are eminently suitable for electric propulsion, most having cavernous bodies for batteries, and featuring bogies with external frames. Electric locos have been modelled with either dummy pantographs, or those collecting power from the overhead line or third rail sliding “shoe” contacts. Whilst much of the aforementioned for steam outline is relevant to “Traction”, some specific differences are discussed below.

Batteries are no longer limited to compact shrink wrapped profiles for insertion into small spaces. They can instead be large heavy Sealed Lead Acid (SLA) units, with ratings of; 6V, 12V, 18V or 24V, being readily achievable. As mentioned earlier their weight can be an advantage for increasing tractive effort. SLA cells are cost effective and can be sourced as high capacity units of 4.5 Amp-hour or greater. It is worth noting that whilst SLAs can be positioned in any orientation during service, unlike NiMH they must be upright during charging to vent gas. The Lead plates are at the base of the SLA.

Unlike a Steam Outline loco where all driving wheels are externally coupled, a Traction prototype most often uses a separate motor on each driven axle. For good tractive effort in the model a similar approach is often needed requiring either; multiple motor gearbox units in each bogie or a geared arrangement whereby one motor drives two axles. An example of the former is the enclosed motor gearbox unit supplied by Apple Tree Railway (pic), or in the latter there are the bogie motor ‘blocks’ supplied by USA Trains. These will require re-gauging from 45mm (pic). A twin-axle (B0 or B) bogie will operate best with both axles driven, and for a three-axle (C0 or C) bogie it will often be sufficient for the first and third axles to be driven. The remaining bogie on the loco could often remain unpowered, but in such cases the weight of batteries should be biased towards the driven bogie for best traction. There is nothing to prevent one driving all powered axles of course, an example of a BR Class 44 ‘Peak’ bogie with three axles independently driven by bogie motors is pictured below (pic). Advice should be sought from the supplier as to; how many drive units are required for your particular weight of train, the wheel diameter and the expected speeds. Some diesels have used inside frames, examples being the Class 08 shunter and the Class 52 ‘Westerns’. These invoke special needs for narrow motor-gearbox units, much as for Steam Outline.

Conclusion

An RC Battery loco has many advantages for the Gauge ‘3’ modeller; those of instant availability, the cleanliness and no requirement for boiler certification. Portability is normally good, with as much detail and realistic sounds that can be added to suit ones desires and pocket. The choice is yours!


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Page last modified on September 10, 2020, at 09:27 AM