DCC Loco Conversions - 6 - Worth the Effort? – Part 4 of 5

14) Motors

Motors fall generally into two groups, discrete and integrated.

Most motors in pre-DCC locos typically have a mechanical (i.e. metal to metal) power connection through the chassis from one rail and an insulated path via wire from the other rail. This is typical of most Tri-ang and all Dublo motors for 2 rail operation, (including integrated) as well as Hornby tender drives, which have a variety of implementations of the mechanical connection to catch out the unwary.

14.1) Discrete

A discrete motor can be removed from the chassis as a totally separate item that may be powered away from the chassis, e.g. the Tri-ang X04, the Dublo Ringfield motors fitted to the larger locos and the so called 'Scalextric' type fitted to some Bachmann locos.

Bachmann split chassis designs often have a discrete motor but the motor terminals are in mechanical contact, not soldered, with the two chassis halves. They must first be isolated from the chassis and then have wires from the decoder routed to them in a tight space. Care is required in routing and insulation or smoke will be released from an unhappy decoder.

An old discrete motor may have one brush holder in permanent connection to its frame and therefore to the chassis. This I have found in a couple of second hand kit locos that I have bought, in that case the motor frame itself will require modifying.

14.2) Integrated

An integrated motor has bearings of at least one end fitted directly to the chassis and only one brush holder is insulated from the chassis. Examples are Tri-ang power bogies, early Dublo locos, and many tender drives,

A tender drive exception to this is a British Trix A3 loco which has a discrete motor mounted in the tender.

It must be remembered at all times that the motor brushes are connected to the output of the decoder by orange and grey wires and they must not make any electrical contact with the track supply normally connected to the decoder by the red and black wires.

Only the more expensive decoders have some protection against this type of mis-connection, otherwise instant destruction of the decoder will result on the main if track wires cross with motor wires. No second chances, puff of smoke, some melted decoder wrapping and £10 - £20 for a non-sound decoder gone just like that and of course much worse for a decoder with sound capabilities.

Converting Hornby Dublo integrated motor chassis can be the most demanding, not to be tried on your mint in box rarity unless you have successfully practised on others.

The most subtle Hornby tender drive nasty is the cover of their pancake motor which holds the motor brushes in place. It is made of plastic but the screws holding the brush spring retaining plates to the plastic face may be of unequal lengths such that one connects the brush plate to the diecast chassis, which in turn is in metal to metal contact with one rail via an axle.

Other variants appear to rely only on individual wires to both brushes, it is imperative to check that both motor bushes are isolated from both rails, I can't emphasise this too often.

I will give details when I write about specific conversions.

14.3) Performance

This is a wide subject but the basics are that the motor must be in good order.

First check its current consumption running light and it runs as fast as it should for a given voltage, this is more of a 'does it feel right' approach. The following needs consideration if it does not feel right.

If it has heavy brush carbon deposits on the commutator it will take excessive current as the carbon dust short circuits the commutator segments. Any oil contamination must also be removed, the nature of the oil may cause partial insulation or shorting depending on its chemical composition. A clean up with a switch cleaning spray chemical like Servisol normally fixes that.

Weak magnets due to instability of early magnet materials whilst ageing or improper dismantling can result in field strength collapse which will increase current and reduce motor torque, (the ability to revolve against a load).

Re-magnetising is required, if that cures the problem fine, otherwise if the motor current is still too high then the armature windings may have overheated and lost insulation by burning off between adjacent windings. This results in short circuits between the affected wires, less armature magnet field strength, greater current flow, more heat, even more insulation is burnt off. In which case an armature rewind is required. I also consider magnets in the next section, 14.2.

All being well with the motor the second step I prefer is to hold a coupling and turn up the power until the loco starts to slip and note its current consumption.

Some say that you should test for maximum current by holding the loco such that the motor cannot turn and turn the controller fully up. Personally I think that is a brutal overkill, for a loco to stall completely as opposed to wheel slip which is visibly noticeable a complete stall is unlikely unless it has a serious mechanical failure.

This could range from valve gear lock due to a screw lost from valve gear, lock due to damage or quartering slippage, a split axle tube gear as described earlier jamming the gear train. But these are rare events and too big a decoder is expensive and potentially difficult to fit. I often fit N gauge loco decoders to OO gauge locos without issue.

Higher end decoders will have protection for over current, but do you want the expense?

Motors will make or break your efforts, so do ensure that the decoder you intend to fit is capable of delivering the current your loco requires at a maximum of 70% of its rated maximum. This is typical percentage for electrical power supplies and motors if you want a decent operating life span.

Remember that lights and other accessories powered from the decoder will also eat into the power rating budget for the decoder.

It is always preferable, but generally not essential that there is space around the decoder for ventilation unless it is going to be run at near rated limits in an overheated loft environment.


Next, I continue assessing motors (14.2) Magnet and then summing up.

Jim

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Image: Airfix (GMR) Brush Type 2 fitted with a Lenz std+ 10231-01 decoder.