ElectronicExposure
Locomotive Controllers -- The Technical

The technical exposure of Locomotive Electronic Speed Controllers

The electronic locomotive speed controller is now well established within the model railway arena.
It is an area of great confusion and even greater technical deception.
The electronics for commercially available model railway locomotive controllers is at a very low level of electronic technology.
There are plenty of British manufactures of electronic controllers most of them are using totally obsolete designs and technology.

To give some good examples without naming brands :-

Thermal mechanical cut outs for short circuit protection.
This technology was around  ninety years ago -- it is TOTALLY unsuitable for anything electronic. It is very slow, unreliable, will cause damage within an electronic circuit -- due to the basic  operation concept -- a wire coil heats up due to electric current flow --  at some point this heat causes a bi metal strip to bend and break a electrical contact. Hence --  extremely slow, insensitive and crude.
The only place these devices find a correct home is replacing a wire fuse.
They are not designed for any other application.
In a locomotive controller they would be suitable to protecting the secondary winding of a transformer -- the auxiliary AC output being a good example. But nothing else.

Most locomotive electronic controllers use power darlington transistors to control the current flow in the locomotive electric motor.
This has been totally obsolete for around twenty five years in the electronics industry -- for any form of electric motor control.
There are about thirty electrical parameters - the key ones being -- high voltage drop across the two transistors ( darlington pair ) in the order of 2 to 3 volts at 1 amp this is 3 watts power loss within the device. They are considered as medium output impedance devices -- this limits the  efficiency and degree of current /voltage control as a power source for a electric motor. The control voltage/current output ratio is not very good -- limits the ability of good slow speed locomotive control.
A good linear motor controller would require at least 20 to 30 transistors to provide good motor control -- most commercial controllers have less than 4 to 6 transistors.  Darlington's and their simple transistor circuits by today's electronic standards are very obsolete and unsuitable for good performance electric motor control.
They have been replaced with at least ten more, better  suited advanced electronic technology components over recent years.
                                                                                                                                 

There is a tendency to overstate the power requirements of electric motors within model railway locomotives.
A typical Z to 00 gauge locomotive has the following voltage and current parameters:-
At 0 to approx. 4 volts -- nothing happens -- no motor rotation.
This is due to the fact that a motor requires current, due to the resistance of the windings of the motor it requires 4 volts to force sufficient current through the motor to produce any torque.
Also the mechanical load and stiction of gears have to be over come by the electric motor.
Typical modern locomotives, and four 1950's vintage Hornby 00 locomotive I tested run well in the 250mA (0.25 Amps) to 400 mA ( 0.4 Amps) range. -- this only increases to about 600 mA ( 0.6 Amps) on full load and at mechanical stall.
Generally this was similar in both Z and N gauges.
On the larger gauges an increase of about 20% on these figures should be expected.
On a integrated power electronic system (none darlington power stage) in
development with a 1 amp output current limit -- it ran five locomotives simultaneously with ease.
A electric motor is reasonable efficient and has considerable reduction gearing to its load.
If a locomotive requires more current than these figures -- it is in need of repair.
The motor/running gear/gears -- are shot.

From  extensive evaluations and experimentation, of typical commercially available simple (voltage feed back) electronics of this type of common feed back controller.
It was found from analysis of the performance of dynamic measurements of electrical  characteristics  --  no measurable advantages were found  over conventional linear voltage controllers.

When locomotives in Z, N, OO, and O gauges in good running order are used as a test benchmark.