A Petrol Electric Loco using Brushless Motors in 71/4" Gauge

Update November 2017 - this loco has run several times this year, all the electronics and mechanics has been found fit for purpose, yet to do - make a body shell and complete alternative 'Radio Control' drive interface

It is winter 2016. The aim is to have this loco running by Easter 2017. When the weather is warm enough progress is made on the mechanics in the workshop. On colder days work continues indoors, on the electronics.

The loco will be of modular, assemble-on-site, construction. The idea is that one person can load all the parts into a typical family car - no van, no trailer, no special handling kit needed.

Progress so far : -

Honda petrol engine mounted with a 1.8kW three-phase brushless motor used as a generator - this forms the easily removable power unit.

This has been run-up driving some incandescent light bulbs wired to two phases, this proved the basic principle.

Honda GX120 Petrol Engine ...

... Drives 1.8kW BLDC Motor as Generator

The brushless motor/generator was chosen over vehicle type alternators to maximise efficiency.

Vehicle alternators, like all mass-market and domestic products, are designed "down to a price" not "up to a standard", and may have a dismal efficiency of around 50%. Using one of these wastes half the power output from the engine before you start!

The brushless motor used as a generator should ahcieve efficiencies over 90%. The point is not so much to 'Save the Planet' (although someone ought to!), as to avoid the need for bulky fans and heatsinks and means to dissipate wasted energy. Another objective is to get as much power delivered to the wheels as needed while little more than tickling the engine!

Yet to do to complete the power unit - the rectifiers to turn 3 phase a.c. into d.c. Again looking to minimise wasted heat, synchronous rectifiers will be used (see here for details).

Bogie with 660 Watt BLDC motor per axle

All steel gear drive (1.5 MOD)

Bogies completed October 2016

Frame length 1440mm. A bit short, but then it has to fit in the car with all the rest.

But what of batteries?

There are plenty of locomotive designs that use some generator with batteries - same idea as hybrid cars. This is justified where batteries alone would unduly limit range, or where the generator has insufficient power for all loads and conditions.

The petrol generator shown here is easily removable - the whole point of modular design is to allow for quick and easy assembly / disassembly for one-man handling and ease of transport.

A pair of 12V 50AH gel batteries (as used in mobility scooters) could be dropped into the well in place of the petrol gen, and there we have a battery-electric locomotive !

There's plenty of space to fit up to another six such batteries, 3 in front of and 3 behind the centre well, however the wheelbarrow full of cash to pay for these is absent.

Another option - batteries could be fitted in front of and behind the petrol generator.

Flexibility is part of the design philosophy, multiple alternative power sources, wider range of possible uses.

Bogies, Chassis, Power Unit ...
... Modules Coming Together Xmas 2016


Electric motors are also usable as brakes. Using a system of 'Regenerative Braking', the motors can also behave as generators, but where is this regenerated power to go?

In a battery-electric design, some of this power can be put back into the batteries - very good, but this is not effective with fully charged, or near fully charged, batteries. Without anywhere else to 'dump' any unusable regenerated power, the system voltage can easily rise beyond safe limits. Any robust electric locomotive design will therefore, include some over-voltage sensing, and voltage limiting, power dump.

Design calculations indicate the petrol gen, electronics and brushless motors alone will give more than enough poke for useful duty hauling passengers around a club track, and a full tank of petrol should last a long afternoon or thereabouts.

Calculations also indicate the torque deliverable to the wheels is well above that likely to induce wheel-slip. Should this prove a problem, batteries may be added - as ballast !

A spreadsheet of design calculations can be seen here As you can see, the loco should be able to accelerate a load of 2 tons up a 2% gradient. Please download and play with loads and gradients etc.

Toothed Belt Drive

Computer Control Panel coming on


Video shows one bricked-up bogie on the test bench.

The bench power supply is only capable of delivering 5 Amp at 31 Volts. In regular use, the bogie may be expected to draw up to about 35 Amp from 48 Volts. Note when the hand appears and slows the axle, this is because the bench supply collapses under the load !

The slider control seen to the right side of the touch-screen display will be the normal drivers control. Moving a finger over the slider towards the top and the loco will move. Taking a finger off the slider causes the control to drift back to the 'neutral' position and the loco will glide gracefully to a halt. This meets the 'Dead Mans' function required of electric locomotives on club tracks. For purposes of this test, the buttons 'hidden' behind the voltmeter and ammeter are used to increase / decrease speed.

Below, a very simple power dump for use with regenerative braking.

When the applied voltage is below the zener diode voltage, no voltage is developed across R1, no voltage is applied to the mosfet gates so nothing conducts. As applied voltage rises, the zener passes current developing a voltage across R1 which is applied to the mosfet gates. Once this voltage exceeds the gate threshold voltage, current starts flowing through the mosfets. Resistors R2, R3 etc tend to equalise current through each, and also tailor a not too vicious response to rising voltage.

Any number of mosfets with equalising resistors may be connected in parallel, for this loco design there are 20, each capable of dumping around 50 Watt. (I had considered designs using high power wire-wound resistors, mosfets are much less expensive !).

Voltage Limiter / Power Dump