I've seen that one in the flesh and it works very well and looks right on the small 2" Gold Medal Tractor.

I presume that what is meant is a model aircraft motor. The usual type has a permanent magnet rotor and a three phase stator arrangement. If the rotor is powered they will generate three phase power which may be rectified and used to charge a battery or power a load. This makes them quite similar to the permanent magnet alternators that used to be common on motorcycles.

The characteristics of a permanent magnet machine like this is that the voltage will rise with speed. The bikes used to get around this by connecting the rectified output to a battery, and by winding the machine with two sets of coils. The battery limited the voltage that could be reached so that the alternator was actually delivering its maximum current. The two windings were then arranged so that one provided enough current for the normal load, and the other provided enough for the lighting load. This was all a bit approximate and these simple arrangements had a tendency to cook their batteries.

In the late sixties and seventies, electronics started to provide better solutions. Lucas used a zener diode and others used various forms of shunt regulator. This is the way I would go if I wanted to use one of these on a traction engine. The alternator has to be turned fast enough to generate a bit more than the voltage required, and the output is rectified with a three phase rectifier. Either a battery or a large capacitor is used to smooth the output, and a shunt regulator (eg the Lucas Zener) is used to dump any excess current.

The model aircraft motors are usually outrunners, eg the rotating armature with the magnets is on the outside. For generator use it might be more convenient to find an inrunner of the desired size. These have the armature inside, more like a conventional motor. Of course for traction engine use it might be desirable to hide the whole thing inside a scale looking casing anyway. The beauty of these motors for this purpose is that there are no brushes to wear out.

John

There is a reason why the motorcycles mostly used a simple shunt regulator. The alternator has to be designed so that it will start charging at some reasonably low rpm. Say 1 quarter of maximum rpm. That means that it will be able to produce 4 times the voltage at maximum, and if a series type of regulator is used, it must cope with that on the input, as must the rectifier. At least one bike I tinkered with was capable of around 80 volts open circuit. In the days of selenium rectifiers it was important not to open circuit the connection to the battery since this could destroy the rectifier. With a shunt regulator and a bridge rectifier nothing sees anything more than the battery voltage. So long as it is connected to the battery then the voltage cannot rise higher, and the alternator will act as a constant current source.

OK, with a showmans engine running its lights, possibly with a working governor, the speed variations are not going to be so great, so a more sophisticated regulator could be used. It will have to be able to cope with variations in the input voltage, as will the rectifier and smoothing capacitor. All quite doable, especially in these days of supplies which will run OK off either 240V or 110 V without adjustment. But I would ask if the relative inefficiency of the regulator is going to matter, when the efficiency of the steam part of the cycle is not likely to exceed 2%. (Going by IMLEC results over the years.)

John

there is one described in the construction articles for the Darjeeling loco based on a hand wound torch i have built on and it works ok

regards

peter

If I was going to do this, I'd use an old-fashioned brushed motor as (1) it would give proper DC – it's a dynamo not an alternator and (2) being able to see the windings and armature through the cage is a big bit of the 'look' of those old dynamos. Efficiency isn't much of an issue if you use LED lights.

Neil

OK, I just dug out a Turnigy G60-500, which is a 500kV motor, and ran it up on my lathe. Unfortunately I don't have a working tachometer at the moment, but working from the pulley sizes it would be doing about 900rpm. It gave 1.2 Volts between phases. Your 1100kv motor could be expected to give about twice that at the same rpm. The higher the kv, the lower the rpm it is going to have to turn at to generate enough voltage. Your 1100 would be a better choice than my 500.

Current will not be a limitation, I tried shorting mine into an ammeter and got 13 Amps. If I wanted to use my one for a generator I would be wanting to rewind it with many more turns of finer wire, to get a higher voltage at lower speed. (The same motor is as mine is available in other kv ratings and that is what the difference is, the lower kv have fewer turns.) This would of course reduce the maximum current, but that is obviously far more than is needed. Your 50 LEDs at 20mA is 1 Amp.

John

Looking at the motor spec sheet, the '500Kv' means 500 rpm per volt. This is when it is running as a motor, so as a generator it will want more speed. The rectifying diodes will drop 1.2v, so I'd expect somewhere around 1000 rpm before it showed any DC voltage, which agrees with John's measurements. It might be worth considering a 'joule thief', but I'll leave it to those more competent in electronics to provide a circuit. Alternatively, and this might be complete rubbish, how about using the LED's as the three phase rectifier. I know each individual LED will only be on for half time, but you can drive them a bit harder because of this, and persistence of vision means they will look a lot more than half brightness.

No, they don't like it up 'em. Max reverse voltage is typically only a few volts and I suspect the reverse recovery time will be very long.

Scrumpy ,

Would the Stuart Models dynamo be suitable , albeit expensive ?

Trevor

I have a number of generators and alternators, this is a 6 pole, shaded pole motor, modified with a permanant magnet claw type armature. The shading rings have been cut, but this makes little, if any difference. It uses 2 Ferrite magnets from old speakers, another source of these magnets is old Micro Wave ovens.

Ian S C

Edited By Ian S C on 11/10/2016 10:06:16