Electric Motors

It is one thing to look at your plans for you latest creation. It is somehow quite another thing to sit down and work out exactly what sort of power you will need to make the wheels go around -and for it to actually pull something along... I am not going to explain the ins and outs of adhesion and limiting friction, these you can look up for yourselves, what I am going to explain to you are the simple methods, (although highly empirical), that I have developed to allow me to size the correct gear ratio and wheel sizes for my models.

Example 1.

In the first case let us assume an 0-6-0 engine with an electric motor -all the wheels provide traction to the rail and thus all the weight of the model is available to pull rolling stock. The question is -how much? Well this relates to how many axles you can pull since it is the friction on the journals that limits the rolling resistance of the axle.

As a guide I have found that: 1Kg of Loco tractive weight to the rails will pull 9Kg of rolling stock

Here I will do my "normal" trick and say that I would like my train to travel at 1mS-1 and accelerate from zero to this speed in 10 seconds...

This means that the rough power required by the motor is: VA=M/10

M=the sum Mass of the loco plus rolling stock in Kilogrammes

V=the Voltage of the supply

A=the Amperage of the supply

I have several electric motors that I can buy if I peruse the MFA catalogue, there are several that are possible candidates. They have the torque shown in Grammes per Centimetre, which is actually a very useful statement as the wheels I normally use are really quite small... I now have to factor in the size of the wheels, this is a freight loco so the wheels will be small and the gearing fairly low. Let us assume that the wheels are 5cm diameter and the loco is not expected to achieve more than 1mS-1. This gives me a wheel rotation of roughly 6.5 revolutions per second and we use a commonly available gearing (worm and spur) to give us a 20:1 reduction, or a shaft speed to the worm gear of 7,800 RPM. My theoretical loco weighs in at 5Kg thus M is 50Kg and VA is thus 5WATTS. But the electric motor is not 100% efficient so, I have to say that this 5Watts is what I need after the power losses. If my motor is a typical 60% efficient then the true size of motor, (or motors), that I require has a rating of 8.3Watts.

So, does it really matter which of them I choose -well actually it matters quite a lot!!! There is the infamous heating effect that means for each ampere you push through the motor the SQUARE of it -is the heating factor...

Thus if I push 1 Ampere through I get a heating factor of 1.

If I push 2 through I get 4

If I push 3 through I get 9...

The situation soon spirals rapidly out of control with an expensive bang! So, the best option is to run at higher voltages rather than higher amperages. There are a lot of simple control circuits that will take 3 amps quite easily -so this could be taken to be a design limit for current. The effective design limit for voltages is 24volts as there are few easily available motors at higher than this voltage rating, however a 12volt motor would be a more practical one in many ways.

Thus far we are looking for a 12V motor of 8.5W with a shaft capability of 7,800RPM -but what about the torque rating? We know our "design" requires it to move 100Kg of Mass...

The equation to tell us this is: F=Ma (Force equals Mass times Acceleration).

Putting the numbers in gives us: F=50Kg x 10cmS-2 which is 500Kgcm

Divide that by the gear ratio (20:1) and we get the seemingly massive figure of: 25Kgcm. BUT this is spread out over 10 seconds so the figure now drops to 2.5Kgcm, or, (more usefully now), stall shaft rating of 2,500 Grammes per Centimetre. The catalogue does not list a single motor with this shaft rating but it does list several motors with half this rating -so we will have to use 2 of them...

Example 2.

The second example is that of a 4-6-4 express loco, (see elsewhere!), the effective tractive weight of the loco is far less then the one in the first example. If we assume that the axles all share the same weight, (i.e. 1/7th of the weight), then only 3/7ths of the weight of the loco is available for traction... This then using my guide means that the 5Kg of loco can only pull 19Kg, (5 x 3.8Kg), of rolling stock, and then have to pull it at a far higher speed than a freight loco.

If I use the same speed 1mS-1 and I double the wheel diameter to 10cm, then the wheel rotates at 3.18 revolutions per second -slightly under half of that used in the first example. However an express train of passengers does not accelerate at the same rate as a freight loco, the load is lighter and it can be usefully higher geared -normally around plus 25% of that of a freight loco seems visually right to me. So, the gearing with twice the size of driving wheels does not use half the gearing, (e.g 10:1 ), but more like 12.5:1

We push the figures into the equations...

The shaft speed to the gearbox is 1,908RPM and the required stall shaft rating of the motor is 1,920 Grammes per Centimetre.

At the end of this exercise we have two locos which weigh the same, move at the same speed and have VASTLY differing motor requirements.... In the first example the motor requires a stall shaft rating of 2,500 Grammes per Centimetre, in the second one of 1,920 Grammes per Centimetre. In the first I would opt for a 3 pole motor and in the second a 5 pole motor and always for the WIDEST motor that I can fit. This gives the windings the biggest flux to work at and wide motors often have fans coupled to the commutator part -thus increasing the cooling possible. How you mount your motors can be advantageous to your electronics as well. I am a great believer in cheating and often mount my motors to blow their coolant air onto the heat fins of the electronics. In my latest model I do the reverse -but then they do have pretty big heat sinks!

Here we see a pair of motors mounted to blow air onto the electronics.

See Picture 1.

And here is a one of my typical motors that I use

See Picture 2.

Steam boilers

What follows now is "empirical" and is not backed up by any "proofs" or any form of "maths" at all!!! I have built two boilers -both using widely differing conceptual techniques. Which is better, I cannot say, but I give the information here -free of charge.

The Porcupine Boiler.

This is what powers the Heilmann steam electric locomotive. It is not very hard to build, but it does suffer from one very poor constructional problem. The heat absorbing quills also act as cooling quills -so rather more heat than would be normally contemplated has to be put into the joint. That said it has a very high surface to water ratio and the quills act as extremely good nucleation points for the production of steam. It is not really suited for boilers with a diameter less than 50mm though. The boiler fitted to the Heilmann may be classed as an extreme form of this type and can boil dry 500ml of water to 40PSI in 10 minutes. Since the latest modification it is running 6 cylinders at 2,400RPM. This limits what can be done with it. I will have to fit some kind of "Goodal/Enots" valve for water filling.

The Brotan Water Tube Boiler.

This is a type of boiler that I think is highly suitable for model locomotive work, even though I may sound like a heretic -I believe that this type of boiler is probably easier for the home constructor than the "conventional" Stephenson type. The original was designed as a boiler that did not use any expensive copper -something that, (nowadays), we are all having problems with. It does seem to produce "wetter" steam than a Stephenson type boiler but this can be cured using a superheater or drier of some sort. The boiler drives a pair of turbines quite successfully at 40PSI for about 12 minutes.

It was Henry Greenly who proposed what has become the std definition of model locomotive boiler requirements:

"1 Cubic inch of Water boiled in 100 Square inches of heating surface at 100 Pounds per square inch -per minute".

This was written in the days of Meths and Coal fired locos. I think that this definition can now be classed as, "Correct For Its Time", however both of my steam boilers use Butane gas and incandescent ceramic heating elements... So perhaps a more modern definition is called for? It is common amongst 16mm scale live steam locomotives to have a single flue with a gas burner inside, this is nowhere near the "Greenly" std in steam production -but I doubt anyone who owns one will complain... "Scenic" models, (ones that simply pull model rolling stock -not people), do not require such high pressures -but instead require longer running times for their owners/builders to admire them in. Thus it might be better to design a boiler that has a very high rate of steaming for a very low rate of fuel. The basics of "short fat boiler" versus "long thin boiler" have been worked out in the 19th century. The square cube rule of heat loss shows that small boilers lose heat faster than big boilers. BUT in a model, do we actually need a large boiler, -could a simple small high production boiler be hidden under the casing of what would look like a large boiler? It should be fairly easy to design some form of flash steam single tube boiler that could be adapted for work in a locomotive. Thus all of the heat from the fuel could be used to generate steam and very little of it used to heat the surrounding parts.

Just a thought!

Diesel and Steam Electric Generation Systems

If you have examined the Heilmann page then you will have seen how I built a dynamo. Would I do this again? NO most definitely not!!! The sources of information that I used for it were very antique to begin with. I had to hand cut the laminations for the field windings, install and epoxy the "activator" magnets (in my case Neodynium Iron Boron ones) and then wind the laminations... I confess that it was a case of "sheer bloody mindedness" that made me do it this way as I felt that I had to prove that a steam electric was possible for me to build. All the time I was doing this I did have "friends" who asked me why I was doing it the hard way and not simply use a DC motor in reverse!!! Well looking back on it they may have been right -but I have the satisfaction of knowing that I build one that worked. If authenticity is not important -then do it that way!!!

In the same vein there is a growing band of people who are experimenting with "Live Diesel" something that is I have to confess totally unknown territory to me. I am investigating mechanical drive for my "Live Diesel". The reason for this has to do with with the fact that I would need to source an alternator... There is however one MAGNIFICENT cheat for this.

It is called "The Permanent Magnet Stepper Motor".

In a std DC motor there are simply the Positive and Negative connections. In a stepper motor there are four electromagnets forming the field system. Sequentially powering each magnet provides rotation, the permanent magnets forming the central shaft. If the outputs from the field winding terminals are linked together, it is possible to have anywhere from 4 times voltage to 4 times amperage when the shaft is rotated -either by a steam motor or a diesel motor. The flux out of the terminals is all AC, so care will have to be taken to "pair match" the windings. These will have to individually rectified, (not a hard job -just 4 bridge rectifiers!), and then the outputs can be combined as required.