Dave Webb
It seems there is a lot of interest in the use of alternators to power traction motors in model locomotives. I am not an electrician nor am I an electrical engineer, electronics technician or any thing like that, I am a fitter and machinist by trade so I will do the best I can to explain what I have done to make my loco work.
Automotive alternators are readily available, and are capable of producing high currents. Alternators are a little different to generators as they produce 3-phase AC power (at a relatively high frequency), and this is converted to DC by a built-in rectifier (Generators produce DC directly). Refer to Fig. 01 at left for typical schematic diagram. As standard, alternators also have a built-in voltage regulator set to the (vehicle) battery voltage. The rotor in an alternator contains a field coil (fed by the regulator) which produces a magnetic field to generate the voltage in the stator windings, and varying the rotor field current varies the output voltage. Detailed descriptions on how alternators function can be found on the Internet, and these sites on Understanding Alternators and Alternator Secrets give good explanations.
I have a small 71/4" gauge 0-4-0 loco using an engine driving an alternator at the moment and it has proven to be very reliable and very simple as well. The main issue I initially had was how to control the motor on my loco. My 12 volt traction motor, which is rated at 1200 watts, is capable of drawing a huge amount of current and I could not come up with a suitable and cost effective electronic speed control that was capable of handling this amount of current. I’m sure it can be done but at what cost? And I don’t have the knowledge to build such an item anyway. I’ve seen in a few posts of people using some of the commercially available speed controllers only to find that they get so hot that they have melted the solder, or burnt out the MOSFETs due to the high currents that the traction motors are drawing.
The reason for the setup I am now using is so the speed control only needs to handle the 2~3 amps required by the alternator field winding. This can be done by using one of the cheap 10 or 20 amp 12 volt speed control kits available from electronic hobby stores etc, or by using a 5 or more position rotary switch running through a series of resistors or a [high-power] rheostat to control the power. I have tried both methods and both are equally successful.
I am using a 5.5HP Honda engine to drive a normal car type 12 volt alternator (100A rating), but with the field connections modified, and in-built regulator disabled or removed. I use the chassis of my loco as the earth just like a car's electrical system. The alternator main output is wired directly to the traction motor via a reversing relay [See Controller article for reversing detail]. This relay must be able to carry the maximum alternator rated current, or greater. The ones I use are for an electric anchor winch or a 4WD recovery winch. They are more than capable of handling the current for most applications. Refer to Fig. 02 at left for circuit details.
The speed control is used to power the field windings of the alternator, which only draw 2~3 amps. The more current that is applied to the field windings - the more output power the alternator will produce. It really is that simple. What you are effectively doing is replacing the car or alternator's voltage regulator with some form of speed control, whether it is an electronic type or just a series of resistors on a rotary switch, and controlling the field current from zero to the maximum 12V.
For this application as a traction power source, the normal voltage regulator associated with the alternator MUST be disabled. If it is an external type, this is straight forward, just disconnect it. If the regulator is internal to the alternator, which is common in modern units, it must be disconnected and disabled.
If you are using a newer type of alternator with an internal regulator then you will need to bypass this and wire your speed control up to the field windings. I did just that, but left the unit in place, now unused, as it was convenient to do so mechanically. I would recommend using the newer types because they tend to have much higher outputs than the older ones. They also have the added bonus of having an internal cooling fan rather than the external type like the older ones which means one less thing spinning around inside your loco to get your fingers caught in.
If you are using the older types of alternators, then just wire your speed control directly to the field windings. The power for the speed control and for the relays comes from a 12 volt car battery or similar. I am using one of the gel cell type batteries. If your loco and its engine are big enough you can have a second alternator to charge this battery. If your engine is an electric start engine, then it may well have a suitable charging system built in for charging the starting battery. In the loco I’ve just explained, I simply don’t have the space for a charging system so I just charge the battery with a normal car battery charger when required. Because of the low amount of power that is required to power the alternators field windings, I find that I only need to charge my battery after 3 or 4 running days.
Using this set up I am able to start my loco moving just as smoothly as a battery electric loco using a sophisticated electronic speed control. This locomotive is a scale model in 3.625"/ft scale (ratio 1:3.31) of a 2ft gauge locomotive built originally by Malcolm Moore Ltd in Melbourne.
As for using a rotary switch and a series of resistors for your speed control, my first setup used ignition coil ballast resistors and a 5 position rotary switch as a means of speed control. This is an almost identical circuit to a Resistive Speed Controller, except the resistor values need to be re-calculated for the alternator field current.
Rather than use a switch to directly select the resistor settings for speed control, I used the switch to drive relays which do the switching. This enables only the small relay coil current to be switched, and permits the use of a small hand held controller. It also permits easy implementation of a "dead man's handle" as a fail safe mechanism. Take your hand off the control, or if the control cable is unplugged or severed, power to the control relays is instantly removed (and alternator output instantly drops to zero), and the loco just coasts to a stop without traction power.
1st speed - push the dead man's button and the power to the alternator's field windings runs from the battery through 4 ballast resistors. 2nd speed - turn the rotary switch to its second notch and the power to the field windings runs through only 3 of the 4 resistor. 3rd speed - turn the rotary switch to its third notch and the power to the field windings runs through only 2 of the 4 resistors and so on.
If you find when pushing the dead man’s button for first gear that your loco lurches forward too quickly, then just add another resistor or two in series with the others. What you are trying to achieve is to reduce the current in the field windings enough so that the alternator is only putting out enough power to get your loco just creeping away from a stand still. Got a big load and there is not enough power to start you moving, then just click it up a notch.
As long as the last notch on your rotary switch has a full 12 volts running from the battery directly to the field windings and not through any resistors, then you will have full power to your traction motor(s). Unless your rotary switch is capable of handling at least the current draw from your alternators field windings, it is advisable for your rotary switch to drive a relay for each notch so the power from the battery through the resistors and to the field windings is not actually running through your rotary switch. Just a normal car horn type relay from one of the auto parts suppliers is fine.
It is probably much simpler to use one of the 12 volt electronic speed control kits because it avoids a lot of extra wiring and the resistors. It will also give you much smother control. These speed control kits typically operate in switch-mode (pulse width modulation) just like a Switch-mode Controller. Just make sure you wind the control back to zero, or select the first notch, before pushing the dead man's button or you might find you have left a few passengers lying on the track behind you! If you use a switch-mode type speed control to power the field coil, just make sure you use an inverse parallel diode across the field coil to protect the controller from the voltage spikes. Be sure to use diodes the same or higher current rating as the field.
One question I am often asked is: what diameter pulleys are required? In the normal automotive environment where these type alternators are used, the engine RPM varies considerably, but is usually in the range of 1000 RPM to 3000 RPM most of the time, with the alternator rotating at typically 1800~5000 RPM. As most single cylinder engines used for this application run at 1000~3500 RPM, you should aim for a motor: alternator pulley ratio of 1.8:1 to 2.5:1 so the alternator is operating near its normal speed. The actual ratio will depend to some extent on the amount of space available. Leave the standard alternator pulley untouched.
I am currently building a larger loco with a larger traction motor which I will be using the same system on, save for the fact that because of the larger size traction motor I will be using a larger alternator. The same setup can be used with any number of traction motors. You just need to make sure that you have enough capacity in alternator output to power the traction motor(s), and make sure you have enough power in your petrol engine to drive the alternator as it is still possible to stall the petrol engine with too much load on the alternator. I have found that a Honda 5.5HP engine is capable of running an alternator of around 100A output capacity without any problems.
You should note that with the full 12V applied to the alternator field windings, the alternator output voltage can be much higher than the nominal 12V. The output can reach 40V or 50V, or even more, and you should make sure that the traction motors can handle the higher voltage (and the resulting higher speed). Normally, the voltage regulator takes care of the output voltage, but now with a separate field supply, the output can vary over a much wider range (which is what we want).
In my opinion this system of powering a petrol-electric loco is both the simplest and probably the most effective. As far as I know a full size diesel electric loco works in much the same manner, although I am happy to be corrected.
I hope that this has been of some help to anyone wishing to build a petrol-electric loco and that I have made it understandable in layman's terms without rambling on to much. Like I said, if anyone has any questions I will try to answer them a best I can.
As an aside, I have found that solid mounting these small 4 stroke engines seems to be much better than mounting them on rubber engine mounts. I find that if they are able to move on flexible mounts then that's exactly what they will do. I also try to fit my locos with the largest engine possible so I can run them a lot slower than full power output speed. That way they sound more realistic. There's nothing more unrealistic than a perfectly built miniature loco that sounds like an old lawn mower screaming its head off! Once again just my opinion!
Well there you have it.
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| Note that the controller resistors (above 2 figures) are high power rating, and mounted away from the other components. Also plenty of space to permit maximum cooling. Note too the multi-pin plug for the hand held controller. | |
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Now for some calculations to work out what size alternators, motors, engines etc are needed. The relationships between voltage, current etc and electrical calculations are quite simple, but probably power (P = E * I) is the most useful here. Another useful factor is 1 HP = 746 Watts (W).
Assume you have a 12V 20A motor, and you are using 2 of these as traction motors. The power required for each motor is 12 * 20 = 240W, or 480W total. You therefore need an alternator of say 50A output (~600W), and you will need at least 600W output available from the petrol engine. There are always mechanical losses in the bearings, power loss for cooling fans, losses in the drive belts etc, and there are conversion losses in the alternators and motors. Conversion losses are say 15%, and drive losses are say 20%, so it we would be losing up to 35% of output power. If we require 600W to drive the alternator, the engine needs to be at least 925W power (or 1.25HP) allowing for losses, so a 3HP or 3.5HP engine would be a good choice with some reserve. For a 100A alternator, a 5HP engine would do.
The locomotive described and pictured above runs on the Boulder Creek Tramway, which is it's home track.
This article is based on a forum posting by and additional material from Dave Webb and is used with permission.
| [Updated: 23 Jun 2010] 9,519 |
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