Build Your Own Battery Locomotive
Now why on earth would I tell you how to build your own locomotive when I’m trying to sell them? Simple, I’m not in this for the money – it’s a hobby I enjoy and hopefully I can share the hobby with others. And if you buy something along the way like the motor controllers then it’s a bonus.
So where to start? First you need to decide what you want out of the hobby and determine your budget. Often the very hardest part is getting the two to come to an agreement. Most likely you are here because you’ve decided your budget says that you want a small locomotive or speeder. I can’t answer the question as to whether gas or electric is better for you. For me, electric wins due to its ease of use, safety for kids and inexperienced operators, price, and of course it’s QUIET.
Designing an electric drive system involves four components: Batteries, Motor Controller, Motor, and Drive Train. Let’s start with the motor.
Motor
The motor is usually the first thing purchased, and this isn’t a bad thing. Sizing the motor is very important, too large and you’ll spend more money than you need and possibly overtax your controller. Too small and you’ll likely burn it up, a very dissapointing thing and possibly dangerous. My rule of thumb is 1/4 horsepower (185 watts) for every 100lbs of locomotive. This is enough power to allow for wheelspin if you put too much train behind the loco. The worst thing you can do is stall an electric motor, this allows the amps to spike and heats the motor very quickly. Heat is the enemy of electric motors. Once the internal insulation melts your motor becomes a paperweight.
There is much debate about the best way to wire multiple motors. I prefer parallel because I understand current divider circuits better than I understand voltage divider circuits. Either way DC motors tend to be self regulating if they are well matched and mechanically connected. I try to avoid multiple motors if possible, more things to go wrong. Some don’t like permanent magnet motors, but I’ve found them to be excellent performers in fractional and small horsepower applications.
Drive Train
Building the drive train seems to be the most confusing portion of building any locomotive. Simple math can take most of the confusion and guesswork out of this. Three things need to be know to design a drivetrain: The output RPM of the motor, the diameter of the drive wheels, and the maximum speed you want the locomotive to travel. Typical motors have an output of 3000 RPM, a typical wheel diameter is 4.125″, and a typical top speed is 5mph. Figure you’ll only get 75% of your motors full speed under load, and the calculation goes like this:
5mi/hr * 5280ft/mile * 1/60 hr/min * 4.125in * pi(3.14) * 1/12 in/ft = 475RPM
475 RPM is the speed that your 4.125″ diameter wheels need to turn to be moving 5 mph.
Take your motor RPM, times 75%, divide by your calculated RPM to get your needed ratio
(3000 * 0.75)/475 = 4.73
4.73:1 is the gear ratio you would want in between your motor and your wheel axles in this example. I would round to 5:1 to make things simple. This can be done in multiple stages. Remember stages muliply, not add. For example, one could take the 11 tooth sprocket on a typical scooter motor to a 28 tooth sprocket on an idler shaft, and then from a 15 tooth sprocket on the idler to a 30 tooth sprocket on the drive axle for a total reduction of 2.5 * 2 = 5.
Motor Controller
This sounds like the most difficult step, but it’s actually the easiest. Two things that your motor controller needs: 1: Solid state reversing and 2: regenerative braking. The Chinese scooter motor controllers don’t offer either of these. Size your controller to be equal or larger than the capacity of your motor. If you’ve sized your motor to the locomotive then your motor should rarely if ever try to draw more power than it’s rated for. So long as the controller can provide that power you will never have any trouble with your locomotive. If your controller is oversized for your motor, install a fuse that is near the maximum amp rating of your motor. If you blow the fuse under normal operating conditions then your motor is undersized for the job or your gear ratio is too low. If you only blow the fuse under extreme circumstances like starting a long train then you’ve sized things right and a larger fuse can be installed as motors can handle high amps for short periods.
Batteries
I prefer sealed lead acid batteries for several reasons. They do well in the occasional deep discharge situation, but most importantly they won’t get battery acid on the inside of your locomotive in a derailment or during transport. The only drawback is that they can’t be kept on a charger as the water will slowly evaporate out causing premature failure. The run-time will need to be determined. Take eight hours as a good baseline. Now for some more math. Of course you’re going to use a regenerating motor controller that will return about 25% of your power, trains are so very efficient. Of course you won’t be pulling full power up grade the entire time. Then assume that you’ll be spending more time at red signals and in the station staring at that articulated steamer that’s worth more than your house. So four hours of full load running, with our 350 watt motor, pulling 15 amps at 24 volts for 4 hours. That is 15 amps * 4 hours = 60 amp/hours at 24 volts. So two 60a/h 12v batteries will keep our locomotive going strong pulling passengers all day.
That pretty much covers it. Some may disagree with my methods or assumptions. That’s fine, this is what works for me and I’ve built more small electric locomotives than just about anyone else.
For examples of what others have done check out these articles. I like some of the ideas in these articles, I disagree with others. I’d be happy to converse with you via email about any questions you may have.
Bill Gardei’s “NB1”, very similar to my own Mini Motor Car in appearance. He takes a different but equally effective approach to the drivetrain and suspension.
https://www.discoverlivesteam.com/magazineold/67.html
Jim O’Connor’s personal rail vechicle, or PRV for short.
https://www.discoverlivesteam.com/magazineold/83.html
Finally this article on building an electric locomotive, the control system is an arc flash waiting to happen. Seriously, do not use a knife switch to run your loco: buy a commercially made motor controller with regen braking.
https://www.discoverlivesteam.com/magazineold/86.html