DC Motors - Parallel or Serial Connections?

Connect them in parallel, powered by a 12V supply. Just make sure you have enough current.

Unless one or more wheels slip on the tracks, the motors will be forced to turn at very nearly the same speed in any event, with slight variation on curves, where the outer wheels and motors will turn a bit faster.

If the motors are in series, failure of one may interrupt power to all (just like series Christmas tree lights). So I think parallel would be better in that respect. However, if you are delivering power through the rails, I²R losses will be a larger percentage on a 12-volt setup versus 48. If the batteries are carried with the engine, this will be (virtually) no problem.

I once got to ride on a 12-inch gauge RR built of 3/8" x 1-1/2" flat bar "rails" set into 2x2 ties over about a half-mile rural course, with various buildings and bridges to like scale. Great fun; sounds similar to your setup. Dinah won't you blow your horn!

I was thinking of connecting 2 motors in series for each truck and wiring the 2 trucks in parallel using 24VDC. Power will be from 2-12V storage batteries carried on-board.

Ronseto -- You've got good advice here so far. I would recommend you connect with the 7-1/4"/7-1/2" gauge model railroad subculture to learn what some of their participants have done with respect to electric locomotives. A lot of those guys start with battery or gasoline powered locomotives just to get running while they get ready or decide whether they are ready to take on the technical challenge of a live steamer. Even the serious live steam guys with actual railroads will sometimes have a battery powered "mule" (so to speak) to do work or car movements on short order when they can't wait for one of their steam locomotives to raise steam. I know of one of these operations on a private estate in my area with over 1/2 mile of 7-1/2" gauge track in a hilly location.

The last time I visited there this fellow had a small battery powered loco with a diesel critter superstructure just for moving cars and hauling tools and material to outlying areas for lineside and track maintenance. Of course among him and his friends they filled the engine house and staging tracks with 5 live steamers including a 3 truck shay. But we were well into an autumn morning before the first one of those beauties actually moved its drivers.

Ed Weldon

I may choose the same solution, Also the motors should be exactly identic and their fields similarly excited

Yes 2 loops is a good method. There are some really good control explanations in recent issues of Australian Model Engineer in the Arcs N Sparks section. www.ameng.com.au

For DC traction applications loops with 2 motors are common as it helps ensure load sharing. 24 volts is easy to control and you have some easy control options with dropping out one loop when the full torque is not needed.

You have to connect them in parallel. In series connection the motors will get the same current. If one of the axles slips the rotation speed of the associated motor will grow so the voltage drop on that motor will also grow. That means that the slipped motor will rotate with its max and all other motors will stop. That stabilize the "slipped axle" situation and the locomotive will stop.

Any motor that will be used here will have a speed far higher unloaded than loaded.

The loading of the Locomotive will bring them all down to the same RPM.

There will be no problems if all are driven in parallel from 12 volts.....all will be running slower due to the load/weight of the loco. Each will take its part of the whole load....

Too much theory can get you in the wrong frame of mind.....

Unless the load, applied to each motor is the same amount, yo can never bu sure the speed and torque of each motor is the same. No matter they are parallel or serial connected..

Speed at those DC motors are strictly dependent with the load.

If you are sure the loads to each motor are identical in each speed range, then the speed will be the same (providing the motors are identical)

Speaking on a scale locomotive, you can easily imagine the speeds and the loads during right and left turns, will be different for each wheel. (Considering one is the right side motor and the other is left side).

Regards

If you connect the motors parallel and measure the current of each motor individual, you have an idea of the torque each motor delivers.

In case one motor is taking less current, is it slipping, so decrease the voltage to this motor. to prevent slipping, remind static wheel friction is "stronger" (better traction) than dynamical wheel friction

In case a motor is taking more current then the rest, it is delivering more torque, increase the voltage to the other motors, to share the load.

I tried connecting some electrical devices in series and discovered that each received a fraction of the voltage of the circuit and the current draw was more than an individual device would take. So if 12 volt motors are run in series, wouldn't you need to provide 48 volts of power? And your current source should be prepared to dish up four times the amps as well.

am I wrong? I am but an egg, eager for instruction.

Only a higher voltage is needed. Assuming that all motors are identical, then the current will be the same as for one motor on just 12 volts.....

You have to check the power, like if the motor takes 1 amp at 12 volts = 12 watts

Therefore 4 motors at 1 amp at 48 volts = 48 watts or 4 x the power needed for one motor.

Simple Ohm's law....

Sounds like parallel is the way to go. Thanks guys.

Assuming the motors don't slip the situation is almost academic.

In industrial situations we will often choose the arrangement depending on the power supply available, higher voltage controls usually being more efficient.

As an example I recently worked on a dock side crane. There are 16 driven gantry wheels running on rails. The DC power source is 500 v dc. The motors are 240vdc, 13kw. The motors were connected 2 series, then in parallel.

Is it possible that the rotors of the two paralleld motors were is serie and de fields were excitated separatly. In this case the two parallel motors would delever the same torque.

Actually, the 2 motor fields were in series also, and those paralleled, so they had exactly the same flux in the series pair. The field regulator for the lot was a current regulator.

In the series armature connection it is common to have both field loss protection and armature overvoltage protection so that if a field shorts (the Cemf goes to zero) the motor without the shorted field is protected against seeing the full supply (twice rated voltage).

I have also used parallel and/or motors with individual field regulators. This can allow for some interesting load share schemes with mismatched motors.

These are all industrial applications with motors typically rated 240vdc up to 600vdc (or VAC depending on application). Hp range typically from 1 to 1500Hp. With this background the application information may not apply to smaller 12vdc motors. The physics however are similar.

It seems to me that a speed control will be required, unless you plan on running full speed all the time.

You may want to consider using four controllers in a master slave arrangement. One would be the master and it would provide voltage to the motor which is proportional to the speed you want to run. The current (proportional to torque) drawn by this motor could be sensed and used to command the three slave controllers. The job of the slave controller would be to drive its motor until it's motor current matches that of the master motor. This torque sharing arrangement would compensate for minor differences in wheel sizes which could result in uneven motor loading or a forced wheel slip.

In case a wheel becomes unloaded because of imperfections in the rails, an additional speed control clamp could be implemented to prevent the unloaded wheel from going to maximum speed. This additional control loop would limit the slave motor voltage (speed) so that it never exceeded the master motor voltage (speed) by a small percentage (say 5%).

Is the plan to provide the voltage through the rails or an on board power source?

A single PWM controller could control all 4 motors simultaneously, depending upon the current drawn.

There are plenty of circuits around on the web, some using 555 chips (I have built some, they worked fine) and a MOSFET of suitable size.....

No, it won't.

They will run at speeds determined by the adhesive weight on each axle, the wheel diameters and the propensity for slipping at any wheelset.

I have been away so sorry for late response.

Some of the early electric locomotives used a switchable series/parallel system to start with the motors in parallel, then switching to series mode at speed. I believe similar series parallel switching was used on earlier multi-voltage locomotives. These techniques are long obsolete with modern drive electronics.