Powered Tender Truck

January 31st, 2019

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This article is about a standard Tom Bee "Bettendorf" freight car truck in 7.5" gauge, that has been powered with an electric motor.

It is intended to be used in a tender with an unpowered steam locomotive (whether actual steam or a fake one).

For example there are times I want to run my 2-8-2 but don't want to or can't fire it. It can also be used as a booster for those who have trouble on the hills. And third, it can power a train "pulled by" a fake or "steam outline" locomotive.


Powered Tom Bee "Bettendorf" Truck

For electric steam engines, Putting the motor in the tender is a better idea than putting it in the locomotive. Remember that the weight of the engineer and the batteries is in the tender, giving it ultimate traction.

Note: The motor can be positioned either behind the truck (as shown) or directly above it - inside the tender carbody.


Elevated view, topside.

This modification requires 4 chains and 8 sprockets. All chains are #35 and are of the same length, namely 46 links. The original truck bolster bar is not used. The motor is a 2.5 HP permanent magnet 130 volt DC treadmill motor. But the truck can be powered using other types of motors as well.

The motor drives an idler (running on one of the axles) which drives the output shaft which drives both axles identically.

Here is a breakdown and description of the chains and their associated drive and load sprocket tooth counts as well as the resulting ratios.

                   Drive   Load
Chain              Teeth   Teeth   Ratio      Reduction
---------------    -----   -----  --------   --------------
Motor to Idler      11      30    .3666667   (2.727273 : 1)
Idler to Output     13      30    .4333333   (2.307692 : 1)
Output to Axle (2)  13      30    .4333333   (2.307692 : 1)
Net ratio                         .06885185  (14.52394 : 1)

End view, inverted.

The idler provides a means for coupling the motor (either overhead or behind the truck) to the common output shaft. It runs on one of the axles. This has some interesting advantages over a fixed shaft. One, it solves the problem of the axle being in the way and two, it distributes the wear on the shaft.

The idler has it's own lubricator as seen with the copper tube and it's oil cup.


End view, topside.

The output shaft uses the same size and type of needle bearings as the axles.

To power the electric motor, a special power supply (called a DC to DC converter) is needed. Shown below is the power supply for the tender truck. It requires 24 volts DC and generates up to 120 volts DC for the motor at up to 1100 watts long-term and much more short term.


Tender Truck Power Supply

View a larger photo HERE
View annotated photo HERE


Tender Truck Power Supply Controller

The power supply is controlled by this small stack of boards. The lower board is a basic Altera FPGA breakout board. The upper board is the I/O interface between the power supply and the FPGA board. The firmware for the controller can be loaded from the SD memory card at powerup.


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