Zero-Loss Motor Speed Control

Most of us build our ships with a simple switched, servo-activated drive motor control system that gives our ships only two speeds: Ahead or reverse Flank and Full Stop. Some people have used speed controls of one type or another to allow for some intermediate speeds, but most of these have some serious drawbacks:

Electronic speed controls can be a little pricey. Typical costs start at $60.00 and go up from there.
Both electronic Pulse Width Modulated (PWM) and resistive speed controllers generate lots of heat, which represents power loss. Most of us can't afford to waste all of this power that gets dissipated as heat.
Electronic speed controls don't usually care for water!
Interfacing an electronic speed control to an R/C receiver can be a challenge, unless the controller is built specifically for R/C use (bringing up the cost issue, again).

Doing without variable speeds is something that most of us have just learned to live with. There probably isn't a great tactical advantage to be able to propel yourself at less than your maximum speed. Faster ships on convoy escort duty might like to be able to slow down to something closer to that of the cargo ships, although they can start/stop or zig-zag to let the convoy keep up. Conserve power? Probably not, since speed controls tend to be wasteful anyway.

Still, if you wanted intermediate speeds, wouldn't it be nice if there were a simple, economical way to achieve it? Well read on, because there is! And it's simpler than you might have expected!

The following circuit was first developed for use in USS Pittsburgh (CA-72), a speedy heavy cruiser of the Baltimore class. During a major overhaul, I wanted to add some extra batteries to allow more endurance. By removing excess weight from the hull, and eliminating a separate pump battery, I was able to go from 2 2V Hawker Cyclon batteries in series (4V total) to 4, doubling my range. But what I also did was to devise a circuit that lets me run the motors off of either a pair of batteries in parallel (2v) OR switch in all 4 batteries (4V) to double the available power. The schematic for this circuit is shown in Figure 1:

Figure 1.
Dual Speed Motor Circuit.

What makes this circuit different from the tried-and-true single-speed reversing circuit is the addition of another lever switch (SW3), and a second battery (B2). With all switches in their Normally Closed (NC) positions, as shown, there is an open circuit condition and no current flows through the battery. When SW1 is closed (connecting C to NO), a circuit is established allowing current to flow from the + terminal of B1, through normally-closed SW3, through SW1, through the motor, through normally-closed SW2, and back to the - terminal of B1. This provides reverse propulsion. Likewise, if SW2 is activated, connecting its C to NO terminals, a circuit is established connecting the + terminal of B1, via SW3, through SW2, through the motor, through the normally-closed SW1, and back to the - terminal of B1. This provides forward propulsion. So far, only one battery has been used. However, if SW2 AND SW3 are both activated, a different circuit is created. This circuit connects B1 and B2 in series, doubling the voltage. Current in this circuit flows from the + terminal of B2, through the speed controlling rheostat VR1, through the NO/C terminals of SW3 (note that the connection from B1+/B2- is broken by activating SW3), through the NO/C terminals of SW2, through the motor, through normally-closed SW1, and back to the - terminal of B1.

SW1, SW2, and SW3 are the usual lever-activated switches in common use in the hobby. Mechanically, SW2 and SW3 are mounted next to one another, and a cam extension - a long arm capable of reaching both switches' levers - is used. The cam extension is trimmed so that only SW2 is activated when the servo has rotated approximately 1/2-way to the forward direction, and both SW2 and SW3 are activated when the servo is rotated fully to the forward position. The physical relationship of the "throttle" servo and switches is shown in Figure 2.:

Figure 2.
Servo and Switch Layout.

It's important to use identical batteries. My personal preference is the Hawker Cyclon X cell. This is a 2V, 5.0 AH gel cell that comes in a cylindrical package about 1-3/4" diameter by 3-1/4" high. Gel cells are sealed and can be mounted in any orientation. They are also very forgiving of abuse such as over-charging, which make them a favorite of mine. Gel cells also are capable of delivering a lot of current, which is nice for motors. Any single-cell battery, such as a 1.2V NiCd or alkaline would work, though.

In CA-72, I've added an additional pair of batteries, each in parallel with B1 and B2. This gives a total of 10 AH available at low speed, plus an additional 10 AH at high speed. If somebody wanted to, they could add additional switched cells, providing even more variations of speed. Also, multiple speeds in reverse could be added. I placed the speed control rheostat so that only the highest speed is affected by it - no sense wasting heat reducing speeds that are already under the limit!

While the jury is still out with regard to the combat effectiveness of this speed control, it definitely does the job. And it does it cheaply, simply, and without costing any wasted energy in a ship that doesn't have any displacement to waste!

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