Which Electric Motor?

Batteries & Speed Contollers

Tips On Motor Mounting

Batteries & Speed Controllers.

Electric motors generate electrical pulses, which can interfere with incoming signals to the receiver. These pulses can cause servos or speed controllers to judder or ‘glitch’. This problem is usually solved by soldering the end wire of a 10nfd (0.01mfd) capacitor to the positive terminal of the motor and the terminating wire onto the motor case itself. The same procedure should also be carried out on the motor’s negative terminal. A 47nfd (0.047mfd) capacitor is then soldered between the two motor terminals. These devices will filter out any unwanted signals from the motor. Most good model shops stock suitable capacitors, which are sometimes also referred to as condensers, and they will usually come with an instruction leaflet of some kind. Any capacitor should have a minimum working voltage of 100vdc, as with a lower voltage, there is a possibility that they could break down due to interference spikes. Some of the higher quality motors may even have electrical suppression already built in. An additional pecaution against unwanted signals is to run the receiver aerial as far away from the motor as possible.

This beautifully detailed Victorian steam launch has a Monoperm electric motor well hidden inside the scale steam engine casing, batteries are all fitted under the cabin floor.

In recent years ni-cad battery technology has improved substantially, but the maximum cell voltage is always 1.2v whatever the capacity. There is a wide choice of ni-cad batteries available in most model shops, but ni-cads should only be charged with a unit that is specifically designed for this type of batteries, and they can be either slow or fast charged.

The best choice of main drive power source for many of the hulls in the Kingston Mouldings line-up, and certainly for anything other than high-speed scale models, is not ni-cad batteries, but rather the type of accumulator known as a ‘gell cell’. These are square shaped objects that look rather like a small brick, and they are available in various sizes and capacities. The standard voltage for charged accumulators is 2.0v per cell for both gel and lead-acid types. Their principle advantage of gel-cells is that the acid in the accumulator is in the form of a jelly, something that allows it to be used in almost any position without any worries over possible spillage of contents, as could happen with the older lead acid type accumulator.

A 6-volt accumulator of 12-amp hour capacity would be a good choice for most hulls of thirty-six inches (900mm) and over. However, an example of this size weighs around 4½ pounds (2kg), obviously not a problem with the average tug, though for lighter displacement models like MTBs, ni-cad batteries would normally be a better choice. Due to the characteristics of both kinds of power source, they can be connected in series to increase the voltage if necessary, but never in parallel to increase the current, as there would then be the possibility of generating circulating currents.

Just like ni-cads, gell cells MUST be charged with the correct charger. DO NOT use a car type battery charger. A point to watch when reading magazine articles and the like, is that people often talk about batteries when they really mean accumulators, which is of course what I’ve just done in the previous sentence, but the two terms are easily confused and not really interchangeable.

Whichever types of battery/accumulator you decide to use, ni-cads or gell cells, their weight can be most useful as moveable ballast to trim the model to the correct scale waterline. Depending on the size of the model, and exactly what you have available, two smaller batteries packs or accumulators linked together can sometimes provide a better solution, and with two fairly heavy items to move around, this can make it simpler to trim out the finished model so that it sits correctly in the water. If there are two drive motors, then you can use individual battery packs or accumulators for each motor. When you install anything at all in your boat, never forget that you’ll almost certainly want to take them out of the hull again some day, so remember to provide adequate access at the building stage, to any items that may require maintenance at some time in the future.

Electronic speed controllers are the norm these days and come in a variety of types and guises. It is not really possible to recommend any particular make or type, but make sure that the current rating given is that for continuous running, and not just the momentary peak current. Whichever controller you select always remember to have suitable fuses between each battery and speed controller. The rating of the fuse should be that of the speed controller’s normal maximum running current, not the peak current. Remember that it is easy to pick up weed on a propeller, and this will slow the motor, or even stop it completely, and this means that the current taken can then be many times the normal full load current. As a rough guide, the current rating of a speed controller should be 50% more than, or even double the normal maximum running current. Not all speed controllers are designed with a reverse function, so do check this with the supplier before buying. You should choose a speed controller with an amp rating greater than the maximum running current, and a voltage rating higher than the battery voltage.

One fairly economical speed controller is the ‘resistance’ type, which fits on the top of a standard servo. They are cheap, but the main failing is that the resistance that they function by will inevitably absorb a certain amount of battery power. The most popular type of resistance speed controller can be purchased under the trade name ‘Bobs Board’, but several other ‘resistance’ type speed controllers are available from other manufacturers. This type of speed control is both simple, and easy to understand and operate, so although they are not much cheaper than some electronic controllers, they are perhaps worth considering as a starting point for newcomers to the hobby.

Another relatively economical but very satisfying way of achieving motor speed control is to build your own fully electronic version. Fairly simple kits to assemble these are available from firms such as Action Controls or Electronize. A fair degree of skill with a soldering iron is an essential requirement here, but results are pretty well guaranteed provided the instructions are adhered to. Both of these companies will assist should the builder run into difficulties.

Finally, some general wiring tips that I’ve found useful over the years. Try to run all power wiring down one side of the hull, and radio wiring down the other wherever possible, and colour code all wires, especially power wiring. If you use connectors for the power wiring rather than soldered connections, do use the polarised kind so that the power can’t be misconnected.

Something I’ve been doing for as long as I can remember, is to ‘tin’ flexible stranded wire leads when they were going to end up clamped under a screw terminal. If you try this, remember to use a really hot soldering iron so that prolonged heating doesn’t melt the plastic insulation on the wire. I thought I was making better contacts by doing this, especially as it gets rid of any long-term corrosion troubles, an very important consideration if like me you often sail on salt water. I’ve never encountered the slightest problem, but perhaps I’ve been lucky. I’ve now been told by someone with many years of electronics industry experience, that the practice isn’t such a good idea, as the tin/solder can flow with pressure and time, to create a possibly troublesome intermittent contact. I can see that this would be something to look out for, so you have been warned.

All of the products and items mentioned in this section and also the one on selecting the most suitable motors, can be obtained from the better modelling retailers, some of whose names you’ll find in the regularly updated Recommended Suppliers section on this website. Many other model shops advertise in monthly magazines like Marine Modelling International www.traplet.com and Model Boats www.modelboats.co.uk Both magazines are published in the UK, and they also obtainable in many English-speaking countries around the world. Either magazine is also available by subscription, so just check their respective websites for details.