SPEED CONTROLS

By Donald Spielberger

So your pride and joy has just stopped dead on the pond and little wisps of smoke are escaping from the portholes. No matter what you try your boat does not move. What could have happened?

You may think that this is funny and could never happen but let me tell you; I see it at least once every summer. One possible source of the smoke is your now dead speed control.

Selection, installation and proper use of speed controls is the topic of this column. Hopefully, after reading this, we will all be able to avoid the all too familiar smell of burned out speed control components.

The primary purpose of a Speed Control Unit ( SCU ) is to provide the modeler with the ability to vary the speed and direction ( forward or reverse ) of his ship. There are basically two methods for doing this. The first involves “burning” the excess power off as heat with a resistor. A variable resistor for controlling the speed and a switch to control the direction of rotation are combined to form this type of speed control. This type of speed control is relatively inexpensive and simple to use. It is also immune to any possible damage that could result from connecting the drive battery backwards. Since it requires a servo to operate, it does take up some room and requires fine adjustment of the connecting linkage to operate properly. A couple of additional drawbacks to this type of speed control are: 1) when not running at full speed the excess power is wasted as heat and 2) this heat can cause other problems if not vented properly. This type of speed control is produced by many well know manufacturers such as Dumas, Kyosho and Tamiya. These units are intended for model boat applications and work well for the great majority of models. The second method to achieve speed control is through the use of drive transistors and electronics to control them. This electronic method provides pulses of full motor battery voltage to the motor. The longer the pulses are the faster the motor runs. The shorter the pulses are the slower the motor runs. With a good electronic speed control it is possible to have the motor turn so slow that an individual blade can be watched as it spins around. This is a feat that is hard to accomplish with a resistance-based unit. Most electronic SCU’s use a second set of transistors for running the motor in the reverse direction, although some units use a relay to perform the switching of direction. If using a speed control with a relay care must be taken to select the proper speed control for the motor load. Failure to due so could cause the relay contacts to burn out. Due to the fact that the electronic type speed controls don’t waste the excess power as heat to slow a motor they are much more efficient. Therefore, more run time will be available from the same motor/battery combination with an electronic speed control than would be possible with a resistance type. There are at least three other advantages to the electronic speed control. The first is there is no need for a servo or it’s linkage as the speed control plugs directly into the receiver. The second is the generally smaller size and simple installation. The third advantage is the greatly reduced generated heat. In most cases the heat produced is so minimal that it does not need to be vented. As is the way of the world, you can't have all of these benefits without having to give up something. The down side to electronic speed controls is their additional cost and dislike for water. To make matters worse, if the battery pack happens to be plugged in backward (reverse polarity) you can plan on buying a new speed control. These problems are easily prevented by mounting the speed control in a dry place (like upside down underneath the deck) and using polarized battery connectors so the battery can never be plugged in backwards.

All speed control units have both voltage and current ratings, which must not be exceeded. The voltage rating refers to the voltage of the motor battery and not necessarily the voltage rating of the motor. As an example, you can run a motor on a voltage greater than it’s rating (within reason) but to do the same to a speed control could ruin it. The amperage rating of the speed control refers to the current draw of the motor(s). To be on the safe side, the stall current of the motors should be used when selecting a speed control.

There is a very large selection of electronic speed controls available, so for purposes of this article I have chosen three size categories. They are mini/micro, standard and large.

The speed controls in the mini/micro grouping all use the receiver battery to power the electronics as well as the motor. Therefore, when using one of these speed controls you will be limited to a maximum of 6V to drive the motor. Also, since the motor power is being drawn from the receiver battery it may be necessary to upgrade the batteries, wiring and on/off switch to handle the extra load. Examples of this size speed control are produced by:

Vantec

model RSPM 6VDC, .5 A

Hunter Systems

model Micro-con 6VDC, 1 A

ACTion Kits

model micro1 6VDC, 1A

In addition to these, the electronics from a servo can be modified to function as a micro speed control. This will be the subject of a future article.

The speed controls in the standard size range need a separate battery pack to power the motor(s) and are rated to handle up to 25 Amps of current. This is the size of speed control that is used in the great majority of model boats. There are some additional useful features found on some of the speed controls available in this class. One such feature is a battery elimination circuit (BEC). This allows you to power the receiver from the motor battery pack and save the weight and space required by a separate receiver pack. Another feature provided in some speed controls is optical isolation of the input and output stages. This prevents electrical noise created by the running motor from feeding back into the receiver and causing interference. Examples of this size speed control are produced by:

Model Control Devices Super MiniconII 20VDC, 10A

Model Ultra Minicon 20VDC, 20A

Vantec

models RET411P 26VDC, 12A

RET512P 26VDC, 18A

Hunter Systems

model Advance 1 36VDC, 25A

ACTion Kits

Model MINI 2 12VDC, 2A

CONDOR 10 12VDC, 10A

CONDOR 20 12VDC, 20A

Electronize

models 43VR 24VDC, 10A

The final size range is the one that I have labeled as large. These are basically standard size units on steroids, which makes them capable of controlling much larger currents/voltages. These units usually find homes in only the largest of projects. In general they have the same base features as the standard size units with varying additional features depending and the manufacturer. Examples of this size speed control are produced by:

Model Control Devices SC-450 28VDC, 50A

models SC-480 28VDC, 80A

Vantec

Model RET713P 26VDC, 33A

Hunter Systems

Models Advance 2 36VDC, 50A

Advance 3 36VDC, 75A

Advance 4 36VDC, 100A

Electronize

models FR30HX 24VDC, 30A

As with many things in life there are always tradeoffs. The same is true when selecting a SCU. There are many factors to consider such as price, size, efficiency and ease of use. The importance of each of these will vary with the particular model being built and builders’ preferences. It is up to you the consumer to decide which purchase is right for you. If in doubt ask around and see what experiences others have had with the equipment that you are considering.

Once the selection is made remember the following tips to help keep everything working smoothly and you happy.

Always add filter capacitors to the motor. The level of electrical noise (created by the motor) that finds its’ way back to the receiver is reduced by these. This will help to eliminate radio interference problems. The best filtering is provided by using a capacitor ( .05uf ) from each motor terminal to the motor case and a third across the two motor terminals.

If the motor runs the wrong direction when moving the stick to the forward position simply swap the wires connected to the motor terminals (or use the servo-reversing switch on your transmitter if you have one).

In most uses the speed control will not even get warm. When this is the case the speed control may be placed anywhere in the hull. If the speed control does get warm make sure to provide ventilation. If you don’t, the heat could build up to the point where damage is caused to other electronics or even the hull (I once saw a hole melted through the plastic deck of a model from the heat generated by the speed control). When trying to select where to mount the speed control remember to provide access to the adjustments and try to keep it in a dry place (don’t forget about the water dripping off your fingers and into the hull).

Finally, it is always a good idea to protect the speed control with a fuse. This fuse should be placed in the circuit between the battery and the speed control and should be large enough to handle the normal load of the motor(s) but should blow if the motor were to stall at full speed (such as being fouled in weeds).