Design Guide - Kit Cars

Motors for Model Solar Competition

There are many small, low voltage, direct current motors available, but not all are suitable for use in the model solar competition. The dilemma is which motors are suitable and which is the best for your vehicle. The data below will give a starting point.

Sheridan Kit Cars, these must be constructed from the components supplied in the kit which includes the motor (Scorpio SM 403). There is no scope for motor selection in this competition.

Within the competition regulations you can choose from a limited number of motors. There are some basic motor features that limits your choice.

  • Motor size and weight. As in all motor sports power to weight ratio of the vehicle is critical to performance. This is particularly so in the boats.
  • Motor power. The motor must be capable of converting the electrical power available from the solar array into mechanical power. It is no use having 6 watts available from the solar array and a motor that can only produce 1 watt. Things are just as bad in the opposite direction if you have a 30 watt motor a significant portion of the solar array output will be used in just running the motor. The additional weight of this higher powered motor will adversely influence the power to weight ratio.
  • Motor rated voltage. This rating must be compatible with the output from the solar array. But do note that in many cases the motors used in the student designed car competition are rated at 6 volts but are being operated at up to 20 volts. Operation at over voltage increases motor RPM and the power it can deliver but will shorten the motor life.
  • Motor efficiency. Obviously the highest efficiency possible is desirable. Generally high efficiency is only available in proper industrial type motors not the typical toy motors available at the hobby shops. High efficiency usually comes with a high price.
  • Motor RPM. This does not matter much in the student designed car event as a gear reduction is required between the motor and drive wheel. The gear ratio is chosen to suit the motor RPM. It is very important in the boats as matching motor RPM to propeller characteristics is critical in obtaining maximum power transfer from solar array to propeller. In the advanced boats a gear reduction is permitted so it can be selected to obtain maximum power transfer. Many advanced boats use a direct drive, junior boats must use direct drive which makes motor RPM critical.

Motor type is another variable, typically permanent magnet DC motors are used. There have been many suggestions that a brush-less DC motor such as those used in model aircraft and drones would be better. So far no one has come up with such a motor that is suitable for the model solar competition. The main issues seem to be RPM and efficiency and possibly more importantly the characteristics of the solar array being so different to the batteries normally used to power these motors.

Keep looking. Just because no one has found a suitable motor yet does not mean there is not one out there somewhere! You never know what will appear in the future.

Solar Panels

In order to get the best out of your vehicle it is important to know what power your panel is delivering. For the student designed cars there is strict limits, the regulations stipulate that the panel power produced at standardised AIM 1.5 sun level must not exceed 10 Watts. And when the car is presented for race scrutineering the panel is checked using calibrated equipment.

While this may seem a complicated process your can perform a Power Test using some relatively simple equipment and still estimate the power your solar panel is generating to within less than 10% of what a calibrated system would measure.

If you also use a Calibrated solar panel at the same time you test you can adjust your measure values to correct the reading to the AIM 1.5 standard to generate a more accurate maximum panel power value.

What is a Solar Panel?

A solar panel is one or more solar cells, or photovoltaic (PV) cells, that coverts the energy in light waves into electrical power.

These cells all share a same trait, they generate a voltage and current flow when a photon of energy in light is absorbed by the cell. This powers devices.

There are MANY different types of construction technique for creating a PV cell and they range from the relatively cheap and common Silicon based unit all the way to the very expensive and uncommon Gallium arsenide devices that are used to power satellites and even the International Space Station.

Testing your Vehicle

There is one re-occurring theme that you will encounter when looking through the technical resource information. It is a task that many first time competitor may be eager to skip, although they do so at their own peril. Yes it is testing.

Regardless of the division in which you are competing if you decide not to test and evaluate the performance of your constructed vehicle then you are running a very large risk that you will encounter major issue when attending the events. You mentors in the design and build phase of the model creation should have talked to about what your vehicle needs to be able to achieve. For a boat the ability to run straight along the guide without submerging. For the both the kit car and student designed it is the ability to run along the track with no loss of guidance events in all levels of sun. Whilst these seem obvious it is important that you perform tests to ensure that these needs are satisfied otherwise it makes no difference if you have created the fastest ever vehicle as it will never complete the race.

Once the basics have been covered then you can move onto testing for satisfy other requirements of your division, the most demanding is finishing first but you can never tell what the actual conditions of the race will present on the day.

The design guides for each division include more information on tests that you should perform and the expected results of those tests. You are encouraged to review that information and the current regulations to ensure that on race day you have as few unexpected occurrences as possible.