The internals of a hobby DC motor

Motor Selection

Motors for Model Solar Competition

There are many small, low voltage, direct current motors available however not all are suitable for use in the mode 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.     

Within the VMSVC competition there are what may be considered four divisions or different competitions, each with differing motor requirements.

  1. Student designed cars, in this event there is no limitation placed on the motor used it is entirely up to the team.
  2. 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.
  3. Junior boats, here the motor used is at the discretion of the team as long as it conforms to the regulations which specify it must be commercially available in Australia at a retail price below that specified in the regulations. Check the current regulations for price details.
  4. Advanced boats, as for the student designed cars in this event there is no limitation placed on the motor used it is entirely up to the team.

Within the limits of the competition regulations you may choose any motor you wish. Your choices however are not particularly wide. There are some basic motor features that limit the choices.Scorpio Technology SM403

  • 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. To date 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, in any case constant development will probably produce one eventually. 

Amid all this confusion what motors do most competitors use. This is probably a good starting point since many years of trial and error and development have influenced this.

Starting with student designed cars, in this competition we record the motor used at scrutineering so have a very good idea as to what motors are being used. 

Faulhaber 2232U006RIn over 90% of cars we see the Faulhaber 2232 6 volt motor. Maxon motors make up most of the remainder.

The Faulhaber and Maxon motors are both proper industrial motors with efficiencies around 86%. From dynamometer tests conducted on both these brands there is nothing in their performance that would suggest one is better than the other. They both perform to their advertised specifications. The range of motors available from Maxon is wider than from Faulhaber. It is most likely that the main reason for the Faulhaber being so prevalent is its increased availability for competitors to acquire with relative ease.

Tests have been performed on motors from two other manufacturers with claimed performance equivalent to the Faulhaber and Maxon. Neither of these motors performed to the quoted specifications one was over 20% below the claimed performance, while the other was quoted to have a no load current the same as the Faulhaber but when tests actually had 4 times this no load current and was so poorly constructed that it exhibited serious vibration at high speed along with low efficiency. The message here is one needs to be careful in selecting motors from relatively unknown manufacturers.

Faulhaber 2232 disassebledAdvanced boats have no motor restrictions imposed on them in the regulations. Many use the Faulhaber motor with or without a gear reduction. It has been stated that some boats have used a motor and gearbox intended for model aircraft. Again tests performed on one of these units and it was found that its no load current to be equivalent to 40% of the solar array output. This means that the boat would not even run below this Sun level. Unfortunately, no information on the manufacturer of this unit is available but use its performance test results as a warning to test any motors you intend to use to verify they perform to specification or at least to a level that gives reasonable performance.  

Junior boats have price and availability restrictions on the motor that can be used listed in the regulations. Any hobby type motor commercially available in Australia with a retail price below that specified dollar value is acceptable. Again caution is required in selecting a motor, many have extremely low efficiency resulting in poor performance when used on the day. 

The motors Scorpio Technology sell (SM 403) for use in the junior boat competition are used by many competitors. These motors are the result of several years testing motors from many suppliers trying to find a motor that performs as best as possible for the junior boats. Unable to find a suitable motor from the then current units available Scorpio Technology has had motors specially manufactured for them. These motors have characteristics that match Scorpio’s solar panels and propellers.