Car Motors

07/06/2023 - New solar boat kits for 2023 are currently being assembled and set to be available shortly.

ΩModel Solar Car Motors

To help promote cutting edge innovation and yield the most efficient solar cars possible, the Tasmanian event and national finals impose no cost limit on any materials, components or construction methods. It may therefore come as no great surprise that teams will aim to seek out the very best motors available for their builds. A motor that converts the most solar panel power into a mechanical form to drive the car will give the greatest results.

Inexpensive brushed permanent magnet DC hobby motors and those recovered from various toys and gadgets like VCRs can be used to successfully power a model solar car but their performance is far from ideal. These have an iron core and are cheaply mass produced at less precise tolerances using sub optimal materials. With peak efficiencies somewhere in the region of only 50-60% this is ok for many household or battery powered applications but a real problem if looking to maximise model solar car performance. Such low efficiencies mean that half and sometimes even more of the solar panel power being lost in the form of heat, vibration and noise.

Top model solar cars are instead powered by a special type of high efficiency coreless brushed DC motor. The coreless architecture of these motors removes the need for an iron core and with it the associated hysteresis and eddy current losses. Coupled with a higher precision manufacturing process and using superior materials such as rare earth magnets and precious metal brushes, these motors can now reach maximum efficiencies of close to 90% at low powers. That’s an extra 30-40% more efficient than a cheap hobby motor.

It’s true that brushless DC motors similar to the ones seen in a lot of RC cars, planes, boats and drones can theoretically be a little more efficient due to the elimination of brush-commutator friction. These however tend to reach such figures when a lot more current and power is involved (ie hundreds or even thousands of Watts). We’re yet to see a suitable low power brushless option for the solar challenge but that’s not to say that one doesn’t now exist with more and more such products hitting the market every year. It’s also important to note that using such a motor will require additional control circuitry since commutation now needs to be carried out electronically rather than with the physical contact of motor brushes (hence the brushless name).

Three manufacturers of such high quality coreless motors are listed below.

These motors are typically designed for use in the medical, automation and aviation industries. They are also readily used in space exploration and feature prominently throughout satellites and interplanetary vehicles. NASA uses dozens of Maxon motors to power their Mars Rovers and carry out martian surface experiments.

All top car teams are now using the Faulhaber 2232 006SR 6V motor. This motor was first released back in 2005 and every winning entry from then on has used this motor. It is still without a doubt the TMSC’s recommended motor of choice and is ideally matched with the solar panels used in the Challenge. Its dimensions are very similar to the previously used 2233 4.5V from the late 90’s and early 2000’s but comes upgraded with rare earth magnets and a lower armature resistance that improve motor efficiency during the early stages of a solar car race.

Perhaps one other motor worth considering is the Maxon 118740 4.5V. This motor is however over twice the weight and cost of the Faulhaber and has yet to prove itself on a winning entry. Dynamometer tests have shown it to have the potential of marginally outperforming the 2232 in some conditions but any possible racing advantage will almost certainly be lost due to the extra weight of the motor.

If datasheets are anything to go by then Portescap’s Athlonix 22N78 319P 6V would also be in the mix. Specifications for this motor are very impressive but initial dynamometer testing has seen its real world performance fall well short of what was expected. Perhaps there was something wrong with the particular motor tested so consider this option at your own risk. We’re yet to see the motor feature on a car at an event and its smaller 1.5mm drive shaft is a mismatch for the 2mm pinion gears used on the 2232.

The datasheets of these three motors have been singled out below for easy download. A full list of the Faulhaber, Maxon and Portescap range can be viewed on their websites.

Faulhaber 2232 6V

Maxon RE 25 118740 4.5V

Portescap 22N78 319P 6V

Not only the winners but every single national finalist at the last eight (8) Australian-International Model Solar Car Challenges has used the Faulhaber 2232 6V motor. In previous years the odd cars out typically used the Maxon 118740 to varying levels of success, the best being a quarter final result. The image below shows the black 2232 with a few older motors alongside.

(L-R: Faulhaber 2233 4.5V, Faulhaber 2232 6V, Maxon 220404 9V, Maxon 221011 9V)

Here, the 221011 was used by Tasmania’s winner and third place at the 2005 national car event (the 221024 through-shaft version with identical performance was actually used). Like the Faulhaber 2233, this motor is now considered to be inferior to the 2232.

The 220404 9V also pictured above actually boasts a maximum efficiency that is 2% higher than the 2232 under the right conditions. This may have been a worthy rival in previous years when teams were able to select their own solar panel voltage. The lower voltage of the solar panels used today however mean this motor is not quite as well suited.

Fauhaber 2232 motors can be purchased from Scorpio Technology and this is the recommended supplier in most cases. They are listed in their 2023 solar catalogue for $122.38. They’re an expensive necessity but can then be reused for many subsequent years of competition if looked after with care. As an example, several of the motors running in a number of TMSC demonstration cars date back to 2006 and are still performing at the very highest level.   

Pending sufficient participant interest the TMSC have sometimes made bulk motor orders straight from Germany. Doing this can see costs drop to around $80-$90 each. Enough sponsor or government funding would ideally allow new motors to be distributed out to teams at no cost but this has not yet been possible in 2023. The TMSC also stocks a small number of spare Faulhabers that we may be able to lend out to a select few teams struggling to source their own for the competition. Please contact us if you’re interested in getting hold of one of these.

Motor Characteristics and Some Quick Calculations

Most solar car teams will simply purchase and use a 2232 6V motor without giving it much thought. All the top cars have been using it for many years so why go with anything else, right? Not many students, teachers or parent mentors will however go the lengths of understanding why they’re using this specific motor. In this next section we’ll cover a few of the basics.
 
Motors are designed for many different applications with different constraints in mind (ie size, cost, etc). Each particular model will have its own set of characteristics and its these characteristics that need to be looked at to determine whether a motor may be any good for a model solar car. You can set up a series of experiments to test for these if you have an unknown motor but otherwise such information can be found in its datasheet. It’s this datasheet that allows engineers to select the most suitable motor for an intended application. If you can’t find a detailed datasheet for a motor that you’re researching then chances are that it won’t be the high quality product needed to power your solar car. You also need to be careful that the figures being stated are accurate. Less creditable manufacturers sometimes give values that make their motors appear much better than they actually are in reality.   
 
Top motor manufacturers like Faulhaber and Maxon have very reliable documentation for all their products but what do all those values really mean? What makes the 2232 6V motor so good for a model solar car? It essentially comes down how the heat and frictional losses balance out but let’s take a brief look at a few of the key parameters in its datasheet below.
 
Firstly, don’t get too caught up with the Nominal voltage when it comes to the solar challenge. We’re not driving the motor with a power supply or battery pack which are capable of delivering much more power than our event solar panels.
 
These motors will happily operate at half or double the specified voltage as long as the speed doesn’t get so high as to fly apart internally. Most coreless motors will comfortably spin up to 20000rpm and beyond and you can do a quick calculation of the free running motor speed by simply using the Speed constant. This won’t be 100% accurate as it doesn’t factor in the frictional losses but should give a reasonable estimate, especially for motors with a low No-load current like the 2232 motor. For example, the 2232 6V has a speed constant of 1190rpm/V so if you wanted to keep it under 20000rpm then the voltage should be no greater than about 20000/1190 = 16.8V. In past years teams would race with their own solar panels and many chose voltages in the 12-14V range in combination with the 2232 6V motor.
 
The first key parameter to always look at initially is the peak motor Efficiency. This gives you some idea of what the motor is capable of at first glance. If the efficiency is anything under 85% then it’s probably not even worth pursuing for a model solar car as you’re going to be losing too much power. The 6V version of the 2232 has a maximum efficiency of 87% which is about as high as Faulhaber go. Maxon’s 118740 also comes in at 87% while their 220405 can reach an even better 89%. Portescap’s Athlonix 22N78 319P goes even higher and can supposedly reach 90%.
 
The next step is to consider the Terminal resistance of the motor coupled with your solar panel current and power. The competition panels we currently use in 2023 have a maximum power of 5.6W, a voltage of approximately 7V, current of 0.8A and the 2232 6V motor has a terminal resistance of 0.81Ω. Using Ohm’s Law P = I2R this is going to result in a power loss of 0.82 x 0.81 = 0.52W in the form of heating up the copper windings. This comes to about 0.52 / 5.6 x 100 = 9.3% and takes the efficiency down to 100 – 9.3 = 91.8% with some frictional losses yet to be factored in. What about the Maxon 220405 with the higher 89% peak efficiency? This instead has a terminal resistance of 3.36Ω so using Ohm’s Law again we get a power loss 0.82 x 3.36 = 2.15W. This comes to 2.15 / 5.6 x 100 = 38% and would take the total motor efficiency down to less 100 – 38 = 62% which is no where near that peak. So what gives? It really comes down to that terminal resistance in combination with the solar panel output. You would need a different panel more suited to the motor to reach that 89% peak. For example, if the same 5.6W panel instead had an output of 14V and 0.4A then things would already be looking much better for the 220405. Here, the heat loss would be reduced to 0.42 x 3.36 = 0.54W or 0.54 / 5.6 x 100 = 9.6%.