4QD have for many years, supplied motor speed controllers to the Greenpower battery powered electric car competition and our controllers are highly efficient. The event has been won more than once using our controllers.We have written this page to assist would be-entrants in choosing a controller for the event, but in essence our recommendation for new teams is our Porter-5. For the teams that are chasing that last 1% of performance we have recently launched the Porter 10XX.
Interpreting the rules
There is a lot of information including the rules, tips etc at the official Greenpower site which you can find here.
Motor and batteries are fixed, so the challenge is how to make best use of the available energy. This means building an efficient design, then getting the most out of the batteries.
Mechanical efficiency; The competition is all about the mechanical design of an efficient vehicle. This, above everything else, will define the result. The areas you must look at are:
Aerodynamic efficiency; Wind resistance increases with the square of the speed. To minimise aero losses the ideal approach is to go as slowly as possible consistent with just running out of battery power exactly at the end of the event [but try telling the driver that!].
Rolling efficiency; If the vehicle does not roll very freely, it is wasting power. Narrow, high pressure tyres will help, as will high quality, well adjusted and lubricated bearings.
Mechanical drive train efficiency; The motor’s mechanical output has to be transferred to the wheels. Gear trains are not 100% efficient and can loose quite a lot of power. Chains are good, but again must be well adjusted and lubricated.
Energy usage; Getting the most out of the batteries is paramount. Plot the discharge curves for your batteries when supplying the current that your vehicle actually takes. Then figure out how you can get that amount of energy to last to the end of the event. Options are a] set your maximum speed appropriately. b] have a monitoring system that lets you monitor your energy use and adjust your speed accordingly. Test each battery, and use the best for the races and the worst ones for practise.
Motor efficiency; The motor’s performance curves (which are available on the official site) include a graph of its efficiency. For this sort of contest you need to run as close as possible to maximum efficiency: (73% at 1800 RPM pulling 18A), and use the gearing to keep the motor running near it’s optimum speed. The speed controller can be used for a smooth start and then to adjust the speed if required during the race for tactical reasons.
Efficiency curves for other voltages are not quoted, but one can expect the same sort of shape of curve – at low motor currents efficiency drops off because of brush pressure and other mechanical friction. At high currents motor heating causes high losses.
Cooling the motor will help keep it’s efficiency up. Additional heatsinks / fins will help, as will ducting some airflow onto it. PC cooling fans have been used but they take power that could be used for going forwards. Some contestants have powered fans from a PP9 battery.
Most of the heat from a motor is generated in the windings on the armature, the shortest thermal path for this heat to escape along is the motor shaft itself. Fitting a fan blade to the shaft is a common ploy but beware that a high blade pitch fan rotating at 1700 rpm will use a lot of energy in shifting the air. Keep the pitch low and paint it black.
Do I need a speed controller? You can get by without one, and if budgets are really tight then you can start without one. But once you start to compete with other teams you will find the ability to regulate the power smoothly a significant advantage. We have also seen motors with broken casings caused by the sudden application of maximum torque when a relay closes at zero speed! If you are going to use a relay then be sure to give the car a push start.
Controller efficiency; Speed controllers are high efficiency devices having efficiency figures in the high 90% area. To be honest, it’s the mechanical design of the machine that’s important, not the controller. Unless your mechanical design is extremely good, whether the controller is 95% or 98% efficient is not important.
As a comparison, the motor has a resistance of 198 milliOhms – that’s when cold, copper resistance increases as the motor heats up. The MOSFETS used in our Porter 5 have a resistance of a mere 4.8 milliOhms, some 40 times less than the motor, so negligible by comparison. But they too, increase their resistance as they heat up, so pay some attention to keeping the controller cool.
Current limiting; We are sometimes asked about limiting the current to maximise battery life. Which current do you wish to limit – current drawn from the battery or current delivered to the motor? Since all the power drawn from the battery is delivered to the motor, motor current is not the same as battery current except at full speed. Full current can be delivered to the motor at low speeds, without drawing high battery current! So limiting battery current has no point. When using a speed controller you simply do not get the initial high surge battery current that happens without a controller. The motor will only draw the current it needs, depending on the mechanical load. Limiting the motor current may mean it cannot draw the current it needs for the acceleration or whatever mechanical load it has at that point. So there is no point in a motor current limit either.
4QD controllers do have a current limit built-in however, but to a current level that is safe for the controller. For the Porter-5 the motor current is limited to approximately 60 amps.