torque by shaft speed (RPM). For a car the power required to drive it can be
calculated by multiplying the drive force applied to the car by the cars
velocity. (When the car is moving at a constant speed the retarding force is the
sum of aerodynamic drag, rolling resistance which includes wheels and guides
and any gravity force such as when on the hill.)- Ignoring friction losses the power from a motor can be converted into the
same power at any nominated RPM by the use of gearing. (Note the torque
will be altered ) - For the DC motors we are using torque varies directly with motor current
(amps). - For the DC motors we are using RPM varies directly with voltage.
To obtain the best performance from any car we need to take the maximum amount
of energy available from the solar panel and use as much of it as possible to drive
the car. To achieve this, a high efficiency motor used in conjunction with a solar
panel whose output current and voltage suits the motor characteristics is essential.
A gear ratio that matches the available motor power to the power required to
propel the car at the highest speed possible is just as important.
The statement above while accurate, is of little help unless we understand the
implications and know what to do in order to achieve the requirements. I hope the
following explanation which is broken down into each major section will make it clear.
Maximum energy from solar panel: Refer to Graph 1 where the power output from the
panel is shown. The maximum power point is clearly visible. This is where we need to
operate.
High efficiency motor: Firstly let us be clear about what we mean by efficiency. For a
motor the efficiency is the power delivered by the motor shaft divided by the input power
to the motor. There is no point in using a motor that is say 50% efficient when at best
we will only get out 50% of the power we put in. We are throwing away all this power
without even having a chance to use it.
Motors of around 85% efficiency although expensive are readily available. But do
remember the motor will not always be operating at its maximum efficiency point.
Panel voltage and current match to motor: Remember power (P) is voltage (V)
multiplied by current in amps (I). Example a power of 10 watts can be obtained from 10
amps at 1 volt or 0.1 amps at 100 volts. Neither of these combinations is of the slightest
use to drive our solar car.
Let me elaborate. Assuming the use of a Faulhaber 2232 6 volt motor which has a rotor
resistance of 0.8 ohms. (this motor type was used by every car in the 2009 Victorian
competition.)