As the motor begins to rotate it generates a back EMF (voltage). The faster it spins, the
higher the back EMF becomes (this can be demonstrated by spinning a motor by hand
with a voltmeter across its output). When the motor speed has stabilized there will only
be a small voltage difference between the supply and the motor back EMF. This small
voltage will drive a small current through the motor producing the power lost in friction
and electrical losses. (In the above description, the motor is running free i.e. not driving
any load.)
When the motor is loaded, the load slows the speed of rotation of the motor. This reduces
the back EMF, resulting in a larger difference between the supply voltage and the motor
voltage. This increase in voltage drives more current through the motor which increases
its power output. The motor speed will stabilize at a new balance point.
- MOTOR AND PANEL IN COMBINATION
A motor that wants more current to drive its load than the panel can supply will pull the
panel voltage down and reduce the power available to drive the car. The car will then run
slower. Depending on the magnitude of the load, the panel voltage could be pulled to
near zero and the panel "STALLED". The panel and motor will stay in this state till the
motor load is reduced or the panel produces sufficient current to get the motor moving
again. (Increasing current will increase the torque produced on the motor shaft.)
Conversely, if the load on the motor is low, it will not take all the current the panel is
capable of producing and will not be using all the power available from the panel to drive
the car. The car will be running slower than its potential. This is why it is critically
important to match the motor load to the panel output.
In real life this means that for a particular light level and car speed there is a gear ratio
from motor to wheel that allows the use of all the power produced by the panel.
Consider a car running at exactly this power matched position:
· an increase in light level means that more current is available from the panel at a
very slightly higher voltage. The very small voltage increase will cause a very
slight increase in car speed and air drag. However the majority of the extra
current available has not been used. To use this extra available current we need
either to increase the motor RPM which requires more voltage which we do not
have or to increase the motor load by changing the drive ratio ie. a higher ratio. If
we do not change the gear ratio there will be more power available from the panel
than is being used. This "available" extra power that is not being used is
effectively wasted. If this extra power was used the car would go faster.
· A decrease in light level or increase in load such as a head wind or climbing the
hill will mean the motor will not have sufficient current available from the
panel to provide the power required to drive the car. The panel voltage will