The speed of rotation of a brushed motor is determined by the load on the motor, the
amount of current passing through the coils, the number of turns or winds in the coils
and the operating voltage. Motors running at high voltage are generally more efficient
because less current is needed to generate power, so less energy is lost as heat.
Also, motors with fewer turns have more power and run faster, but are generally less
efficient than motors with more turns.
Brushed motors are easy to manufacture, but are inefficient to operate. Only about
75% of the electric enery entering the motor is converted to mechanical energy. The
rest is converted to heat by the electric resistance of the brushes and by conflicting
magnetic fields inside the motor.
O-Brushed eff graph
As shown in Fig. 3-2, which is a graph of motor efficiency at various motor speeds,
brushed motors are most efficient within a very small speed range. Outside of this
range efficiency drops quickly, and most of the electric energy entering the motor is
converted to heat. When a motor operates outside its efficient speed range the motor
body becomes very hot (sometimes too hot to touch), and the motor coils can burn
out.
Adjustable Timing
Motor coils cannot energize instantly when current is applied. This poses a problem
for brushed motors running at high speed because the coils energize at the wrong
time and generate conflicting magnetic fields that push on the permanent magnets
when they should be pulling (and vice versa). Some brushed motors, such as the
Electro-Fusion
motor on your
Corona
helicopter, have adjustable timing to address
Op er a tor's Guide
Electric Motors
3-4
LITE MA CHINES
Figure 3-2.
