Abstract :
A BLDC
motor has permanent magnets which rotate and a fixed armature, eliminating the problems of
connecting current to the moving armature. An electronic controller replaces
the brush/commutator assembly of the brushed DC motor, which continually
switches the phase to the windings to keep the motor turning. The controller
performs similar timed power distribution by using a solid-state circuit rather
than the brush/commutator system.
A
microprocessor-controlled BLDC motor powering a micro remote-controlled
airplane. This external rotor
motor weighs 5 grams, consumes approximately 11 watts (15 millihorsepower)
and produces thrust of more than twice the weight of the plane.
Brushless
DC motors (BLDC motors, BL motors) also known as electronically commutated
motors (ECMs, EC motors) are synchronous electric motors
powered by direct-current (DC) electricity and having
electronic commutation systems, rather than mechanical commutators and brushes. The current-to-torque and
voltage-to-speed relationships of BLDC motors are linear.
BLDC motors
may be described as stepper motors, with fixed permanent magnets
and possibly more poles on the stator
than the rotor, or reluctance motors. The latter may be without
permanent magnets, just poles that are induced on the rotor then pulled into
alignment by timed stator windings. However, the term stepper motor tends to be
used for motors that are designed specifically to be operated in a mode where
they are frequently stopped with the rotor in a defined angular position; this
page describes more general BLDC motor principles, though there is overlap.
BLDC motors
offer several advantages over brushed DC motors, including more torque per
weight and efficiency, reliability, reduced
noise, longer lifetime (no brush and commutator erosion), elimination of
ionizing sparks from the commutator, more power, and overall reduction of electromagnetic interference (EMI). With
no windings on the rotor, they are not subjected to centrifugal forces, and
because the windings are supported by the housing, they can be cooled by
conduction, requiring no airflow inside the motor for cooling.
This in turn
means that the motor's internals can be entirely enclosed and protected from
dirt or other foreign matter. The maximum
power that can be applied to a BLDC motor is exceptionally high, limited almost
exclusively by heat, which can weaken the magnets. (Magnets demagnetize at high
temperatures, the Curie point, and for neodymium-iron-boron magnets this
temperature is lower than for other types.)
A BLDC motor's
main disadvantage is higher cost, which arises from two issues. First, BLDC
motors require complex electronic speed controllers to run. Brushed DC
motors can be regulated by a comparatively simple controller, such as a rheostat
(variable resistor). However, this reduces efficiency because power is wasted
in the rheostat. Second, some practical uses have not been well developed in
the commercial sector. For example, in the Radio Control (RC) hobby, even
commercial brushless motors are often hand-wound while brushed motors use
armature coils which can be inexpensively machine-wound.
BLDC motors
are often more efficient at converting electricity into mechanical power than
brushed DC motors. This improvement is largely due to the absence of electrical
and friction losses due to brushes. The enhanced efficiency is greatest in the
no-load and low-load region of the motor's performance curve. Under high
mechanical loads, BLDC motors and high-quality brushed motors are comparable in
efficiency.