Brushless
Direct Current (BLDC) motors are one of the motor types rapidly gaining
popularity. BLDC motors are used in industries such as Appliances, Automotive,
Aerospace, Consumer, Medical, Industrial Automation Equipment and
Instrumentation.
As the
name implies, BLDC motors do not use brushes for commutation; instead, they are
electronically commutated. BLDC motors have many advantages over brushed DC
motors and induction[a1]
motors. A few of these are:
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Better
speed versus torque characteristics
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High
dynamic response
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High
efficiency
?
Long
operating life
?
Noiseless
operation
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Higher
speed ranges
In
addition, the ratio of torque delivered to the size of the motor is higher,
making it useful in applications where space and weight are critical factors.
In
this application note, we will discuss in detail the construction, working
principle, characteristics and typical applications of BLDC motors. Refer to Appendix
B: “Glossary” for a glossary of terms commonly used when describing BLDC
motors.
CONSTRUCTION
AND OPERATING PRINCIPLE
BLDC
motors are a type of synchronous motor. This means the magnetic field generated
by the stator and the magnetic field generated by the rotor rotate at the same
frequency. BLDC motors do not experience the “slip” that is normally seen in
induction motors.
BLDC motors come in
single-phase, 2-phase and 3-phase configurations. Corresponding to its type,
the stator has the same number of windings. Out of these, 3-phase motors are
the most popular and widely used. This application note focuses on 3-phase
motors.
Stator
The
stator of a BLDC motor consists of stacked steel laminations with windings
placed in the slots that are axially cut along the inner periphery (as shown in
Figure 3). Traditionally, the stator resembles that of an induction motor;
however, the windings are distributed in a different manner. Most BLDC motors
have three stator windings connected in star fashion. Each of these windings
are constructed with numerous coils interconnected to form a winding. One or
more coils are placed in the slots and they are interconnected to make a
winding. Each of these windings are distributed over the stator periphery to
form an even numbers of poles.
There
are two types of stator windings variants: trapezoidal and sinusoidal motors.
This differentiation is made on the basis of the interconnection of coils in the
stator windings to give the different types of back Electromotive Force (EMF).
Refer to the “What is Back EMF?” section for more information.
As
their names indicate, the
trapezoidal motor gives a [a2] back
EMF in trapezoidal fashion and the sinusoidal motor’s back EMF is sinusoidal,[a3]
as shown in Figure 1 and Figure 2. In addition to the back EMF, the phase
current also has trapezoidal and sinusoidal variations in the respective types
of motor. This makes the torque [a4] output
by a sinusoidal motor smoother than that of a trapezoidal motor. However,
this comes with an extra cost, as the sinusoidal motors take extra winding
interconnections because of the coils distribution on the stator periphery,
thereby increasing the copper intake by the stator windings.
[a5] Depending
upon the control power supply capability, the motor with the correct voltage
rating of the stator can be chosen. Forty-eight volts, or less voltage rated
motors are used in automotive, robotics, small arm movements and so on. Motors
with 100 volts, or higher ratings, are used in appliances, automation and in
industrial applications.
