馬達有分直流交流、有刷無刷。
基本元件包括定子 (stator)、轉子 (rotor，也就是電樞 (armature))、電刷 (brushes)、及換相器 (commutator)。由於電刷跟換相器是機械式的，用久了會磨損。
Permanent magnet
Shunt-Wound: the field coil in parallel (shunt) with the armature
Series-Wound: the field coil in series with the armature
Compound-Wound: employ both a series and a shunt field.
Drive Circuits and Speed Control
Reference
換相不用電刷，改用電子式。
advantages over brushed DC motors and induction motors:
• Better speed versus torque characteristics
• High dynamic response
• High efficiency
• Long operating life
• Noiseless operation
• 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.
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.
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).
As their names indicate, the trapezoidal motor gives a
back EMF in trapezoidal fashion and the sinusoidal
motor’s back EMF is sinusoidal, 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
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.
Rotor
The rotor is made of permanent magnet and can vary from two to eight pole pairs with alternate North (N) and South (S) poles.
magnetic materials for rotor
Hall Sensors
The commutation of a BLDC motor is controlled electronically. To rotate the BLDC motor, the stator windings should be energized in a sequence. It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence. Rotor position is sensed using Hall effect sensors embedded into the stator.
Most BLDC motors have three Hall sensors embedded into the stator on the non-driving end of the motor. Whenever the rotor magnetic poles pass near the Hall sensors, they give a high or low signal, indicating the N or S pole is passing near the sensors. Based on the combination of these three Hall sensor signals, the exact sequence of commutation can be determined.
Figure 5 shows a transverse section of a BLDC motor
with a rotor that has alternate N and S permanent magnets.
Hall sensors are embedded into the stationary part
of the motor. Embedding the Hall sensors into the stator
is a complex process because any misalignment in
these Hall sensors, with respect to the rotor magnets,
will generate an error in determination of the rotor position.
To simplify the process of mounting the Hall
sensors onto the stator, some motors may have the Hall
sensor magnets on the rotor, in addition to the main rotor
magnets. These are a scaled down replica version of the
rotor. Therefore, whenever the rotor rotates, the Hall
sensor magnets give the same effect as the main magnets.
The Hall sensors are normally mounted on a PC
board and fixed to the enclosure cap on the non-driving
end. This enables users to adjust the complete assembly
of Hall sensors, to align with the rotor magnets, in
order to achieve the best performance.
Based on the physical position of the Hall sensors,
there are two versions of output. The Hall sensors may
be at 60° or 120° phase shift to each other. Based on
this, the motor manufacturer defines the commutation
sequence, which should be followed when controlling
the motor.
Theory of Operation
Each commutation sequence has one of the windings
energized to positive power (current enters into the
winding), the second winding is negative (current exits
the winding) and the third is in a non-energized condition.
Torque is produced because of the interaction
between the magnetic field generated by the stator
coils and the permanent magnets. Ideally, the peak
torque occurs when these two fields are at 90° to each
other and falls off as the fields move together. In order
to keep the motor running, the magnetic field produced
by the windings should shift position, as the rotor
moves to catch up with the stator field. What is known
as “Six-Step Commutation” defines the sequence of
energizing the windings. See the “Commutation
Sequence” section for detailed information and an
example on six-step commutation.
TORQUE/SPEED CHARACTERISTICS
Figure 6 shows an example of torque/speed characteristics.
There are two torque parameters used to define
a BLDC motor, peak torque (TP) and rated torque (TR).
(Refer to Appendix A: “Typical Motor Technical
Specification” for a complete list of parameters.) During
continuous operations, the motor can be loaded up
to the rated torque. As discussed earlier, in a BLDC
motor, the torque remains constant for a speed range
up to the rated speed. The motor can be run up to the
maximum speed, which can be up to 150% of the rated
speed, but the torque starts dropping.
Applications that have frequent starts and stops and
frequent reversals of rotation with load on the motor,
demand more torque than the rated torque. This
requirement comes for a brief period, especially when
the motor starts from a standstill and during acceleration.
During this period, extra torque is required to overcome
the inertia of the load and the rotor itself. The
motor can deliver a higher torque, maximum up to peak
torque, as long as it follows the speed torque curve.
Refer to the “Selecting a Suitable Motor Rating for
the Application” section to understand how to select
these parameters for an application.
COMPARING BLDC MOTORS TO OTHER MOTOR TYPES
Compared to brushed DC motors and induction
motors, BLDC motors have many advantages and few
disadvantages. Brushless motors require less maintenance,
so they have a longer life compared with
brushed DC motors. BLDC motors produce more output
power per frame size than brushed DC motors and
induction motors. Because the rotor is made of permanent
magnets, the rotor inertia is less, compared with
other types of motors. This improves acceleration and
deceleration characteristics, shortening operating
cycles. Their linear speed/torque characteristics produce
predictable speed regulation. With brushless
motors, brush inspection is eliminated, making them
ideal for limited access areas and applications where
servicing is difficult. BLDC motors operate much more
quietly than brushed DC motors, reducing
Electromagnetic Interference (EMI). Low-voltage
models are ideal for battery operation, portable
equipment or medical applications.
[待續]
References