Brushless DC Motor
Brushless DC motors were developed from conventional brushed DC
motors with the availability of solid state power semiconductors. So,
why do we discuss brushless DC motors in a chapter on AC motors?
Brushless DC motors are similar to AC synchronous motors. The major
difference is that synchronous motors develop a sinusoidal back EMF, as
compared to a rectangular, or trapezoidal, back EMF for brushless DC
motors. Both have stator created rotating magnetic fields producing
torque in a magnetic rotor.
Synchronous motors are usually large multi-kilowatt size, often with
electromagnet rotors. True synchronous motors are considered to be
single speed, a submultiple of the powerline frequency. Brushless DC
motors tend to be small-- a few watts to tens of watts, with permanent
magnet rotors. The speed of a brushless DC motor is not fixed unless
driven by a phased locked loop slaved to a reference frequency. The
style of construction is either cylindrical or pancake. (Figures
and below)
Cylindrical construction: (a) outside rotor, (b)
inside rotor.
The most usual construction, cylindrical, can take on two forms
(Figure above). The most common cylindrical style is with the rotor on
the inside, above right. This style motor is used in hard disk drives.
It is also possible the put the rotor on the outside surrounding the
stator. Such is the case with brushless DC fan motors, sans the shaft.
This style of construction may be short and fat. However, the direction
of the magnetic flux is radial with respect to the rotational axis.
Pancake motor construction: (a) single stator, (b)
double stator.
High torque pancake motors may have stator coils on both sides of the
rotor (Figure above-b).
Lower torque applications like floppy disk drive motors suffice with a
stator coil on one side of the rotor, (Figure above-a). The direction of
the magnetic flux is axial, that is, parallel to the axis of rotation.
Hall effect magnetic sensors
The commutation function may be performed by various shaft position
sensors: optical encoder, magnetic encoder (resolver, synchro, etc), or
Hall effect magnetic sensors. Small inexpensive motors use Hall effect
sensors. (Figure below) A Hall effect sensor is a semiconductor device
where the electron flow is affected by a magnetic field perpendicular to
the direction of current flow. It looks like a four terminal variable
resistor network. The voltages at the two outputs are complementary.
Application of a magnetic field to the sensor causes a small voltage
change at the output. The Hall output may drive a comparator to provide
for more stable drive to the power device. Or, it may drive a compound
transistor stage if properly biased. More modern Hall effect sensors may
contain an integrated amplifier, and digital circuitry. This 3-lead
device may directly drive the power transistor feeding a phase winding.
The sensor must be mounted close to the permanent magnet rotor to sense
its position.
Hall effect sensors commutate 3-φ brushless DC
motor.
The simple cylindrical 3-φ motor Figure above is commutated by a Hall
effect device for each of the three stator phases. The changing position
of the permanent magnet rotor is sensed by the Hall device as the
polarity of the passing rotor pole changes. This Hall signal is
amplified so that the stator coils are driven with the proper current.
Not shown here, the Hall signals may be processed by combinatorial logic
for more efficient drive waveforms.
The above cylindrical motor could drive a hard drive if it were
equipped with a phased locked loop (PLL) to maintain constant speed.
Similar circuitry could drive the pancake floppy disk drive motor
(Figure below). Again, it would need a PLL to maintain constant speed.
Brushless pancake motor
The 3-φ pancake motor (Figure above) has 6-stator poles and 8-rotor
poles. The rotor is a flat ferrite ring magnetized with eight axially
magnetized alternating poles. We do not show that the rotor is capped by
a mild steel plate for mounting to the bearing in the middle of the
stator. The steel plate also helps complete the magnetic circuit. The
stator poles are also mounted atop a steel plate, helping to close the
magnetic circuit. The flat stator coils are trapezoidal to more closely
fit the coils, and approximate the rotor poles. The 6-stator coils
comprise three winding phases.
If the three stator phases were successively energized, a rotating
magnetic field would be generated. The permanent magnet rotor would
follow as in the case of a synchronous motor. A two pole rotor would
follow this field at the same rotation rate as the rotating field.
However, our 8-pole rotor will rotate at a submultiple of this rate due
the the extra poles in the rotor.
The brushless DC fan motor (Figure below) has these feature:
Brushless fan motor, 2-φ.
- The stator has 2-phases distributed between 4-poles
- There are 4-salient poles with no windings to eliminate zero
torque points.
- The rotor has four main drive poles.
- The rotor has 8-poles superimposed to help eliminate zero torque
points.
- The Hall effect sensors are spaced at 45o physical.
- The fan housing is placed atop the rotor, which is placed over the
stator.
The goal of a brushless fan motor is to minimize the cost of
manufacture. This is an incentive to move lower performance products
from a 3-φ to a 2-φ configuration. Depending on how it is driven, it may
be called a 4-φ motor.
You may recall that conventional DC motors cannot have an even number
of armature poles (2,4, etc) if they are to be self-starting, 3,5,7
being common. Thus, it is possible for a hypothetical 4-pole motor to
come to rest at a torque minima, where it cannot be started from rest.
The addition of the four small salient poles with no windings
superimposes a ripple torque upon the torque vs. position curve. When
this ripple torque is added to normal energized-torque curve, the result
is that torque minima are partially removed. This makes it possible to
start the motor for all possible stopping positions. The addition of
eight permanent magnet poles to the normal 4-pole permanent magnet rotor
superimposes a small second harmonic ripple torque upon the normal
4-pole ripple torque. This further removes the torque minima. As long as
the torque minima does not drop to zero, we should be able to start the
motor. The more successful we are in removing the torque minima, the
easier the motor starting.
The 2-φ stator requires that the Hall sensors be spaced apart by 90o
electrical. If the rotor was a 2-pole rotor, the Hall sensors would be
placed 90o physical. Since we have a 4-pole permanent magnet
rotor, the sensors must be placed 45o physical to achieve the
90o electrical spacing. Note Hall spacing above. The majority
of the torque is due to the interaction of the inside stator 2-φ coils
with the 4-pole section of the rotor. Moreover, the 4-pole section of
the rotor must be on the bottom so that the Hall sensors will sense the
proper commutation signals. The 8-poles rotor section is only for
improving motor starting.
2-φ push-pull drive
Brushless DC motor 2-φ push-pull drive.
In Figure above, the 2-φ push-pull drive (also known as 4-φ drive)
uses two Hall effect sensors to drive four windings. The sensors are
spaced 90o electrical apart, which is 90o physical
for a single pole rotor. Since the Hall sensor has two complementary
outputs, one sensor provides commutation for two opposing windings.
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