BLY’s motor interface wires are terminations for the three
stator phases. The remaining five wires service the BLDC
motor’s built-in Hall effect sensors. Here’s how our BLY
BLDC motor is wired:
- Phase A: Yellow
- Phase B: Red
- Phase C: Black
Hall Effect Sensors:
- Hall Effect Supply: Red
- Hall Effect Sensor A: Blue
- Hall Effect Sensor B: Green
- Hall Effect Sensor C: White
- Hall Effect Ground: Black
If we want to deploy the BLDC motor is a
standard manner, the Hall effect sensors are used in
the commutation process. In a sensorless BLDC motor
implementation, we’re only interested in the motor phase
wiring. Sensorless motor control commutation is a product
of the BLDC motor’s back electromotive force (BEMF),
which is produced in the motor’s stator windings as a
result of the movement of the rotor’s permanent magnets
past the stator coils. Before we formulate a method of
getting rid of the Hall effect sensors, it would be helpful to
understand how a BLDC motor works with them.
BLDC Operation 101
Although we have identified only three major coils in
our BLDC motor, a typical three-phase BLDC motor contains
a multi-coiled stator and a permanent magnet rotor. The
more stator coils one can cram into the stator assembly of
a BLDC motor, the smaller the rotational steps. Smaller
rotational steps result in less torque ripple. The same
principle applies to the rotor. A BLDC motor’s rotor supports
an even number of permanent magnets. The more magnetic
poles associated with the rotor, the smaller the rotational
steps. You know the rest. Regardless of how many stator
coils and rotor magnets a BLDC motor has, we can still
gain an understanding of how a BLDC motor works by
examining only three coils.
Like a stepper motor, a BLDC motor commutates
according to a predetermined coil activation sequence.
That’s about where the similarity ends. Stepper motors have
higher step counts and require a higher operating voltage.
BLDC motors are designed to operate with Hall effect
commutation and variable voltage drive. Precise rotor
alignment is not something a BLDC is particularly good at.
Conversely, a stepper wants a constant voltage drive and is
designed for precise angular positioning of its rotor.
A three phase BLDC motor has six discrete states of
commutation. When the correct coil activation sequence is
presented to a BLDC, the motor shaft will rotate. Reversing
a valid BLDC motor coil activation sequence will reverse the
Motors and Motor Drivers
Figure 1. This reminds me of a stepper motor coil activation
chart. An application that uses the Hall-Effect sensors would
trigger a commutation at every sensor logic level change.
direction of the shaft’s rotation. I have labeled the BLDC
motor coils A, B, and C in Figure 1. As you can see, each
commutation state consists of one coil driven positive, one
coil grounded, and one coil open. This type of commutation
is called block commutation.
Let’s walk through a BLDC motor commutation
sequence visually. Figure 2 is a simplified view of a three-phase BLDC motor’s coils and its rotor. The simulated rotor
is centered in the figure as an arrow. The pointed end of
the arrow is positive while the opposite end of the arrow
is negative. This positive/negative arrow arrangement
simulates the magnetic fields generated by the rotor’s
permanent magnets. The stator coils in Figure 2 are shown
as rectangles, which are spaced at 120° intervals. The
arrow (rotor) will move from commutation state to adjacent
commutation state depending on the direction of the
current applied to the energized stator coils.
Reference the coil voltage drive levels in Figure 1 and
apply them to Figure 2. Commutation state 1 is a product
of stator coil A being driven positively, stator coil B being
driven negatively, and stator coil C floating. Driving stator
coil B negatively actually means that stator coil B is grounded.
Thus, the stator coil current is flowing between stator coils
A and B while stator coil C is electrically disconnected. The
positive magnetic pole of the rotor will be attracted to the
negatively-charged stator coil B while the negative magnetic
pole of the rotor will be drawn towards the positively-charged stator coil A. If we specify the positive pole of the
rotor as our commutation state reference, the positive pole
of the BLDC motor rotor is now positioned in BLDC motor
commutation state 1.
Let’s transpose the next set of coil drive voltages in
Figure 1 to our imaginary BLDC motor in Figure 3. Moving
from left to right in Figure 1, the coil drive voltage level for
stator coil A remains positive. Stator coil B becomes the
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