Motors and Motor Drivers
Figure 2. The Hall effect sensors
(which are not depicted here) are
mounted in such a way as to sense
the position of the rotor’s magnetic
fields in relation to the magnetic fields
emanated by the stator coils.
floater and the stator coil C drive voltage level transitions
from floating to negative. This stator coil drive voltage
pattern forces the positive field of the rotor to move
towards the negatively charged stator coil C. The negative
field of the rotor is still under the influence of positively-charged stator coil A.
I think you get the idea. However, to set the BLDC
commutation concept in stone, we’ll walk through one
more commutation using Figure 4. Again, moving 60° to
the right in the Figure 1 commutation chart, we assign
stator coil A as the floater. Stator coil B is no longer the
floater and moves to a positive drive state. The coil current
direction in stator coil C remains as it was in the last
commutation state illustrated in Figure 3. Stator coil A is
electrically null and is powerless to influence the motion of
the rotor. The magnetic forces of stator coils B and C now
Figure 3. We could reverse the BLDC
motor’s rotor to commutation position
1 by backtracking the drive voltage
chart in Figure 1 by 60°.
Figure 4. We’ve gone halfway around
electrically and I think you know where
the next set of stator coil voltages
will take us. The commutation table
in Figure 1 represents electrical
revolutions, not physical revolutions.
The number of magnetic poles in the
rotor and stator determine the physical
number of revolutions versus the
number of electrical revolutions.
determine the
position of the
BLDC motor’s rotor,
which is now
positioned in what
we have defined as commutation state 5.
I’ve pinned the Hall effect sensor states to the bottom
of Figure 1. Note that the sensors generate a unique binary
code for each commutation state. The binary code is used
by a BLDC controller or microcontroller to determine the
commutation position of the rotor which, in turn,
determines the present and next set of commutation
drive voltage levels. Now that we possess the knowledge
necessary to drive the stator coils of a BLDC motor, let’s look
at the hardware required to process those drive signals.
BLDC Hardware 101
Figure 5. You’re used to seeing the Phase A and Phase B
circuitry as a pair of half H-bridges commonly used to drive
stepper and universal brushed DC motors. Phase C is a
necessary part of spinning the rotor of a BLDC motor.
32 SERVO 12.2008
A typical BLDC motor’s coils are driven by a
three-phase half-bridge circuit like the one you see
in Figure 5. Block commutation with a twist is used
to drive this multi-phase collection of MOSFETs. The
twist is PWM, which is applied only to the bottom
MOSFETs. Applying the PWM there leaves the
possibility of only one of the top transistors to be
totally ON at any time. I’ve used the coil drive
levels from the commutation chart in Figure 1 to
formulate the hardware commutation chart you
see in Figure 6. Just as in Figure 1, each hardware
commutation state in Figure 6 is associated with
a binary Hall effect sensor value. So, the BLDC
motor controller knows exactly where to pick up
the commutation sequence it needs to rotate the
BLDC’s rotor.
From what we’ve seen thus far, the BLDC
motor’s built-in Hall effect sensors are always
available to tell the BLDC controller how to correctly
commutate the motor. So, today’s million-dollar
question is how do we get rid of the Hall effect
