in this video we'll discuss what kind of a control algorithm you need to control
a BLDC motor will explore the behavior of different signals of this control
algorithm and also discussed the concept of inductive fly back in the previous
video we introduced the BLDC motor that has three coil windings in the stator
and a single pole pair in the rotor we also talked about how motion is
generated in the BRDC motor through the stick step commutation or trapezoidal
control where the correct phases are accommodated every 60 degrees for
continuous rotation of the motor here we have a DC voltage source that provides a
constant voltage to the three-phase inverter which converts the DC power to
three phase currents to energize different coil pairs when they applied
voltage is constant the motor turns at a constant speed due to the proportional
relationship between voltage and speed but if we want to control the motor at
different speeds then we need to build a controller that will adjust the
magnitude of the applied voltage let's first build a diagram of this controller
here's our motor to control it we first need to measure its angular position and
speed by using sensors such as the Hall effect sensor note that the hall sensor
doesn't provide information on precisely where the rotor is within a sector but
instead it allows to detect when the rotor transitions from one sector to
another and actually sector information is all we need to know to determine when
to commentate the motor but we still don't know which two out of three phases
to commentate the correct phases are specified by a commutation logic circuit
that computes the switching pattern for the three-phase inverter let's also see
visually how these blocks interact with each other
in the commutation logic table the letters a B and C stand for the three
phases of the motor the high side of the three-phase inverter is labeled with H
and low side would L to see how the switches of the inverter and the BLDC
commutation work based on the commutation logic let's replace this
part with our previous animation if the water is within the first sector the
commutation logic selects this switching pattern which dictates an on state for
the high side switch of phase a and the low side switch of phase C as the water
transitions to other sectors a switching pattern is selected accordingly and sent
to the three-phase inverter we got the motor spinning because now we know when
to commutator water and which phrases to energize during each commutation our
next goal is to make the motor spin at different speeds
currently we input a constant DC voltage to the inverter which leads to constant
speed as we discussed before we can adjust this voltage by closing the loop
with a suitable controller based on the difference between the desired and
measured speed the controller will adjust the voltage to bring the motor
speed close to the desired value here note that the commutation logic also
belongs to the control algorithm which is shown with light blue on the diagram
and all the components of the physical system are shown with light gray such a
motor control algorithm can be implemented in a simulation environment
as seen here note that here we assume were changing the voltage in an ideal
manner to demonstrate the operation but in reality we need to use PWM which
we'll discuss in the next video if you want to learn how to build this model
don't forget to check out the links below this video
coloured boxes show how different parts of the control algorithm map to
subsystems in the Simulink model this model logs signals such as speed
voltage current and torque to explore these let's run the model and first look
at the desired speed as you see here it ramps up from 100 to 500 rpm in 100 rpm
increments if we now look at the voltage we see how it's adjusted by the
controller to make the motor rotate at the desired speed as seen on the
measured speed one thing we immediately notice as the ripples in the speed
signal here we have another plot showing when the rotor transitions between
sectors looking at the sector and the speed plots together shows us that speed
ripples have something to do with commutation because the ripple pattern
aligns with the beginning of each commutation cycle remember what happens
during commutation one of the phases is pulled high while the other one is
pulled low and the third phase is open if the three phase currents changed as
seen in this plot during commutation then we wouldn't observe in a ripple
pattern in the speed but in reality when we drive a face the current doesn't
change instantaneously looking at the three phase currents we see how they
rise over time which in turn leads to ripples in the speed and speed is not
the only affected signal but we observe ripples in the torque response as well
as current and torque are proportionally related the ripples in the torque
response are considered one of the drawbacks with trapezoidal control of
BLDC motors did something catch your eye when we looked at the three phase
currents when a face is pulled high from an open-face state a sudden jump occurs
in the phase current before it starts to rise again at the same time another jump
occurs in the face that's being kept law during competition to better
understand the reason behind this instantaneous changes in the energized
phases let's look at this animation during commutation the magnetic fields
get established on the energized phrases as the phase currents grow at
commutation one of these phases in this case phase a becomes an open circuit and
therefore the established field on this opened phase collapses so the phase
current drops to zero let's rewind us to see what happens to phase B and C right
at the commutation when phase C that has a fully established field connects to
phase B nearly instantaneously the established field on C collapses while
at the same time phase B establishes a field in response to the collapse due to
this suddenly established field at phase B we observe a jump in the face big
current and due to the collapse at phase C we observe a sudden decrease in the
current which drops to half of its magnitude since the fields at B and C
find an equilibrium at 50% fill strength as a result of these instantaneous
changes in the phase currents we observe spikes in the three phase voltages this
phenomenon we just described is called inductive fly back let's wrap up what
we've seen in this video we first discussed the different elements of the
control algorithm that decides when to commentate the rotor and which phrases
energized during commutation we then showed how the speed of the motor can be
controlled by adjusting the voltage by a controller here we assume then ideally
controllable voltage source but in reality we need a method such as PWM to
convert the constant DC voltage to AC voltage in the next video we'll update
our control algorithm to control the motor speed
using PWM for more information on motor control don't forget to check out the
links below this video