in this video we're going to learn what PWM or pulse width modulation is and how
it's used to control the speed of a BLDC motor previously we discussed how we can
control a BLDC motor at varying speeds by adjusting the DC voltage provided to
the three-phase inverter in this simulation we use an ideal voltage
source which let us generate different DC voltage levels commanded by the
controller but in reality the DC voltage source we have supplies a fixed voltage
which we need to modulate using a technique called PWM or pulse width
modulation before providing it to the three-phase inverter here's what a pwm
signal looks like it's basically a square wave signal that repeats itself
at a certain frequency to understand how PWM helps with voltage modulation let's
look at an example say that we have a DC voltage source that can supply either
zero or hundred volts for controlling a motor at varying speeds we need voltage
values ranging from zero to hundred volts PWM acts like a switch that takes
the DC voltage and applies it to the motor with a series of on-and-off pauses
at a certain frequency each PWM cycle is called a period and
the person of the time the PWM signal is on during a given period gives us the
duty cycle for example if we have a duty cycle of 50 percent this means that in
each period half of the time the signal is on and in the other half it's off
when we drive a motor with us the effective voltage seen by the motor will
be the average of this PWM signal which is 50 volts we were able to take the DC
voltage of hundred volts and pass it on and off with a 50 percent duty cycle
to create 50 volts now if you keep changing the duty cycle you can
continuously modulate the signal and create the whole range of different
values between zero and hundred volts to control your motor at varying speeds
note that the longer the duty cycle the higher the voltage we get now we know
PWM control has an averaging effect on the output voltage that is seen by the
motor to get this averaging effect right we should be careful when selecting the
PWM frequency which is computed by one over period if the switching frequency
is too low instead of seeing an averaged voltage the model will see a voltage
that tries to follow the square wave shape this will lead to poor tracking of
the reference speed and the motor will keep speeding up and slowing down
however when we increase the PWM frequency to a certain reasonable value
the voltage will be averaged out which will improve the speed control
performance note that the ripples will occur due to
the switching nature of PWM typically PWM frequencies to control BLDC motors
are on the order of a few kilohertz and need to be selected to be much higher
than the reciprocal of the motor time constant now that we discuss PWM
conceptually we'll look at two common architectures used to implement PWM
here's the first one in this model we want the BLDC to track a desired speed
that gradually ramps up from 0 to 600 rpm this model consists of similar
blocks that we saw in the previous video except this part where we implement PWM
control by using a buck converter the buck converter is used to adjust the DC
source voltage to different voltage levels in order to be able to control
the bialys the motor at varying speeds in this
simulation the input to the buck converter is provided by this DC voltage
source block which supplies 500 watts let's look inside the subsystem to
understand how the buck converter works what we see here is a PWM generator that
generates a square wave signal at 1 kilohertz if we go up we see that the
input to the PWM generator is the duty cycle which is determined by the
controller the signal created by the PWM generator fluctuates between 0 and 1 and
controls the on and off duration of the two switches of the buck converter
depending on this duration we observe a different amount of voltage drop at the
output of the buck converter here we measure two voltages one at the input of
the buck converter which is the DC source voltage and the second one at the
output of the buck converter which gives us the modulated DC voltage that is done
provided to the three-phase inverter now we'll run this model and take a look at
both of these voltages as well as the reference and measured speeds in the
upper plots we see that the DC source voltages 500 volts the second plot shows
us the modulated DC voltage by the buck converter as a result of the voltage
modulation we're able to control the motor at the different speeds we see in
here here the measured speed is shown in orange which successfully tracks the
desired speed shown in green we discussed be a DC speed control using
this architecture where PWM generator is used along with a buck converter to
provide a modulated DC voltage to the three-phase inverter let's take a look
at the second architecture to see how PWM control is implemented and this one
the first thing we notice here is that this model doesn't use a buck converter
in the first model we were modulating the voltage provided to the three-phase
inverter but in this model we modulate the phase voltages directly here the PWM
is used under commutation logic subsystem which we'll take a closer look
at next here is the PWM generator according to
this logic we see here the PWM generator output makes sure that the DC source
voltages pass on and off to energize the correct phases based on the sector the
rotor isn't the easiest way to understand how voltage modulation is
being performed here is to simulate this model and observe the phase voltages now
we're on the model and take a look at the speed sector phase a and phase C
voltages as seen in the speed plot in this region the speed is constant so
let's zoom into here to better see how phase voltages change as the motor is
running at a constant speed according to this logic when the water is let's say
in sector one these two inputs are selected this one commands a high and a
low signal for a and C phases respectively and this input does exactly
the opposite by sending a low signal to a and a high signal to see the PWM
generators switches between these two states based on its students icon and as
a result here we see how the phase a and phase C voltages are passed on and off
between plus minus 250 volts which is plus minus DC source voltage divided by
2 when the phase voltages are modulated like this the effective voltage seen by
the motor will be averaged remember in the previous videos we discussed how the
back EMF is induced during the non accommodating face the Beckham of
voltages seen in the plots gives us a clue about the averaged voltage that's
seen by the motor for example when the phase a is not being accommodated this
is the back EMF voltage which tells us that the phase a voltage seen by the
motor will be around approximately 25 volts throughout this region whereas the
phase C voltage seen by the motor will be around minus 25 volts using this clue
we can simply figure out what effective voltages during the rest of the
commutated phases in summary we discussed the concept of PWM and how
it's used to control a BLDC motor at varying speeds we also talked about two
common implementations of PWM and simulated those models to take a closer
look at the voltage and speed characteristics during PWM control you
can also check out our other video series where we show how to build the
models that we use in these Tech Talk videos please find the link to the
series below in the next Tech Talk videos we'll discuss EMS app motors and
field oriented control for more information on BL DC and PMS and motor
control don't forget to check out links below this video