Motor Control, Part 3: BLDC Speed Control Using PWM

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

Channel: MATLAB

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Keywords: MATLAB, Simulink, MathWorks, 매트랩, 시뮬링크, 매스웍스, 모터제어, motor control, bldc, bldc모터, 브러시리스모터, 브러시모터, 모터제어알고리즘, dc모터, モーター制御, ブラシレスDCモーター, ベクトル制御, 永久磁石同期機, 三相インバーター, コンバーター, 電動化, パワーエレクトロニクス, 制御設計, Simulink Control Design, Simscape Electrical, Motor Control Blockset, 电机控制, PWM, 脉宽调制, 三箱逆变器, 汽车, 机器人, Simscape

Id: b5J5qkR7msc

Length: 10min 22sec (622 seconds)

Published: Mon Jan 13 2020