Explaining the function of certain blocks

In this topic Speed Control of a DC Motor Using BJT H-Bridge - MATLAB & Simulink (mathworks.com)

I need to understand what is the function of the blocks arrangement of the pulse generator and the armature voltage control.

How it works? what is the function of the armature voltage control block? and why using mux with these inputs? and what is the block that comes after the armature voltage control?

Answer 1

[Mohamad Nazir]

When a DC motor is supplied with a fixed value DC voltage, it runs at a constant speed (assuming constant load). To control its speed, the voltage needs to be controlled which is generally done by chopping the DC source voltage by means of a switch (in this case IGBT) controlled by a PWM signal with controlled duty cycle (this signal is input to the Gate of the switch, in this example it is P1 P2 P3 P4 for the switches Q1 Q2 Q3 Q4 respectively).

PWM is a simply a repeated pulse at a specified frequency which switches between ON (logic 1 - switch conducting) and OFF (logic 0 - switch not conducting), the ON and OFF time are determined by the duty cycle: for example if duty cycle is 40% the switch is ON 40% of the PWM period and is OFF 60% of that same period.

In this example the pulse generator block generates the PWM signal.

Now if we want to run the motor in both directions (clockwise and anti-clockwise), we need to invert the DC supply at its terminals (+/- of dc source to A+/A- of motor then +/- to A-/A+), which can be done using an H-bridge converter depending on which switch is ordered to open and which one is ordered to close. P.S: Two switches on the same arm of the inverter cannot be conducting at the same time because they will cause a shortcircuit (Q1 & Q2 or Q3 & Q4 in this example).

In this example, to run the motor in the positive direction, Q1 and Q4 must be controlled by the PWM signal while Q3 & Q3 must remain open and vice versa for the other direction.

The Mux blocks are used to gather the gate (control) signals of the four switches Q1 Q2 Q3 Q4, and the armature voltage control block here is simply a signal that controls a switch (the block that comes after it) to reverse the direction of motor rotation after 0.5s (during 0.5s Q1 & Q4 are controlled by PWM while Q3 & Q2 are open (constant block 0) then after 0.5s it reverses the operation).

Answer 2

[Oguz Kaan Hancioglu]

The example explains how the H bridge works. If the armature voltage control is bigger than the threshold, the PWM signal directly goes to Q3 and Q2 igbts. If armature voltage control is lower than the threshold, the PWM signal directly goes to Q1 and Q4 igbts. Armature voltage control changes the dc motor direction.

why using mux with these inputs? - to send pwm signals to the igbts for H bridge.

what is the block that comes after the armature voltage control? - to switch the signal.

It would be more better to change "armature voltage control" as clockwise/counterclockwise.

From the description:

"The H-bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). Two transistors are switched simultaneously: Q1 and Q4 or Q2 and Q3. When Q1 and Q4 are fired, a positive voltage is applied to the motor and diodes D2-D3 operate as free-wheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are fired, a negative voltage is applied to the motor and diodes D1-D4 operate as free-wheeling diodes when Q2 and Q3 are switched off."

Answer 3

[Dr Narayanaswamy P R Iyer]

Hi: Assume that pulse generator output is vg and armature voltage control signal output is vcon. The top mux output, vcon, bottom mux output form the inputs u(1), u(2) and u(3) to the Switch (Threshold switch). The mux inputs from top to bottom are the first, second, third and fourth. The mux output passes first input first followed by second and then third and so on. The switch output is u(1) if u(2) >= Threshold else its output is u(3). Other combinations such as u(2) > Threshold, u(2) ~= 0 are also available. This can be set by clicking the Switch icon. The Demux generates output of Switch, first output top followed by second, third etc. one below the other. Assuming a threshold value of 0.5 for the Swich if vcon is HIGH (logic 1), gate pulse P1 to P4 are 0, vg, vg, 0 thus switching Q2 and Q3 only with negative voltage applied to A+ with respect to A- terminal causing the DC motor to rotate in the anti-clockwise direction. If vcon is LOW (logic 0), P1 to P4 are vg, 0, 0, vg thus switching Q1 and Q4 only with positive voltage applied to A+ with respecto to A- terminal causing DC motor to rotate in the clockwise direction. Thus acceleration and braking of the DC motor can be achieved. To understand more about the application of Switch to model inverters, please refer to the following text book: Narayanaswamy P R Iyer: "Power Electronic Converters: Interactive Modelling Using Simulink", CRC Press, 2018, Pages 63 - 68 and 76 - 80.