Week-7 Challenge: DC Motor Control
1. A. Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’.
B. Comment on the armature current shoot-up from the scope results.
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results. (Refer to article - Multiquadrant operation of motor )
3. Explain in a brief about operation of BLDC motor.
1. H-Bridge ,in terms of H-Bridge is a rather very simple circuit,consists of four switching elements ,with the load at the center in alphabet "H" like configuration.
such as follows
The general perpose step by step operation of the ckt is given below
Motor rotates in forward direction
Motor rotates in Backward direction
The positive gate signal is given the switches Q1 and Q4 are turn on and the motor rotates in forward direction D1 and D4 aew on off condition during changing of the +ve and -ve time period and D1 and are in on condition.
whenever the gate signal is given -ve then the switch Q3 and Q2 are turn on and the motor rotates in reverse direction and D3 and D4 are in off condition ,we should not trigger the two switches at a time ,otherwise that results in a short circuit.
in general the circuit is shown like as mentioned below
The simulink model
1st PWM- pulse width modulation signal technics generates variable width pulses to exhibits the amplitude of an analog input signal ,depending on the time duration lasts.
pulse width of 75% and period of 2e-3 seconds and the torque ,field voltage and the armature voltage of the motor taken as default value of the model.according to the model ,by tapping on the run button ,the graphs are appear as follows
we noticed 0 to 0.5 time the igbt current is 0 and the diode current is on after the 0.5 to 1 the time the igbt starts the current flow ,the current of the iode is 0.
the output waveform of the load torque ,armature curerent and speed
above graph ,the speed is initially starts from 0 and it reaces up to 1000rpm ,decreases after 0.5 time. The current of the armature starts from 0 and decreases immediately and reaches to the nearer to the 0 value,speed decreases the armature current is also starts flow in the reverse direction,the value of the speed comes to the nearer value -1000rpm .
1.b comment on the armatrue scope result
load torque follows the speed waveform in the +ve direction,with the help of BJT H-bridge model,the result of the simulation as follows
C. Refer to the help section of ‘The Four-Quadrant Chopper DC Drive (DC7) block’. Compare it with the H-bridge model.
It presents DC - DC pwm drive for dc motor with the 4 quadrant chopper ,features closed loop speed control with 4 quadrant operation ,the speed control loop outputs the reference armature of the machine using pi current controller duty cycle of the chopper corresponding to the commanded armature current is derived in a simplistic manner ,the duty cycle is compared with saw tooth carrier signal to obtain the required pwm signals for the chopper circuit .
The machine is seperately excited with the help of DC field voltage source.
WORKING :
The circuit of the 4 Quadrant chopper also known as class E chopper basically consists of four semiconductor switches CH1 and CH4 and 4 diodes D1 to D4 connected in anty-parrallel manner At the first quadrant operation CH1 is made on ,for 2nd quadrant operation CH2 is made on and continues as cyclic order as shown below
1st quadrant operation ,CH4 is kept on and CH3 is kept off and CH1 is operatd ,Both CH1 and CH4 are on simultaneously load gets directly connected to the source ,so the output voltage becomes equal to the source voltage,CH1 is switched off.
2nd quadrant operation: To understand quadrant ,while keeping the CH1,CH3 and CH4 off,when CH2 is on ,the DC source voltage vs ,as load current through CH2,D4,E and L.inductor L stores energy during the on period of CH2.
3rd Quadrant operation:in this quadrant operation both the load voltage and load current should be -ve,The current and voltage are assumed +ve if their direction mathes with what shown in the circuit diagram.
4th Quadrant operation: by obtaning CH4,D2,L and E inductance L energy during the time CH4 is on,when CH4 is made off,current is fed off,current is fed back to the source through diodes D2,D3 ,here load voltage is -ve but the load current is always +ve.This leads to chopper operation operation in fourth quadrant.
2. Develop a 2-quadrant chopper using simulink & explain the working of the same with the relevant results.
schematic for two quadrant chopper
the two quadrant chopper is also called c chopper,that is obtained by connecting type A and type B choppers in parallel,we will always get a positive output voltage V0 as the freewheeling diode Fd is present across the load,when the chopper is on the freeheeling diode starts conducting and the output voltage V0 will be equal to Vs.
two quadrant chopper model using simulink.
let's take two mosfet blocks and connect it as schematic.
voltage source 24volts.
The scope result
The current increases and decreases ,the current may be positive as well as negative.
BLDC motor :Brushless DC motor also known as electronically commutated motors,a brushless DC motor is a permant magnet synchronous electric system electric motor which is driven by direct current dc electrically ,BLDC motors are also referred as trapezoidal permanent magnet motors.
unlike conventional brushed type dc motor,where the brush is connected with armature,its not there in the BLDC motor
in the BLDC motor there is stator ,consist=s of three phase configurable coil arrangement connected to it and a rotor which consists of the permanent magnet, in the BLDC motor hall effect sensor is connected for the feedback for the status of the magnet polarity,
by the help of Electronic BLDC controller ,the BLDC motor can be operated by the 3phase ac kind of signal like shown in the figure
The internal structure of the BLDC motor is presented as follows
