Week-7 Challenge: DC Motor Control

 AIM:- 1) To modify the  MATLAB demo ‘Speed control of a DC motor using a BJT H-bridge model such that armature current doesn’t shoot up when motor changes direction from forward to reverse.

           2) To compare ‘The Four-Quadrant Chopper DC Drive (DC7) block'  with the 'H-bridge model' from MATLAB.

           3) To make a suitable EV model using DC7 block. 

1) Speed control of a DC motor using BJT H-bridge :

Below is the default model:-

Description

The Bipolar Junction Transistor (BJT) when used for power switching applications, operates as an IGBT. When it is conducting (BJT operating in the saturated region), a forward voltage Vf is developed between collector and emitter (in the range of 1 V). Therefore, the IGBT block can be used to model the BJT device.

The IGBT block does not simulate the gate current controlling the BJT or IGBT. The switch is controlled by a Simulink® signal (1/0). The DC motor uses the preset model (5 HP 24V 1750 rpm). It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%.

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.

 Simulation

The motor starts in the positive direction with a duty cycle of 75% (mean DC voltage of 180V). At t= 0.5 sec., the armature voltage is suddenly reversed and the motor runs in the negative direction.

'Scope' shows motor speed, armature current and load torque and 'Currents' shows currents flowing in BJT Q3 and diode D3.

Output:-

                  In the above figure, you can see that there is a shoot up in armature current after 5 sec when the motor sudden changes from forward to reverse motion. Here the default duty cycle is 75%.

Modified Model:-

                To reduce the shoot up, we are reducing the duty cycle.

    Here the duty cycle changed to 45% from 75 % which was set as default.

Modified Output:-

            In the above figure, the armature current shoot-up is reduced as we have applied the average duty cycle which is near to 50%.

2) The Four-Quadrant Chopper DC Drive (DC7) block:-

                  Below is the Four-Quadrant Chopper Dc Drive block default model.

Description

The 200 HP DC motor is separately excited with a constant 150 V DC field voltage source. The armature voltage is provided by an IGBT converter controlled by two PI regulators. The converter is fed by a 515 V DC bus obtained by rectification of a 380 V AC 50 Hz voltage source. In order to limit the DC bus voltage during dynamic braking mode, a braking chopper has been added between the diode rectifier and the DC7 block.

The first regulator is a speed regulator, followed by a current regulator. The speed regulator outputs the armature current reference (in p.u.) used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed. The speed reference change rate follows acceleration and deceleration ramps in order to avoid sudden reference changes that could cause armature over-current and destabilize the system. The current regulator controls the armature current by computing the appropriate duty ratios of the 5 kHz pulses of the four IGBT devices (Pulse Width Modulation). For proper system behavior, the instantaneous pulse values of IGBT devices 1 and 4 are opposite to those of IGBT devices 2 and 3. This generates the average armature voltage needed to obtain the desired armature current. In order to limit the amplitude of the current oscillations, a smoothing inductance is placed in series with the armature circuit.

OUTPUT:- Below is the result of the given speed variation.

COMPARISON:- H-Bridge & DC Chopper

1) Four Quadrant chopper can be used for both motoring & braking purposes. But H-Bridge can only use for motoring purposes.

2) H-Bridge motor can rotate in both forward and backward directions. Four quadrants can do the same but in addition,regenerative braking is also possible through it. Hence it is operated in 4 quadrants and called 4 quadrant DC chopper.

3) In a Four-quadrant chopper the PI controller is used (for speed & current control) but it is absent in H- Bridge.

4) PWM is generated separately in H-Bridge whereas in DC Chopper it is generated automatically by varying the speed.

5) H-Bridge can control the speed of the motor with the motion. Four quadrant can do the same but it is mainly used for varying the DC voltage at the output. 

3) EV model using DC7 block:-

                        Below is the modified model for EV;

                        Here DC voltage is provided. We can provide the battery also with a suitable configuration of a battery whose result will differ from that of DC voltage supply. Inputs are provided through the drive cycle in which the speed is in rpm.

                               Above is the torque characteristics subsystem. The equation used is `24.7+0.0051omega^2`

OUTPUT:-

                    From the above result, we can see that there is an increase in the duty cycle of IGBT 1 & 4 with an increase in motor speed and it is the mirror image to the IGBT 2 & 3. Motor voltage is also showing the increase in the value as the speed increases. When speed reduces suddenly the current requirement is low so it goes down.

LINK:-

 https://drive.google.com/file/d/1IeMNJdfLMF4jKvi1e0fzUoYvr4UooAHw/view?usp=sharing 

CONCLUSION:-

                        The study of the H-Bridge Model & DC Chopper model is successfully carried out & compared. Also, the new EV model is built by using the DC& chopper block.

REFERENCE:-

https://www.mathworks.com/help/physmod/sps/ug/speed-control-of-a-dc-motor-using-bjt-h-bridge.html;jsessionid=91b8a06046eff02dba4c66dda7d3#:~:text=It%20simulates%20a%20fan%20type,BJT%20simulated%20by%20IGBT%20models). 

https://www.mathworks.com/help/physmod/sps/ug/dc7-four-quadrant-chopper-200-hp-dc-drive.html  

https://skill-lync.freshdesk.com/support/solutions/articles/43000571899-how-to-model-dc7-four-quadrant-chopper-200-hp-dc-drive 

https://www.build-electronic-circuits.com/h-bridge/ 

https://www.etechnog.com/2019/03/h-bridge-circuit-working-application-advantage.html