Week-7 Challenge: DC Motor Control

DC Motor Control

Aim:

To simulate and study about DC drive of electric vehicle(EV)

OBJECTIVES

• To run MATLAB demo “speed control od a DC motor using BJT H-Bridge”. Modify this model armature current doesn’t shoot up when motor changes direction from forward to reverse.
• To simulate the model “The four-quadrant chopper DC drive (DC7) block” and to compare it with the H-bridge model.
• To make a suitable EV model using DC7 block and make a result report.

1. BJT H-bridge

Permanent magnet AC machines trapezoidal back emf waveforms are known as the PM brushless DC machines. The concentrated windings of the machine instead of sinusoidally distributed windings on the stator are the reason for the trapezoidal shaped back emf waveforms. In the case of DC motors, it can be easily controlled. If the supply voltage is reduced, then maximum torque falls in proportion and the slope of the graph is unchanged. Any torque and speed can be achieved below maximum values. Changing magnetic flux is also a way to control the DC motor speed. Speed control of a DC motor using BJT H-bridge shows the simulation of an H bridge used to generate a chopped voltage and to control the speed of a DC motor. We can load this model using help in mat lab and the model is represented below.

The bipolar junction transistor (BJT) when used for power switching applications operates as an IGBT. When it is conducting, a forward voltage V_f is developed between collector and emitter. The IGBT block can be used to model the BJT device. The IGBT block doesn’t simulate the gate current controlling the BJT or IGBT. The DC motor uses the present model. It simulates a fan-type load, where load torque is proportional to the square of speed. The armature means voltage can be varied from 0 to 240V when the duty cycle which is specified in the pulse generator block is varied from 0 to 100%. The H bridge consists of four BJT/Diode pairs. Two transistors are switched simultaneously when Q1 and Q4 are fired, a positive voltage is applied to the monitor diodes D2-D3 operates as freewheeling diodes when Q1 and Q4 are switched off. When Q2 and Q3 are switched off. The motor starts in a positive direction with a duty cycle of 75% (mean DC voltage of 180V). at t =0.5sec, the armature voltage is suddenly reverse and the motor runs in the negative direction. The scope block shows motor speed, armature current, and load torque and current blocks show currents flowing in BJT Q3 and diode D3.

In the result, the armature current shoots up when the motor changes direction. It can be noticed when the time is 0.5 seconds. Now we want to modify this model so that the armature current doesn’t shoot up when the motor changes direction from forward to reverse. Consider the pulse generator block in the model and it is represented below.

Here the pulse width is 75%. The pulse width has changed to 50% and runs the simulation, and the result was

From the result, we can notice that the current change was not that severe when compared to the previous case. Now run the simulation with 40% pulse width.

The result shows very little change in armature current. Load torque and rpm are almost constant.

2. Four quadrant chopper DC drive

Here we simulate the model ‘The four-quadrant chopper DC drive (DC7) block’ and compare it with the H-bridge model which was represented above. DC7 block is also loaded from the help.

200HP DC motor is separately excited with a constant 150V DC field voltage source. Armature voltage is from the IGBT converter and it is fed with 515V DC. The braking chopper limits the DC bus voltage during the dynamic mode. All the other description of the model and simulation conditions is given in the mat lab documentation. The result of the simulation is represented as,

When we compare the result with the previous model it is clear that there is no sudden change in armature current and the efficiency of the model is hugh. But the simulation time is more than the other model. It's working in all four quadrants is also an advantage.

3. EV model using DC7 block

Here, the DC7 block is used to make a suitable EV model. For that, a DC source battery and a current sensor which is placed in series for AC to DC converter and rectifier in the existing model of DC7 block. AC source is also replaced with a DC battery to simulate the electric vehicle. We must also consider the torque-speed equation and must provide a drive cycle externally. These changes were made in the model and the model is,

In this model, the torque-speed relation is added as a subsystem.

The given input drive cycle is given as,

While simulating the model with the above-mentioned conditions for 7 seconds time, the result can be obtained as

From the above result, it can be observed with a steady electric vehicle motor.

The Matlab file has been uploaded to google drive and the access link to the file is

https://drive.google.com/file/d/1_TEuqSFiXhJ480DOuaWUGQvr3WfAtxv0/view?usp=sharing

Conclusions:

• MATLAB demo ‘speed control of a DC motor using BJT H-bridge’ has been successfully simulated and then pulse width in the pulse generator is modified in order to achieve the required result.
• The model ‘The four-quadrant chopper DC drive (DC7) block’ has been simulated successfully and then it is compared with the H-bridge model.
• A suitable electric vehicle is made using DC7 block and the results are observed.