Week-7 Challenge: DC Motor Control

Explain a MATLAB demo model named ‘Speed control of a DC motor using BJT H-bridge’

The Bipolar junction Transistor(BJT) when used for power swithching application, operates as an IGBT when it is conducting, a forward voltage Vf is developed between collector and emitter. 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 parameters of DC motor are  5 HP 24V 1750rpm. The armature mean voltage can be varied from 0 to 240V when the duty cycle is varied from 0 to 100%. The H-Bridge consists of four Diodes. Two transistors are switched simultaneously one is Q1 and Q4 and other is Q2 and Q3. When Q1 and Q4 are switchen on, 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 switched on a negative voltage is applied to the motor and diodes D1-D4 operate as free wheeling diodes when Q2 and Q3 are switched off.

                  

The above model consists of DC source ,DC motor block, Diodes, Pulse generator, Square block, multiply block, BJT H-bridge transistor Block, scope block and powerGUI block. 

                                   

The motor starts in the positive direction with a duty cycle of 75%. The sitmulation time is 1sec.

The above plot is obtained form the current scope which is connected to Q3 and it shows the output of Diode current and IGBT current of Q3 block. 

The is output of armature current , load torque, rotation speed. Here we can see that there is the sudden fall in all these three parameters this is due to the duty cycle percantage is higher.

Then the duty cycle is changed to 50% and the stimulation time is same.

The above plot is obtained form the current scope which is connected to Q3 and it shows the output of Diode current and IGBT current of Q3 block. 

The is output of armature current , load torque, rotation speed , here we can see the sudden fall is reduced due to reduce of duty cycle to 50%.

Then the duty cycle is changed to 25% and the stimulation time is same.

The above plot is obtained form the current scope which is connected to Q3 and it shows the output of Diode current and IGBT current of Q3 block. 

The is output of armature current , load torque, rotation speed , here we can see that there no sudden fall when we changed the duty cycle to 25%.

FOUR QUADRANT CHOPPER	BJT H-BRIDGE
From the name itself we can say that it operates in four quadrants	BJT H-Bridge can only operates in Two quadrants
Four quadrant are Forward motoring, reverse motoring, forward braking, reverse braking	Two quadrant are Forward motoring, Reverse motoring.
Regenerative braking is possible	Regenerative braking is not possible
This is used in automobile application	This is used in robotics application
There is no armature current shoot up	There armature current shoot up
It has high switching frequency	It has low switching frequency

Two quadrant chopper:

This is the type of chopper were the average voltage will be always positive but the average load current will be positive or negative. The two swithces CH1 and CH2 should not be turned on simultaneously as it will lead to short circuit. The output voltage will be positive always as the diode FD present across the load. When CH1 is on condition the current flows from source to load and the indicator stores the energy and after CH1 is turned off the diode starts conduction voltage which means output voltage will be equal to source voltage. Then the direction of current is reversed and it flows towards source and the load current will be negative when CH2 is turned on and then the current passes through the diode D2 which completes its path.

                    

The below image is the model of two quadrant chopper model which is developed in simulink. This model has pulse generator, NOT gate, MOSFET block, diode, DC voltage source block, current measurement block, voltage measurement block, Series RLC block and scope block, power GUI block. 

The main DC volatge source is 24v and other which is connected to series RLC is 12V source. The current measurement and volatge measurement block is connected to two quadrant chopper and it is connected to scope. The NOT Gate is connected to mosfet to ignore short circuit and to that do not allow to conduct both mosfet at same time. 

This the plot which is obtained from scope block and it give the output of pulse generator, and the current measured and the volatge measured The amplitude value is 20 and duty cycle is 50% and the stimulation time is 0.1sec.

 

Brushless DC Motor:

Brushless DC motor consits of two or more permanent magnets which is present in rotor that generates magnetic field which reacts with current flowing in winding which produce torque. All the electrical energy produces are then converted to mechanical energy. Brushless Dc motor has a cooling system so the heat produced by the motor is low so there is less loss of energy in form of heat 

                                               

Working of Brushless DC Motor:

The brushless DC motor consists of stator which is made up with armature winding were current passes through it and rotor which is nothing but permanent magnet which is the rotating part in this motor. So this motor is constructed in two ways one is in such a way that permanent magnent is placed in center and surrounded by many armature winded conductor which means rotor is placed in center of all stator and other way is all stator is placed in center and the rotor rotates over stator.

From the above picture we can see that how both stator and rotor is placed and how armature winding is done. Hall sensor places a main role in this motor. If positive current passed through stator it will get magnatized and then it will attract the poles of permanent magnet and then repul it. The work of Hall sensor which is known as electronic commutator is to send the requied positive and negative current to the stator of armature windings so then only it will get excited and attract the permanent motor and makes it rotate faster so this passage of positive and negative electrical current is done by hall sensor.In above image we can see the graph were it indicates the workinf of hall sensor while rotor is roatating and also says about current passed to armature windings.