Week-7 Challenge: DC Motor Control
TASK 1.
SPEED CONTROL A DC MOTOR USING BJT-H BRIDGE:
BJT-H BRIDGE:
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 1V).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 pre-set model (5 HP 24V 1750RPM). 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 when D2-D3 operates as free-wheeling diodes when Q1 and Q4 are switched off.When Q2 and Q3 are fired,a nagetive voltage is applied to the motor and diodes D1-D4 operates as free wheeling diodes when Q2 and Q3 are switched off.
CHOPPER:
A DC chopper is also known DC-DC converter.It is a static device which is used to obtain a variable DC voltage sourc.A DC -DC converter is an electronic circuit or electromechnical device that converts a source of direct current (DC) from one voltage level to another .It is a type of electric power converter.Power levels range from very low ( small batteries) to very high (high voltage power transmission).Choppers are of two types step down chopper and step up choppers.
PROCESS:
The model is opened in a simulink workspace.
Run the model as it is and observe the graphs of the armature current.
Now,at the bottom part of the model,there's a pulse generator blocks,click on it and run the model.
Check the outputs.the same for 75% to 50% just to check if the armature voltage is decrease sharply after half of the simulation time.
As pulse width decrease the steap value of the armature current decrease.
C. The four-Quadrant chopper DC drive (DC7) block.compare it with H bridge model.
The 200 HP DC motor is separately excited with a constant 150v DC filed voltage source.The armature voltage is provided by an IGBT converter controlled by two PI regulators.The converter is fed by a 515v DC bus obtained by rectification of a 380v AC 50 hz voltage source.In order to limit the DC bus voltage during dynamic braking mode,a braking chopper has been added betwwen 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 used by the current controller in order to obtain the electromagnetic torque needed to reach the desired speed.The speed reference change rate follows accelerations and decelaration 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 approprite duty ratos of the 5 khz pulses of the four IGBT devices.For proper system behaviour,the instaantaneous 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
- The stability of the voltage and current supply is higher in the four quadrant compared to BJT as the four quadrant uses the step by step ramping strategy for the change in the stability is lost.
- Regenerative braking is possible in four quadrant but not in BJT.
- It is widely used in automobile sector to control DC motor speed and direction in four quadrant .Whereas the BJT is used in robotics and electro-mechanical devices to control DC motor speed and direction.
- The current values are continous in BJT and in four quadrants,it is chopped so that the simulation takes less time.
- To change the direction of the motor , current needs to be changed.
- The main advantage pf four quadrant is that it operates in all the quadrant but BJT can movie only is two directions.
- Four quadrants provides better efficiency and operates at high power levels when compared to BJT that operates at moderates power leves.
TASK 2.
TWO QUADRANT CHOPPER DC DRIVE BLOCK:
WORKING OF CLASS C
A class A class B chopper operates in first and second quadrant respectively. If we connect both these types of chopper in parallel than it is possible to have chopper operation in first as well as second quadrant.Class c chopper is obtained by the parallel connection of class A and class B chopper.
We can notice that choppeer CH1 free wheeling diode(FD) and load are forming class A chopper whereas chopper CH2,D2 and load are forming class B chopper.Both these choppers are connected in parallel.To obtain first quadrant operation we should switch ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2.
CASE 1.
When chopper CH1 is switched ON,source Vs directly gets connected to the load and hence, load voltage Vo is equal to source voltage.The direction of load current is form source to load.which is assumed positive.
When CH1 is switched OFF,the free wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load.The output voltage Vo becames zero.The Io continues to die down through the FD and L in the same direction.The average output voltage Vo and current Io are positive and hence operation of chopper is in first quadrant.
CASE 2.
When chopper CH2 is switched ON,load DC source E drives current through CH2 and load.he direction of this current Io will be opposite to that and hence is assumed negative.Output voltage Vo is zero during this time.When CH2 is made OFF,diode D2 gets forward biased and hence the current into the source from the load.The output is Vs in this time as the load is connected to the source through D2 during OFF time of chopper CH2.The load current is always negative i.e. operation of chopper is with in second quadrant.
The average load voltage is always positive but the average load current may be positive or negative.Power flow may be from source to load or load to source .The operating region of class C chopper .
CAUTION:
Choppers CH1 and CH2 should not be ON simulatneosly as would lead to direct short circuit on the supply lines.
APPLICATION:
Class C choppers are used for motoring and regenerative braking of DC motors.
IN SIMULINK:
The simulink model corresponding to the circuit diagram is build using basic simulink blocks.The oulse generator is used with the NOT gate to prevent misfiring of the MOSFET so that they do not conduct at the same time whuch results in the short circuit.
An amplitude of 20 is chosen so that we could see the pulses clearly.
DC voltage source parameters:
source side:24V
Load side:12V
OUTPUT:
We could see clearly that the voltage is always positive but the current becomes both positive and negative.The type c chopper is simulated and verified.
TASK 3.
EXPLAIN IN A BRIEF ABOUT OPERATION OF BLDC MOTOR.
Brushless DC electric motors also know as electronically commutated motor.Primary efficiency is a most important feature foor BLDC motors.Because the rotor is the sole bearer of the magnets and it doesn't require any power.i.e.no connections,no commutator and no brushes.in place of these,the motor employs control circuitry.To detect where the rotor is at certain times,BLDC motors employ along with controllers,rotary encoders or a hall sensor.
CONTRUCTION:
In this motor,the parmanent magnets attach to the rotor.The current carrying conductors or armature winding are located on the stator.They use electrical commutation to converte electrical energy into mechnical energy.The main design difference between a brushed and brshless motors is the replacement of mechanical commutator with an electric switch circuit.A BLDC motor is a type of synchronous motor in the sense that the magnetic filed generated by the stator and the rotor revolve at the same frequency.
brushless motor does not have any current carrying commutatirs.The filed inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.
WORKING:
BLDC motor works on the principle similar to that of a brushed DC motor.The lorentz force law which states that whenever a current carrying conductor placed in a magnetic filed it experiwnces a force .As a consequence of reaction force,the magnet will experience an equel and opposite force.In the BLDC motor,The current carring conductor is stationary and the permanent magnet is moving.
When the stator coils get a supply from source,it becomes electromagnet and starts producting the uniform filed in the air gap.Though the source of supply is DC,switching makes to generate an AC voltage waveform with trapezoidal shape.Due to the force of interaction between electromagnet stator and permanent magnet rotor,the rotor continues to rotate.
With the switching of winding as high and low signals,corresponding winding energized as north and south poles.the permanents magnet rotor with north and south poles align with stator poles which causes the motor to rotate.