Week-7 Challenge: DC Motor Control
1)In this question we study the speed control of DC motor using BJT h-bridge. But we understand the basic concept of the speed control using H bridge configuration.
Most DC motors have two terminals because they are series type (armature and field windings are connected in series)DC motors.
As shown in this figure there are two terminals A and B of DC motor.Now if we are connected to the terminals of A with +ve supply and terminal B with -ve supply or ground the current will flow from motor from A to B and motor will rotate in one direction – say clockwise(cw)or forward direction.Now as shown in second figure we change the supply terminals.Now B is connected with +ve and A is connected to ground.The current flow from motor from B to A and motor will rotate in other direction (counter clockwise -CCW or reverse)
The arrangement to get reverse supply is shown in right side of the figure.Four switches are connected in between +ve supply and ground and DC motor is connected between two switches as shown.such circuit arrangement is known as H bridge rectifier because it looks like letter H(H bridge circuits are most widely used in DC motor drivers).
If SW1 and SW4 are pressed simultaneously the current wil flow from +ve to SW1 to A to B to Gnd(ground). So motor will rotate in one direction.open(release)SW1 and SW4 to stop motor.Now if SW2 and SW3 are pressed current will flow from +ve to B to A to SW3 to Gnd(ground). So the motor get reverse supply and it will rotate in another direction.
The circuit replaces with NPN transistors Diodes are given for the current to free wheel as the DC motor is a RLE load were as the current change direction instantenously.we all know that transistor work as a switch. For NPN transistor if we give enough +ve input to base it will turn ON (saturation region)and we give 0 input will be turn OFF(cut region).so in this circuit if q1 and q4 are turned on simulataneously the motor rotates forward direction.if Q2 and Q3 are turned on then the motor rotates in reverse direction.
In order to vary the speed of the DC motor,we have to change the voltage applied to the DC motor.This could be achieved with the help of pulse with modulation
Pulse width modulation means varying the width(duty) of pulse.Width means ON time to Ton of pulse.The Vaverage voltage of (Vdc or V avg) is given by equation.
V dc=Ton/Ton +Toff
Vdc = α*Vα where α is the duty cycle in this expression.
The voltage is directly depends upon Ton.if the duty cycle is 0% the average output voltage Vdc is also be zero of if duty cycle is increased to 50% Vdc is also increased by 75% of Vs and the duty cycle is 100% then the Vdc is reduced as 25% so we apply to PWM to DC motor to vary its speed.
Then the example shows the Simulink model an H bridge used to generate a chopped voltage and to control the speed of a DC motor.
BJT-H bridge :
The bipolar junction transistor (BJT) when used for power switching applications,operates as an in IGBT.when it is conducting (BJT operating in the saturated region),a forward voltage Vf is developed between collector and emitter(in the reange of 1v).Therefore the IGBT block can be used in to the model of BJT device.
The IGBT block does not simulate the gate current controlling the BJT or IGBT switch.the switch is controlled by a Simulink signal (1/10).The DC motor uses the pre set model (5HP 24V 1750 RPM).It simulates a fan type load(where load torque is proportional to square of speed).The armature means voltage can be varied from 0 to 240 volts when the duty cycle is (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,Q2,Q3 andQ4 .When Q1and Q4 are fired,a positive voltage is applied to the motor and diodes D2-D3 operates as a free wheeling diodes when Q1 and Q4 are switched off when Q2and Q3 are fired,negative voltage is applied to the motor and the diodes D1 and D4 operates as free wheeling diodes when Q2 and Q3 are switched off.
Note : the DC motor used in the Simulink model is a separately excited DC motor and NOT a DC SERIES motor.That is the reason why a DC voltage of 300v is given to the field winding
1b) comment on the armature current shootup from the scope results
simulation output:
1)pulse generator parameters:
Period : 2 milli seconds
Pulse width (% of period) : 75%
Intailly there is a spike in armature current.This is because in a DC motor,
Ia = V-Eb/Ra where Ia is the armature current V is the supply voltage Eb is the back emf and Ra is the armature resistance
AT starting Eb = 0 and speed = 0
Therefore Ia =Va/Ra and Ra is a very small value hence Ia is huge and reflects the graph
2)Period : 2milli seconds
Pulse width (% of period) : 50%
Therefore when the pulse width decreases,the voltage applied across the DC motor decreases and the armature current finally decreases
1c) refer to the help section of the four Quadrant chopper DC Drive (DC7 ) Block compare with the H-bridge model :
The four quadrant chopper DC drive(DC7) block represents a four-quadrant,DC supplied chopped (or DC- pwm converter) drive for DC motors.the drive features closed loop speed control with four quadrant operation the speed control loop outputs the reference armature current of the machine.Using a PI current controller,the chopper duty cycle is then compared with a sawtooth carrier signals to obtain the required PWM signals for the chopper.
The Four Quadrant chopper DC drive block uses these blocks from the Electric Drives/Fundamental drives blocks library:
Speed controller(DC)
Regulation switch
Current controller(DC)
Chopper
The above figure shows the schematic of the 4 quadrant chopper DC drive
Hbridge :
Only two quadrants of operations are possible they are forward motoring and reverse motoring
High armature current ripple
Less complex continuous and simulation time is more widely used in Robotics
Four Quadrant chopper DC drive :
It can operate in all four quadrants (forward motoring,reverse regeneration,reverse motoring motoring,and forward regeneration)
Lower armature current ripple due to highg switching frequency of DC-dc converters
More complex
Discrete and simulation time is less
Widely used in Automobile
2) Develop a 2 quadrant chopper using Simulink &explain the working of the same with the relevant results:
Type C chopper or 2 quadrant class A chopper :
Class C or Type C chopper is a category of chopper which can operate in first as well as a second Quandrant .This basically means that,the power can either flow from source to load or load to source in this chopper . This kind of chopper is also known as 2 quadrant class-A chopper.
Operation and working of class-C ot type C:
As we know that class A and class B chopper operates in first and second Quadrant respectively.therefore if we connect both these types of chopper in parallel then it is possible to have chopper operation in first as well as second quadrant in fact,class-C or type-C chopper is obtained by the parallel connection of class-A and class-B chopper.Fig shows the circuit diagram of this type of chopper.
We can notice that chopper CH1 free wheeling diode (FD) and load are forming class A chopper whereas SH2.DH2 and load are forming class-B chopper Both these choppers are connectged in parallel.To obtain the first quadrant operation we should switch ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2 let us discuss in detail.
Case 1: when CH1 is switched ON/OFF:
When chopper CH1 is switched ON ,source voltage Vs directly gets connected to the load and hence load voltage Vo is equal to the source voltage .The direction of the load current is from source to load as shown in the circuit diagram which is assumed positive.
When CH1 is switched OFF the free wheeling diode FD comes in to the circuit as its get forward bias and hence shorts the load.Therefore the output voltage V0 becomes zero.However the Io continues to die down through the FD and L is in the same direction as shown in the circuit.Thus the average voltage Vo and current Io are positive and hence the operation of the chopper is in the first quadrant.
Case 2: when CH2 is switched ON/OFF:
When chopper CH2 is switched ON load DC source E drives current through CH2 and load the direction of them current Io will be opposite to the circuit hence this assumed in as negative.Output voltage Vo is zero during this time.when CH2 is mode OFF. Diode D2 gets forward biased and hence the current in to the source from the load.the output voltage is Vs in this time as the load is connected to the source D2 during OFF time of chopper CH2.Thus the load current always negative i.e operation of chopper is within second quadrant.
The average load voltage is always positive but the average load current may comes positive or negative.Therefor power flow may be from source to load (first quadrant) or load to source (second quadrant) operations.the operating region of the class C or type C chopper.
CAUTION: chopper CH1 and CH2 should not be ON simultaneously as this would lead to direct short circuit on the supply lines.
APPLICATIONS: Class C or type C chopper are used for motoring and regenerative braaking of DC motors.
Hence the Simulink model corresponding to the circuit to the circuit diagram is built using basic Simulink blocks.The pulse generator is used with the NOT gate to prevent misfiring of the MOSFET so they do not conduct at the same time which 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 : 24 volts
Load side : 12 volts
Hence we could see clearly that the voltage is always positive but the current becomes both positive and negative hence the type C chopper is simulated and verified.
3)Explain in a brief about operation of BLDC motor :
Brushless DC motor also known as electronically commutated motord(ECMs,EC motors) primary efficiency is a most important feauture of BLDC motors because the rotor is the sole bearer of the magnets and it odesnot require any power ie no connections no commutation and no brushes.In place of these,the motor employs the control circuitry.To detect where the rotor is at certain times,BLDC motor employ along with controllers rotary encoders or a Hall sensor.
Construction of BLDC MOTOR:
In this motor the permanent magnets attached to the rotor The current carrying conductor or armature windings are located on the stator.They use electrical communication to convert electrical energy in to mechanical energy.The main design difference between a brushed and brushless DC motors is the replacement of mechanical commutator with an electrical switch circuit.A BLDC motor is a type of synchronous motor in the sense that the magnetic field generated by the stator and the rotor at the same frequency.
Brushless DC motor does not have any current carrying commutators the field inside a brushless motor is switched through an amplifier which is triggered by the commutating device like an optical encoder.
WORKING OF BLDC MOTOR:
BLDC motor works on the princicple similar to that of a brushed DC motor.The Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force.As a consequence of reaction force the magnet will experience an equal and opposite force.In the BLDC motor,the current carrying conductor is stationary and the permanent magnet is moving.
When the stator coils get a supply from source it becomes electromagnet and starts producing the uniform field in the airgap.through the source of supply is DC switching makes to generate an AC voltage waveform with trapezoidal shape.Due to the force od interaction between electromagnet and the permanent magnet rotor,the rotor continues to rotate.
With the switching of windings as high and low signals corresponding winding nergized as noth and south poles.the permanent magnet rotor with north and south poles align with stator poles which causes the motor to rotate.