Explanation about H-bridge and Four Quadrant Chopper DC Drive (DC7) block

AIM :

1. By Modifying "Speed Control of a DC motor BJT H-bridge model" in Run MATLAB demo, such that armature current doesn’t shoot up when motor changes direction from forward to reverse.

2. Four-Quadrant Chopper DC drive (DC7) block is to be compared with H-bridge model.

3. To make a suitable EV model using DC7 block and to make result report.

EXPLANATION :

STUDY - 1:

Speed Control of a DC motor BJT H-bridge model in MATLAB :

In MATLAB, Open "Speed Control of a DC motor BJT H-bridge model" by using Help section.

"Speed Control of a DC motor BJT H-bridge model" has shown in above figure. H-bridge consists of four BJT/Diode pairs. Duty cycle and Step time are inputs given from 'Pulse Generator Block' and 'Armature Voltage Control Block' to Bipolar Junction Transistor H-bridge.

Bipolar Junction Transistor H-bridge :

The H-bridge model consists of four Bipolar Junction Transistor(BJT) and four Flyback diodes that control the flow of current to the load.Using one current source, DC motor can drive in two directions by closing two switches.

- If Switch 1 and 4 is closed, this will cause the current to flow from the source, through Switch 1 and then through the load, then through Switch 4, then down to the ground.This is known as Forward motion of the motor.

- If Switch 2 and 3 is closed, this will cause the current to flow from the source, through Switch 3 and then through the load, then through Switch 2, then down to the ground.This is known as Reverse motion of the motor.

- If Switch 1,3 or 2,4 is Closed, then both sides of the motor have same voltage and this is called as Brake mode.

- If Switch 1,2 or 3,4 is closed, then it will cause a short and burn out the H-bridge.

Working Principle or Operation of Speed Control of DC motor using BJT H-bridge Model:

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.

'Currents' shows currents flowing in Bipolar Junction Transistor Q3 and diode D3. Adding of diodes in the Forward direction for the transistors, to give a path for the current to take to release this energy. Without diodes, it risk that the voltage rises and damages the transistors.

From the above plot, observed that '0s -0.5 s' Current flow Through 'Diode D3' and '0.5s - 1s' Current will flow through 'BJT Q3'.

The Ouput from the DC motor is Speed ("rad/s"is converterd into "rpm") and Armature Current(A) is connected with 'Scope'. where, Load Torque(T) is directly proportional to Square of Speed.

T∝ω^2

T=k⋅ω^2

Output from the Speed(ω) is multiplied with constant(k) gives Load Torque(T) and it is also connected with 'Scope'. 'Scope' shows motor speed, armature current and load torque.

In the above plot, Armature current has sudden rise and drop when the motor changes direction from forward to reverse motion.

A Pulse Width Modulation (PWM) Signal is a method for generating an analog signal using a digital source. A PWM signal consists of two main components that define its behavior: a duty cycle and a frequency. By cycling a digital signal off and on at a fast enough rate, and with a certain duty cycle, the output will appear to behave like a constant voltage and current analog signal when providing power to devices.

Fig : Pulse Width Modulation (PWM)

The average value of Voltage (and Current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate.The PWM switching frequency has to be high enough not to affect the load, which is to say that the resultant waveform perceived by the load must be as smooth as possible.

Here Duty Cycle is reffered as Pulse width. In Pulse generator block, decrease the Pulse width to stop the fluctuations of Armature Current during forward and reverse motion.

Decreasing the Pulse width and run the model to test the sope results until the armature current flow without any fluctuations by trial and error method.

Result :

Pulse-width modulation (PWM) is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts.PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.

Here Pulse width is refered as Duty Cycle. Duty Cycle is directly proportional to Voltage. While Changing the Duty Cycle it changes the Voltage. By reducing the Duty cycle from 75% to 45%,means Average Voltage and current will be produced. There is no Sudden rise and drop occurs and constant current will be produced. Therefore armature current does not shoot up when motor changes direction from forward to reverse.

STUDY - 2 :

Four-Quadrant Chopper DC drive (DC7) Block :

In MATLAB, Open "Four-Quadrant Chopper DC drive (DC7) block" model using Help Section.

In above Four-Quadrant CHopper DC drive DC7 block model, 3 Phase Voltage source and AC to DC Converter contains Rectifier bridge and Braking Chopper is added to Four-Quadrant Chopper DC drive DC7 block. The armature Voltage is provided by IGBT converter controlled by two PI regulators. The first regulator is Speed regulator and followed by Current regulator.

Controller Schematics :

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 armature current by computing the appropriate duty ratios of the 5kHz pulses of the four IGBT devices (Pulse Width Modulation).

Working Principle of Four-Quadrant DC drive DC7 block :

A four quadrant chopper is a chopper which can operated in all the four quadrants. The power can flow either from source to load or load to source in this chopper. In first quadrant, a Class-E chopper acts as a Step-down-chopper, whereas in second quadrant it behaves as a Step-up-chopper. This type of chopper is also known as Class-E or Type-E chopper.

Four-Quadrant Chopper block represents a four-quadrant controlled chopper for converting a fixed DC input to Variable DC Output. The block contains two bridge arms. Each bridge arm has two Switching devices.

Fig : Four-Quadrant Chopper or Type E chopper

The circuit of a four quadrant chopper or class-E chopper basically consists of four semiconductor switches CH1 to CH4 and four diodes D1 to D4. The four diodes are connected in anti-parallel.

First Quadrant :

For first quadrant operation, thyristor CH4 is kept on, thyristor CH3 is kept off and thyristor switch CH1 is operated. With CH1, CH4 ON,the load gets directly connected to the source and armature voltage (Vo = Vs) and armature Current (Io) begins flow. Here both Vo and Io are positive giving first quadrant operation.

when CH1 is turned off, positive current freewheels through CH4, D2. In this manner, Vo, Io can be controlled in this first quadrant, and Motor rotates in the forward direction, so it is called Forward Motoring.

Second Quadrant :

Here thyristor CH2 is operated and CH1 , CH3 and CH4 are kept off. With CH4 on, reverse or negative current flows through L , CH2, D4 and E. During the operation time of CH2, the armature inductance ‘L ’ stores energy during the time CH2 is on.

When CH2 is turned off, current is fed back to source through diodes D1 , D4 .Note that here (E+L(di/dt)) is more than the source voltage Vo .As the Vs is positive and Io is negative, it is a second quadrant operation gives forward braking mode. In that power is fed back from armature to source.It is called as Forward Regenerative Braking.

Third Quadrant :

For third quadrant operation, thyristor CH1 is kept off, CH2 is kept on and CH3 is operated, polarity of armature back emf (E) must be reversed for this quadrant operation. With thyristor CH3 is on, armature gets connected to source V. so that both Vo , Io are negative, leading to third quadrant operation.

When CH3 is turned off,negative current free wheels through CH2,D4. In this manner only Vo and Io can be controlled in the third quadrant. Motor rotates in reverse direction and it is called Reverse Motoring.

Fourth Quadrant :

Here thyristor CH4 is operated and other devices kept off, back emf (E) must have its polarity reversed as in third quadrant operation. With CH4 on, positive current flows through CH4,D2,L and E (armature). Armature inductance L stores energy during the time CH4 is on.

When CH4 is turned off, current is fed back to source through diodes D2, D3.Here armature voltage Va is negative, but Ia is positive, leading to the chopper drive operation in the fourth quadrant. Also power is fed back from armature to source. This is Known as Reverse Regenerative Braking.

Results :

Scope Result :

The speed reference is set at 500 rpm at t = 0 s. Observe that the motor speed follows the reference ramp accurately (+400 rpm/s) and reaches steady state around t = 1.3 s.

The armature current follows the current reference very well, with fast response time and small ripples. Notice that the current ripple frequency is 5 kHz.

At t = 2 s, speed reference drops to -1184 rpm. The current reference decreases to reduce the electromagnetic torque and causes the motor to decelerate with the help of the load torque.

At t = 2.2 s, the current reverses in order to produce a braking electromagnetic torque (dynamic braking mode). This causes the DC bus voltage to increase.

At t = 3.25 s, the motor reaches 0 rpm and the load torque reverses and becomes negative. The negative current now produces an accelerating electromagnetic torque to allow the motor to follow the negative speed ramp (-400 rpm/s). At t = 6.3 s, the speed reaches -1184 rpm and stabilizes around its reference.

Comparision of Four-Quadrant chopper DC7 block vs H-bridge model :

1. Four-Quadrant Chopper can operates in all four modes (Forward Motoring,Forward Braking, Reverse Motoring and Reverse Braking), but H-bridge can operate only in two modes (Forward and Reverse).

2. In Four-Quadrant Chopper Regenerative Braking is possible, but H-bridge Regenerative Braking is not possible.

3. In Four-Quadrant Chopper Two PI regulator (Speed Regulator and Current Regulator) is used to obtain electromagnetic torque needed to reach the desired speed and control the armature current and stabilize the system, but in H-bridge model PI regulator is not used.

4. H-bridge is a electronic circuit that switches the polarity of voltage applied to the load and Four-quadrant Chopper is electronic switch that is used to interupt one signal under the control of another.

5. H-bridge circuit are often used in Robotics and other applications to allow dc motor to run forward and backward.Four-Quadrant Chopper circuit is used to refer numerous type of electronic sitching devices and circuits used in power control and signal applications.

6. Four-Quadrant Chopper model gives better efficiency and high energy(power) storage than H-bridge model.

Study - 3 :

EV Model using Four-Quadrant Chopper DC drive DC7 Block :

The above Fig shows EV model using Four-Quadrant Chopper DC drive DC7 block. Four Quadrant Chopper DC drive DC7 block which is already discussed in Study-2. For making a Suitable EV model using Four-Quadrant Chopper DC drive DC7 block following steps are to be followed.

Subsystem of DC Voltage Source :

Energy source of an EV is a battery or a fuel cell. Simple DC Voltage Source is taken as a energy source.3Phase Voltage source and AC-DC rectifier is replaced with Simple DC Voltage source and Resistor is connected in series with DC Voltage source. In order to limit the DC bus Voltage during dynamic braking mode, a braking chopper is connected with DC Voltage source. In DC voltage source 440V is applied as a appropriate Voltage value for an EV.

Drive Cycle Source :

Speed reference block is replaced with Drive Cycle Source.Drive Cycle data is prepared in excel sheet and imported in workspace. Drive Cycle data includes Time and Speed is input to Drive Cycle source.

Subsystem of Vehicle Torque-Speed Characteristics :

Next Linear load Torque is replaced with Vehicle load characteristics for the relating Torque and Speed of the Vehicle. Vehicle Torque-Speed Characteristics equation is give below,

T = 24.7+0.0051ω^2

The above equation is created in Simulink model and made subsystem of Vehicle Torque-Speed in the model.

Results :

Drive Cycle Imported data in Drive Cycle Source :

Drive Cycle imported data has shown above. It includes Time (Sec) and Speed (RPM), Motor Speed requirement is in RPM.

Plot Result :

- From the above plot-1, observed that when the motor Speed increases, IGBT 1&4 duty cycle also increases.

- From the above plot-2, IGBT 2&3 is mirror image of IGBT 1&4. when the motor Speed increases, IGBT 2&3 of duty cycle decreases.

- In Plot-3, According to the motor speed Voltage level will be supplied from the DC Voltage Source. Whenever Speed increases more voltage will be required.

- In Plot-4, Observed that Voltage is proportional to Current. when Voltage increases Current increases and current in positive, when voltage decreases current also decreases and current goes in negative.

- In Plot-5, Motor Speed has shown, which was input reference speed is given in Drive Cycle Source. Motor Speed rises and reached maximum speed of 1000rpm in (11-12sec) and decreases to 0rpm in 20sec as per given input reference speed.