Lecture Notes
EEE 360
George G. Karady

TOPIC 7
Power Electronics

Read Mohan Chapter: 6.1-6,5, 6.7

LECTURE 23
SINGLE PHASE CONTROLLED RECTIFIER
The rectifier converts the ac voltages to dc voltage.
The most frequent applications are:
Battery charger
DC motor drive (speed and torque control)
Power supplies for appliances, computers e.t.c.

SINGLE PHASE CONTROLLED RECTIFIER
The rectifier converts the ac voltages to dc voltage.
The rectifier is supplied by an ac source, which is represented by its thevenin equivalent: AC voltage source and a reactance connected in series
At the dc side the load can be:
Inductive load, which is represented by a constant current source
Capacitive, which is represented by a voltage source
Resistive, represented by a resistance
SINGLE PHASE CONTROLLED RECTIFIER
Basic single phase rectifier circuit contains:
AC Supply
Switch
Load ( inductive, resistive or capacitive)
SINGLE PHASE CONTROLLED RECTIFIER
The most frequently used circuit is the single phase bridge.
This circuit has four switching devices
The circuit provides full-wave rectification
AC
DC
SINGLE PHASE CONTROLLED RECTIFIER
The main element of the rectifier is the directional switch.
The most frequently used switches are:
Thyristor,
GTO (Gate Turn off- Thyristor),
IGBT (Insulated Gate Bipolar Transistor),
Power MOSFET
The rectifier operation will be demonstrated using thyristor switches
THYRISTOR
Thyristor, is a switch that conduct only in one direction when:
The voltage between anode and cathode is positive
The gate is triggered, by a short pulse
This is illustrated on the next slide
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
In the circuit above the thyristor is triggered by a short square pulse, during the positive cycle
The turn on the devices switches the ac voltage to the load and drives current through the resistance
The turn on delay controls the average dc voltage
Simplified rectifier circuit, supply and load reactance is zero
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
DC voltage with gate control

DC voltage without gate control

Gate control pulse
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
The turn on delay (a) controls the average dc voltage.
The average voltage is the integral of the bleu curve for a cycle

SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
Effect of supply and load inductance
The loop equation for the simplified circuit, if the thyristor conducts, is:

This differential equation is solved by using MATCAD. Student switch to MATCAD
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
The MATCAD solution of the differential equation produced the following results

Voltage
Current
The current flows after the voltage zero in case of inductive load
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
The method of analytical solution has been demonstrated
The analytical solution for capacitive and inductive or other loads are complicated.
The PSPICE simulation is used to analyze the rectifier operation
Student switch to PSPICE
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
PSPICE simulation of inductive loaded rectifier
Circuit diagram


Vac = 120V (peek), 60Hz
Ls = 0.1mH
Lload = 10H
Rload = 20 ohm
Co = 10F,
Initial voltage Vc_in = 100V
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
PSPICE simulation inductive load

Voltage source and inductance supply the thyristor
The thyristor is represented by an ideal switch, diode and a gate pulse generator
The load is a resistance and reactance
The firing signal can be delayed by software command
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
The result of the transient run

Voltage
Current
Firing Pulse
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
The result of the transient run
The firing pulse starts the current flow.
The current is intermittent and flows only one direction. No negative current flows in this circuit
Because of inductance the current flows after the voltage zero crossing.
The firing delay controls the average dc voltage.
The next figure shows the increase of inductance increases the duration of the current pulse, but reduces the amplitude
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
The effect of increasing inductance

Voltage
Current
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
PSPICE simulation of capacitive loaded rectifier
Circuit diagram

Vac = 120V (peek), 60Hz
Ls =0.01uH
Rload = 20 ohm
Co = 10F,
Initial voltage Vc_in = 100V

SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
PSPice simulation capacitive load

Voltage source and inductance supply the thyristor
The thyristor is represented by an ideal switch, diode and a gate pulse generator
The load is a resistance and capacitor
The current flows only if the AC voltage is higher than the capacitor voltage and the tyristor is triggered
SINGLE PHASE RECTIFIER WITH THYRISTOR SWITCH
The result of the transient run, capacitive load

AC Voltage
Current
Firing Pulse
Capacitor voltage
TEST 5

DC Machine
LECTURE 24
SINGLE PHASE CONTROLLED RECTIFIER
The single phase bridge circuit provides full-wave rectification.
This circuit has four thyristors
The delay of firing controls the average dc voltage
AC
DC
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
The single phase bridge circuit operation is demonstrated using a simplified circuit.
Zero source impedance and pure resistive load is assumed
The voltage is controlled by the firing delay angle
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
In the positive cycle Th1 and Th2 conduct.
The current path is shown in the figure.
The current and DC voltage is controlled by the firing delay
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
In the negative cycle Th3 and Th4 conduct.
The current path is shown in the figure.
Load current and voltage is positive. Full wave rectifier
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
PSPICE simulation of the operation. Inductive load
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Inductive load with small inductance.
Firing pulse
AC voltage
DC voltage and current
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Inductive load with small inductance.
The firing pulse initiates the current. The current flows only positive direction.
The current flow stops when the current becomes zero
Because of the inductive load the intermittent current flows after voltage zero crossing.
The firing delay controls the dc current and voltages.
The dc voltage and dc current shape is identical.
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Inductive load with large inductance.

SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Inductive load with large inductance.
The firing pulse initiates commutation from T1&T2 to T3&T4 or T3&T4 to T1&T2.
The commutation occurs after voltage zero crossing
The dc current flows continuously, but ac modulation, ripples can be observed
Firing delay controls the dc voltage across the resistance.
The dc voltage and current has identical shape. Vdc = IdcR
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
PSPICE simulation of the operation. Capacitive load
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Capacitive & Resistance load.
The current flows only if the AC voltage is higher than the capacitor voltage and the thyristor is fired
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Capacitive & Resistance load.
The firing pulse initiates the current, if the AC voltage is larger than the capacitor dc voltage
The current flows till the AC voltage is higher than the capacitor voltage
The intermittent current charges the capacitor and increases the dc voltage.
DC capacitor voltage is continuous and controlled by the firing delay.
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Source inductance effect.
The source inductance delays the commutation from Th1 to Th3
Th1 current decreases slowly as Th3 increases. The total current is constant.
The simultaneous conduction of Th1 and Th3 produces a short circuit. The circulating current increases Th3 and decreases Th1
The circulating current is limited by the source inductance
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Source inductance effect.
The voltage is negative whenTh3 is triggered
This negative voltage drives a circulating current in the short circuited loop
The circulating current reduces the current of Th1 and increases of the current Th3
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
PSPICE simulation effect of source inductance
Load is : resistance and inductance, source inductance varies
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Source inductance effect. Lsource = 12mH

Large inductance causes long commutation time
Th3
Th1
IAC
SINGLE PHASE CONTROLLED BRIDGE RECTIFIER
Source inductance effect. Lsource = 0.2mH
LECTURE 25
SINGLE PHASE BRIDGE RECTIFIER INVERTER OPERATION
The inverter converts the DC voltage to AC

In inverter operation the DC source supply power to the AC source.

Solar power generated by photovoltaic cells is inverted to supply the ac network

In an un-interruptible power supply stores the energy in a battery. Energy stored in this battery is inverted to supply the ac network
SINGLE PHASE BRIDGE RECTIFIER INVERTER OPERATION
The adjustment of firing angle delay between 90-180deg in a thyristor bridge rectifier results in inverter operation

Inverter operation requires large inductance connected in series with the converter at the dc side

The large inductance maintains more or less constant dc current

The inverter operation requires an AC source at the AC side to generate sinusoidal voltage
SINGLE PHASE BRIDGE RECTIFIER INVERTER OPERATION
The inverter operation is simulated by PSPICE by adjusting the firing delay angle between 90-180 deg.
SINGLE PHASE BRIDGE RECTIFIER INVERTER OPERATION
Inverter operation firing delay angle is 135 degree
AC Voltage
AC Current
Delay angle
SINGLE PHASE BRIDGE RECTIFIER INVERTER OPERATION
The simulation shows that the AC voltage and current polarity is opposite.
When the voltage is positive current is negative and vice versa

This indicates generator operation, the power transferred from DC to AC

The power is negative, because the power factor (cos f) is negative between 90 and 180 degrees
SINGLE PHASE BRIDGE RECTIFIER INVERTER OPERATION
The nearly square shape current produces undesirable harmonics in the ac system
The FFT analyses shows 3th,5th,7th …harmonics
SINGLE PHASE BRIDGE RECTIFIER WITH PULSE WIDTH MODULATION
The bridge rectifier is controlled by the firing delay
The firing delay cuts out a part of the voltage wave. This reduces the average dc voltage.
Similar effect can be achieved by distributing the off periods along the half cycle
The distribution of the delay improves performance, reduces the harmonics
SINGLE PHASE BRIDGE RECTIFIER WITH PW MODULATION
Demonstration of delayed firing and the distributed firing delay
The system performance can be further improved by changing the width of the on and off periods.
Delayed firing Distributed firing delay
SINGLE PHASE BRIDGE RECTIFIER WITH PULSE WIDTH MODULATION
Concept of Pulse-Width modulation: The firing delay is distributed along period and the on and off times are varied to reduce harmonics
The most frequently used PW method is the sinusoidal PW modulation
The switches in the converter are controlled by pulses.
The width of each pulse is varied in proportion to the amplitude of a sine wave
SINGLE PHASE BRIDGE RECTIFIER WITH PULSE WIDTH MODULATION
Generation of the PWM waveform
A triangular carrier wave is generated
A sinusoidal reference signal is generated
The two signals are compared, when the carrier wave is larger than the reference signal the gate signal is positive
When the carrier wave is smaller than the reference signal the gate signal is zero

SINGLE PHASE BRIDGE CONVERTER WITH PULSE WIDTH MODULATION
Gate signal generation for PWM converter
Carrier wave
Reference signal
SINGLE PHASE BRIDGE CONVERTER WITH PULSE WIDTH MODULATION
Gate signal generation for PWM converter. Firing pulse with variable width
SINGLE PHASE BRIDGE CONVERTER WITH PULSE WIDTH MODULATION
PWM Converter
The generated firing signal controls the converter switches
The converter switches have to switch on and off the current
This can be achieved by GTO (Gate turn on transistor) or GTO, gate turn off thyristor, or MOSFET
The switches have to be shunted by a diode to avoid the over voltages generated by the interruption of inductive current
SINGLE PHASE BRIDGE CONVERTER WITH PULSE WIDTH MODULATION
PWM Converter
The converter can operate both in inverter or in rectifier mode
Advantage of this circuit are :
in inverter mode it can supply passive load.
significant reduction of current and voltage harmonics
SINGLE PHASE BRIDGE CONVERTER WITH PULSE WIDTH MODULATION
PWM Converter
SINGLE PHASE BRIDGE CONVERTER WITH PULSE WIDTH MODULATION
Operation analysis with PS SPICE
The students are provided with the model, they change the parameters and observe the results:
Load inductance and resistance
Switching frequency
Calculate harmonics and RMS values
SINGLE PHASE BRIDGE CONVERTER WITH PULSE WIDTH MODULATION
Operation analysis with PS SPICE
PWM Inverter supplies AC network with voltage source
SINGLE PHASE BRIDGE RECTIFIER WITH PULSE WIDTH MODULATION
PWM Output Voltage
Load Current
Load Voltage
Current, voltage waveforms in bipolar operation.
SINGLE PHASE BRIDGE RECTIFIER WITH PULSE WIDTH MODULATION
L1 = 10mH, C = 100uF, R1 (in series with L1) = 0.1ohm and load resistance (across C) = 10 ohm.. (fundamental - 40 Hz, sw. frequency 1kHz).
Current and voltage waveforms in mono-polar operation mode
SINGLE PHASE BRIDGE RECTIFIER WITH PULSE WIDTH MODULATION
Monopolar operation Loading increases the current ripples
MOTOR DRIVE CONCEPT WITH PWM INVERTER
MOTOR DRIVE CONCEPT WITH PWM INVERTER
The ac voltage of the supply is rectified
The dc link filters the harmonics and produces smooth DC
The PW inverter produces a variable frequency and voltage sine wave that drives the motor
The frequency regulates the motor speed.
The voltage and frequency ratio is kept constant to avoid saturation at low frequencies