Power Electronics
5th semester
Course content:
Unit 1. Power Electronics Devices (10)
1.1 Power
diode – Construction, Characteristic and ratings.
1.2 Power
Transistor - Construction, Characteristic, use as power switch
1.3
Thyristor – Construction, Characteristics, Turn on mechanism, Turn-on process
with gate signal, thyristor firing circuit, thyristor commutation and its type
1.4 GTO -
Construction, Characteristics,
1.5 TRIAC –
Construction and Characteristics.
1.6 MOSFET -
Construction and Characteristics.
1.7 IGBT -
Construction and Characteristics.
Unit 2.
Power Rectifier (11)
2.1 Half
wave and full wave single-phase rectifier – Circuit diagram, operating
principle, calculation of average value, rms value, ripple factor, efficiency,
filtering – C, L and LC filters.
2.2
Single-phase full wave controlled rectifier with resistive load, RL load,
inversion mode of operation.
2.3
Three-phase rectifier with three numbers of diode.
2.4
Three-phase bridge rectifier with six numbers of diode.
Unit 3. DC
Chopper (6)
3.1 Step
down chopper – Circuit diagram, operation, constant and variable chopping
frequency operation.
3.2 Step up
chopper – Circuit diagram and operation.
3.3
Application in speed control dc motor.
Unit 4.
Inverter (10)
4.1 Single
phase square wave inverter – Circuit diagram, operating principle, rms value of
output voltage, operation with resistive load. Step down chopper – Circuit
diagram, operation, constant and variable chopping frequency operation.
4.2
Three-phase bridge inverter with six-step output voltage wavefrorm – Circuit
diagram, operating principle, rms value of output voltage, operation with
resistive load.
4.3
Application of inverter in speed control of induction motor and synchronous
motor.
Unit 5. AC
voltage controller (8)
5.1.
Single-phase ac voltage controller – Circuit diagram, operation with resistive
load.
5.2. Three
-phase ac voltage controller – Circuit diagram, operation with resistive load.
5.3.
Applications in speed control of induction motor, Electronic load controller
for MHP generator, light dimmer.
Introduction
Power
electronics is an enabling technology, providing the needed interface between
an electrical source and an electrical load, as depicted in Figure.
The electrical source and the electrical load
can, and often do, differ in frequency, voltage amplitudes, and the number of
phases.
The power electronics interface facilitates
the transfer of power from the source to the load by converting voltages and
currents from one form to another, in which it is possible for the source and
load to reverse roles.
Click here for Thyristor introduction notes
Turn on methods of SCR
The turning on Process of the SCR is known
as Triggering. In other words, turning the SCR from
Forward-Blocking state to
Forward-Conduction state is known as Triggering. The various methods of SCR
triggering are discussed here.
The various SCR triggering methods are
§ Forward Voltage
Triggering
§ Thermal or Temperature
Triggering
§ Radiation or Light
triggering
§ dv/dt Triggering
SCR Turn OFF Methods
SCR can be turned ON by applying appropriate positive gate voltage between the gate and cathode
terminals, but it cannot be turned OFF through the gate terminal. The SCR can be brought back to the
forward blocking state from the forward conduction state by reducing the anode or forward current below
the holding current level.
The turn OFF process of an SCR is called commutation. The term commutation means the transfer of
currents from one path to another. So the commutation circuit does this job by reducing the forward
current to zero so as to turn OFF the SCR or Thyristor.
The reverse voltage which causes to
commutate the SCR is called commutation voltage. Depending on
the commutation voltage located, the
commutation methods are classified into two major types.
Those are 1) Forced commutation and 2)
Natural commutation. Let us discuss in brief about these
methods.
Forced Commutation
In case of DC circuits, there is no
natural current zero to turn OFF the SCR. In such circuits, forward
current must be forced to zero with an
external circuit to commutate the SCR hence named as forced
commutation.
Natural Commutation
In natural commutation, the source of
commutation voltage is the supply source itself. If the SCR is
connected to an AC supply, at every end of
the positive half cycle the anode current goes through the
natural current zero and also immediately
a reverse voltage is applied across the SCR. These are the
conditions
to turn OFF the SCR.
The load is assumed resistive for the sake of simplicity. The input source is VmSinωt.
For the positive half cycle of input source, thyristor T1 is forward biased and hence it is able to conduct provided gate signal is applied. This means that T1 will remain OFF until gate signal is applied. Now suppose, at some angle α (called the firing angle), thyristor T1 is gated. As soon as T1 is fired / gated, it starts conducting and hence, load gets directly connected to the source. This makes load voltage Vo = VmSinα and load current Io = (VmSinα / R) at the instant T1 is fired. From ωt = α to π, the load voltage and current follows the input voltage waveform VmSinωt and (VmSinωt / R) respectively.
After ωt = π, thyristor T1 becomes reversed biased and the load current becomes zero (note that load voltage and current are in phase, hence as soon as load voltage becomes zero, load current also becomes zero) and hence thyristor T1 is commutated naturally.
After ωt = π, diode D1 becomes forward biased and hence starts conducting. This makes load voltage & current to follow the supply voltage VmSinωt and (VmSinωt / R) respectively for the negative half cycle.
The output waveform for load voltage & current is shown below.
Following points may be noted from the above waveforms:
- By having a control on the firing angle α, the load voltage may be controlled. It may be seen from the output waveform that, there is no control on the negative half cycle of the input voltage. This is the reason, a single phase half wave AC voltage controller is also known as single phase unidirectional voltage controller.
- The positive and negative half cycle of the load voltage & current are not identical. As a result, DC component is introduced in the supply and load circuit which is undesirable
 |
| ......... |
I want the permission of access
ReplyDelete