Power electronics involves the use of semiconductor devices to control a range of applications in rectification, DC and AC motor control and controlled power supplies. To meet the challenges expected of a modern 'heavy current' engineer the unit carries an emphasis on the application of power electronics to variable speed controllers. The focus is on the power aspects rather than the associated detail of the electronic control and firing circuitry.

The unit has the following learning outcomes

1 Understand common configurations for controlled and uncontrolled rectification

2 Understand the methods used for AC motor control

3 Understand the methods used for DC motor control

4 Understand other applications of power electronics

 PART 1:

Understand common configurations for controlled and uncontrolled rectification

(1) The circuit above shows a simple half wave rectification circuit and you are required to determine the following parameters at the output of the load

a) The current wave form in the load

b) The average DC voltage

c) The RMS Voltage

d) The current wave form when an inductor is added to the load

e) How freewheeling diodes connected across the load improve the performance of the rectifier

In order to improve the above parameters a full-wave rectifier bridge is proposed and you have asked to quantify the improvements obtained by comparing the above parameters in both circuits.

2)  A single phase full wave ac controlled rectifier has an input voltage of 120 V (RMS) and a load resistance of 6 ohm. The firing angle (α) of thyristor is 45º. Find

a.RMS output voltage 
b. Power output
c. Input power factor

d. Average and RMS thyristor current.

3)

a) Explain methods of device protection. Investigate different device protection methods such as crowbar, clamping, MOV and snubber circuit. Compare and evaluate these protection circuits.

b) Discuss safe measurement techniques when measuring current and voltage in heavy current power environment specifically highlighting Hall Effect current probes.

PART 2

Understand the methods used for DC and AC motor control

(2) The circuit above shows a simple DC control circuit that is to be used to control the speed of a DC motor using complementary transistors, you are required to produce a report on the operation of the above identifying the current and voltage waveforms across each of the transistors and identify the major disadvantage of such a circuit.

(3) The circuit shown below shows the principle components of DC speed controller using Pulse Width Modulation techniques you are required to produce a report on the operation of the circuit your report should include

a) The circuit diagrams for the MOSFETBridge

b) A discussion of how the speed is controlled by varying the mark/space ratio of the pulse width modulator

c) A discussion of how bridge may be used for regenerative breaking 

d) Comment on the power dissipated in the MOSFETS

(4) The diagram below shows the arrange of two thyristor bridge rectifiers that are to be used to control the speed of DC motor that is to be used for rolling steel which requires control in all four quadrants. You are required to produce a report discussing

a) What is meant by four quadrant motor control?

b) What is meant by regenerative breaking and how the above bridges are capable of performing regenerative breaking?

c) Discuss the operation of the above thyristor bridges when the motor speed is reversed under full control.

d) The disadvantage of using the above configuration for when operating at low speed and high torque.

e) Identify the proposed advantages and disadvantages of using a DC chopper circuit fed from a three phase bridge rectifier.

f)  What type of devices can be to measure the High DC currents required by the motor.

(5) Variable frequency drives are used to control the speed of AC induction motors. Investigate the design of variable frequency designs and identify the difference and advantages of using the following methods of producing variable frequency drives

a) Cyclometer variable frequency converters.

b) A bridge rectifier feeding a three phase inverter based on PWM techniques.

c) The advantages of high power induction motors over DC drives.

6) In the figure below, A 6 pulse converter connected to 415 V ac supply is controlling a 440 V dc motor is presented. Find the angle at which the converter must be triggered so that the voltage drop in the circuit is 12% of the motor rated voltage.

Where, R_a- Armature resistance of motor.L_a- Armature Inductance