All Questions need to be answered TASK 1: Understand common ...

tangybuyerElectronics - Devices

Oct 7, 2013 (3 years and 8 months ago)

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All Questions need to be answered


TASK 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 Form Factor

e)

The Ripple Factor

f)

The efficiency of the rectifier

g)

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

h)

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)

For motors up to a few
kilowatts the armature current

can be supplied from
either single
-
phase or three
-
phase mains, but for larger motors three
-
phase is
always used. A separate thyristor or diode rectifier is used to supply the field
current
of the mot
or: the power is much less than the armature power, so the
supply is often single
-
phase, as shown

below





You are required to investigate the operation of the three phase rectifier bridge
circuit shown above indentifying the following.


a)

The circuit diag
ram for a three phase rectifier bridge connected to a delta/star
Delta transformer.

b)

The current wave forms on the on the AC side of the rectifier, when the motor
is operating at low speed and high torque.

c)

The current feedback loop requires the measurement
of very large DC
currents and you are required to investigate how this may be achieved.

d)

The motor is to be used at low speeds and high torques you are required to
comment on this and in particular what effect this has on the AC supply
system suggesting how

its performance may be improved.

e)

A discussion of regenerative braking and w
hy the system shown cannot be
used for regenerative breaking


TASK 2 (l02,lo3)
Understand the methods used for DC

and AC

motor control




(3)

The circuit above shows a simple DC contr
ol 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 t
he
major disadvantage of such a circuit.


(4)

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 MOSFET Bridge


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



(5)

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 d
iscussing


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 revers
ed 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.




(6)

Variable frequency drives are used to control the speed of AC induction
motors. Investigate the design of variable frequency designs and identify the
di
fference 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.




TASK 3
Understand other applications of power electronics


(7)

Investigate the use of Power Electronic devices used in the modern Hybrid
automobiles you are to produce:


a)

A block diagram of the various power electronic devices and a diagram
indicating how they are connected?

b)

The Power Electronic devices used.

c)

How Regenerative breaking is achieved identifying the energy flows during
this mode of operation.

d)

Automatic drive assist and how it is achieved.

e)

Automatic shutdown/shutoff


(8)

Investigate t
he use of Power Electronic Devices operation of UPS
systems identifying the key parameters used in the design of the power
electronic devices.