Lecture 9 - Power Electronics

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Power Electronics

Lecture 9

9 October 2003

MMME2104

Design & Selection of Mining Equipment

Electrical Component


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Lecture Outline

What are power electronics

Power electronics building blocks

Rectifiers

Converters

Inverters

Applications in motor drives

Efficiency in power electronics


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Power Electronics

The task of power electronics is to process and control theflow of electric energy by supplying voltages andcurrents in a form that is optimally suited to the load.

Power electronics allows you to:

convert from AC DC (rectifiers)

convert from DCAC (inverters)

convert from DC DC or ACAC (converters)

control the frequency of AC supplies

control the voltage and current output of AC and DCsupplies


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Power Electronics Building Blocks

Inductors Capacitors

Semiconductors

Diodes Switches


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Inductors and Capacitors

Inductor: V = L di/dt

The current in an inductor cannot change instantaneously!

Capacitor: i = C dV/dt

The voltage across a capacitor cannot change instantaneously!

Thesepassive components are fundamental to the

operation of all power electronics.


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Rectifiers (AC-DC)


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The Most Basic Diode Rectifier


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Full Bridge Diode Rectifier


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Single-Phase Full Bridge Rectifier

Converts single-phase AC to DC: VDC = 2 VAC

e.g.

240VAC

e.g.

340VDC


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Three-Phase Full Bridge Rectifier

Converts 3-phase AC to DC: VDC = 2 VAC

Example application: Brushless excitation in

synchronous machines

e.g.

415VAC

e.g.

590VDC


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Three-Phase Full Bridge Rectifier

Input

Output


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DC-DC Converters


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The Most Basic DC-DC Converter

This switching technique is known aspulse-width

modulation (PWM)

The duty ratio D = ton/Ts


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Step-Down (Buck) Converter

Converts DC to a lower DC voltage: Vo = DVd

Example application: field control in DC motors


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Step-Down (Buck) Converter


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Step-Up (Boost) Converter

Converts DC to higher voltage DC: Vo = Vd/(1-D)

There are also buck-boostDC-DC converters, whichconvert to higher and lower voltages


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Full-Bridge DC-DC Converter

Provides bi-directional DC-DC power flow

Application: 4-quadrant control of DC motors


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Inverters (DC-AC)


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Single-Phase Full-Bridge Inverter

Converts DC to single-phase AC

Same topology as DC-DC full bridge


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Single-Phase Full-Bridge Inverter

Control

signals

Output

= filtered output

Switching frequency

Carrier frequency


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Three-Phase Full-Bridge Inverter

Converts DC to 3-phase AC

Example application: AC motor drives


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Variable Speed AC Drive

E.g. VVVF control of induction motor

3-phase

rectifier

3-phase

inverter

AC

motor

415V

3-phase

supply

600V DC link


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AC-AC Conversion via Power Electronics

Instead of using a transformer:

AC-AC conversion can be performed indirectly

via a DC link (as per previous slide). AC-AC conversion can also be achieved directly

with a matrix converter topology (beyond the

scope of this course)


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Semiconductor Devices


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Losses in Power Electronics

In semiconductor components:

Switching losses

Conduction losses

In passive components (C & L):

Effective series resistance (ESR)

Power electronics are normally designed as efficient as practical for two

reasons:

Cost of wasted energy

Difficulty in heat removal

Typical efficiencies are in the range of 90-99% for each conversion

stage, depending on the exact converter topology.

f


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Reference

N. Mohan, T. Undeland & W. Robbins (1995)

Power Electronics: Converters, Applications and

Design, 2nd Edition, Wiley, New York.

Lecture 9 - Power Electronics

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