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Inverter Experiments

An inverter converts DC power to AC power by switching the DC input voltage in a predetermined sequence. It has applications in AC motor control and uninterruptible power supplies. The document discusses width control and pulse-width modulation techniques for single-phase and three-phase inverters to control output voltage and frequency. It also describes how variable-frequency drives use inverters connected to AC motors to control motor speed by varying the frequency of the electrical power supplied.

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312 views4 pages

Inverter Experiments

An inverter converts DC power to AC power by switching the DC input voltage in a predetermined sequence. It has applications in AC motor control and uninterruptible power supplies. The document discusses width control and pulse-width modulation techniques for single-phase and three-phase inverters to control output voltage and frequency. It also describes how variable-frequency drives use inverters connected to AC motors to control motor speed by varying the frequency of the electrical power supplied.

Uploaded by

visal
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOC, PDF, TXT or read online on Scribd
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Inverter

Laboratory exercise 3
Inverter (DC AC)
An inverter is an electrical device that converts DC to AC power by switching the DC
input voltage in a pre-determined sequence so as to generate AC output voltage.
Applications: AC motor control, uninterruptible power supply (UPS), etc.
3.1 Width control of Single-phase inverter

V1

V11

V3

V13

V2

V12

up
U

ip
V4

V14

Pic 1 Single-phase inverter (RL load, = , UpRMS = U)


control angle

Pic 2 Single-phase inverter (RL load, < , UpRMS < U)


An effective load voltage (Single-phase inverter):
U pRMS U

(3.1)

Inverter

3.2 Width control of Three-phase inverter


A basic three-phase inverter consists of three single-phase inverter switches each
connected to one of the three load terminals.

V1

V01

V3

V03

V5

i p1

V05

L1

up1

u1- 2

L2
L3

V4

V04

V6

V06

V2

3xR

V02

Pic 3 Three-phase inverter

Pic 4 Six-step switching sequence and waveform of voltage ( = 180 = )


Up1 phase voltage, U1-2 line voltage

Inverter

An effective load voltage only for R load (Three-phase inverter):


U 3 pRMS 180

1
u 2 t dt
T

2
3

2
2

1
2
U
U dt U dt ...
3
3
3

2
U 0,47 U
3
7
U 3 pRMS 150
U 0,44 U
6
1
U 3 pRMS 120
U 0,41 U
6
U 3 pRMS 180

(3.2)

3.3 Pulse-width modulation (PWM)


Amplitudes of the triangular wave (carrier) and sine wave (modulating, desired) are
compared to obtain PWM waveform (output voltage of inverter).
PWM is the usual method used to achieve variable voltage and frequency (can be
controlled independently) of AC motor. Change of frequency means change of motor speed.

Pic 5 PWM
3.4 Variable-frequency drive (VFD)
A VFD is a system for controlling the rotational speed of an AC motor by controlling the
frequency of the electrical power supplied to the motor.
A VFD system generally consists of an operator interface, a frequency controller and an
AC motor.

Inverter

Pic 6 VFD system


Tasks
1. Width control of single-phase inverter with RL load: display waveforms of voltage
and current. Measure and calculate URMS (3.1) for = 180, 150, 120.
2. PWM control of single-phase inverter with RL load: display waveforms of voltage and
current.
3. Width control of three-phase inverter with R load and AC motor load: display
waveforms of voltage and current. Measure and calculate URMS (3.2) for R load and
= 180, 150, 120.
4. PWM control of three-phase inverter with AC motor load: display waveforms of
voltage and current for various switching frequencies.

D1

D3

D5

V1

V01

V3

V03

V5

V05

L1
L2

L1
L2
L3

L3
TR

V4
D4

D6

V04

V6

V06

V2

D2

Pic 7 Wiring diagram of Variable frequency controller (see Pic 6)


Device:
L1 L3
D1 D6
C

3-phase supply
Rectifier (6-pulse)
electrolyte capacitor

V1 V6
V01 V06
M 3~

Three-phase inverter
AC motor

Optional reference:
http://en.wikipedia.org/wiki/Variable-frequency_drive

VFD system description

V02

M
3

3 x 400 V
50 Hz

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