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
up
up
U
R
C
ip
L
ip
V4
V14
V2
V12
Pic 1 Single-phase inverter (RL load, Ψ = π, UpRMS = U)
Ψ – control angle
up
ip
Ψ
Pic 2 Single-phase inverter (RL load, Ψ < π, UpRMS < U)
An effective load voltage (Single-phase inverter):
U pRMS  U 


(3.1)
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
U
L2
L3
V4
V04
V6
V06
V2
3xR
V02
Pic 3 Three-phase inverter
0°
60° 120° 180° 240° 300° 360°
Switch-on
V1
Switch-off
V2
V3
V4
V5
V6
2/3U
1/3U
Up1
t
-1/3U
U
-2/3U
U1-2
t
-U
Period
Pic 4 Six-step switching sequence and waveform of voltage (Ψ = 180° = π)
Up1 – phase voltage, U1-2 – line voltage
2
Inverter
An effective load voltage only for R load (Three-phase inverter):

 23

2
2
3
1 2
1  1 
2

2 
U 3 pRMS   180 
u t dt 
U
 U  dt    U  dt   ... 



T
 0 3 
3 
3

0




U 3 pRMS   180 
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
(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.
Output voltage
U/2
-U/2
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.
3
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 Ψ = 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
Ψ = 180°, 150°, 120°.
4. PWM control of three-phase inverter with AC motor load: display waveforms of
voltage and current for various switching frequencies.
D3
D5
V1
V01
V3
V03
V5
L1
L2
C
V05
L1
L2
L3
U
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
V1 – V6
V01 – V06
M 3~
3-phase supply
Rectifier (6-pulse)
electrolyte capacitor
Three-phase inverter
AC motor
Optional reference:
http://en.wikipedia.org/wiki/Variable-frequency_drive
4
 VFD system description
V02
M
3
D1
3 x 400 V
50 Hz