Download CONTROL OF AC LOADS FROM AN AC SUPPLY

INVERTERS
Definition
An inverter converts d.c. to a.c. An inverter is an electronic device, which the normal
direction of power is from a d.c. source to an a.c. load. Invariably it is necessary for a
thyristor inverter to be forced commutated unless the load happens to be a leading power
factor.
Force Commutation
I commutation
Commutation
circuit
V d.c.
load
Inverter grade thyristors turn off in 6-8s whereas ordinary thyristors turn off in 50-100s.
Ordinary thyristors are quite adequate for rectifiers and inverters working with an ac supply
of 50 Hz, however with an inverter running at 2 kHz it is vital that inverter grade thyristors
are used in order to reduce switching losses, i.e. increase the efficiency of the power
conversion stage.
The turn off process itself is concerned with obtaining a current, which is able to pass in the
reverse direction through the thyristor such that the anode current falls below the holding
current for the required period of time. During this time minority charge carriers have to be
cleared away by recombination with and diffusion process before the device can block
reapplied forward volts. The time from the instant the anode current passes through zero to
the instant it is capable of blocking reapplied volts is defined as the turn-off time.
Single-Phase Bridge Inverters
D1
TH1
TH3
A
D3
The diodes are necessary
for free-wheel paths for
inductive loads
B
load
D2
TH2
TH4
D4
1
A simplified diagrammatic version
A
load
B
The sketch shown is a single-phase bridge inverter; the gating and circuits have been omitted
for clarity. The circuit can be divided into two sections TH1 and TH4, TH3 the load and TH2.
When TH1 and TH4 are gated, current passes through the load from A to B, when TH3 and
TH2 are gated current passes from b to A. In this way terminal A is alternatively connect to
the positive and negative terminals of the battery, and likewise for terminal B. If the time
taken to switch the thyristors on and off is negligible and the volt drop across each thyristor is
zero in the forward direction the following waveforms will occur.
Voltage Waveforms For A Single-Phase Inverter
VA –N
Vdc
Th1 and TH4 on
VB –N
Vdc
Th1 and TH4 on
Th3 and TH2 on
VAB = VAN -VBN
VAB
Vdc
Current Waveforms
(a)
Resistive Load
IR
2
(b)
Inductive Load
IL
If the load is resistive then the output current will be the same shape as the voltage. If the load
is inductive, i.e. an induction motor then the load current will lag the supply voltage.
The output voltage is a square wave of constant magnitude and maybe produced by varying
frequencies by control of the gate and commutation circuits. If a sinusoidal ac waveform is
required in preference to a square wave this can be achieved by the use of appropriate filters,
however this is very expensive.
3 Phase Inverters
TH1
TH3
TH5
TH2
TH6
TH4
a
c
b
1
3
5
4
6
2
c
a
R
R
R
3
Vdc
3
o
a
b
Vdc
R
Step 1
o
2Vdc
2R
3
3
b
c
3
1
3
5
4
6
2
a
2Vdc
R
3
Vdc
a
o
b
R
Step 2
o
2R
3
Vdc
R
b
R
3
3
c
c
3
1
5
b
6
4
a
2
R
Vdc
R
R
3
3
o
a
b
Vdc
2Vdc
R
Step 3
o
2R
3
3
c
c
1
3
5
6
2
1,2,3,4,5 & 6 represent TH1, TH2, TH3, TH4, TH5,
& TH6.
b
4
2Vdc
R
3
2R
3
o
a
b
Vdc
R
Step 4
o
c
Vdc
R
R
3
3
a
c
4
Vao
2.5
2
1.5
1
0.5
0
-0.5 0
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
-1
-1.5
-2
-2.5
2.5
2
Vbo
1.5
2.5
21
0.5
1.5
10
-0.5
0.5 0
-1
0
-1.5 0
-0.5
1
2
3
4
5
6
7
8
-2
-1
-1.5
-2.5
-2
-2.5
2.5
Vab= 2Vao - Vbo
2.5
1.5
2
1
1.5
0.5
1
0
0.5
-0.5 0
1
2
3
4
5
6
7
8
0
-1
-0.5 0
1
2
3
4
5
6
7
8
-1.5
-1
-2
-1.5
-2.5
-2
-2.5
5
Inverters for providing power at variable frequencies of a.c. motor drives are usually threef
phase inverters, (speed of an a.c. motor =
), cyclo-converters may also be used;
pole pairs
however they have a limited frequency range. Like the single-phase inverter the three phase
takes the form of an a.c. supply with a rectifier or thyristor bridge or a d.c voltage controller.
The pattern in which the thyristors are turned on and off is fixed in the electronic control, this
pattern syntheses the desired three phase waveforms as shown.
Sequence
1
1
2
3
2
2
3
4
3
3
4
5
4
4
5
6
5
5
6
1
6
6
1
2
The manner in which TH1 is turned off and TH4 is turned on is part of the commutation
process and involves components which have not been shown. The same sequence occurs for
TH3 and TH6 and TH5 and TH2. The output voltage of each stage (a), (b) and (c) with
reference to the neutral point O, can be obtained from the switching diagrams above. These
line to neutral voltages are called six step waveforms and approximate to a sine wave while
the line voltages are 120o apart having positive and negative pulses.
Cyclo-convetors AC to AC – Single-Phase
TH1
load
TH2
E
TH3
TH4
The manner in which TH1 is turned off and TH4 is turned on is part of the commutation
process and involves components which have not been shown. The same sequence occurs for
6
TH3 and TH6 and TH5 and TH2. The output voltage of each stage (a), (b) and (c) with
reference to the neutral point O, can be obtained from the switching diagrams above. These
line to neutral voltages are called six step waveforms and approximate to a sine wave while
the line voltages are 120o apart having positive and negative pulses.
Cycloconvetors AC to AC – Single-Phase
TH1
load
TH2
E
TH3
TH4
The cycloconvertor is a means of changing the frequency of the alternating supply using
controlled rectifiers; this is achieved by ‘steering’ the waveform in the required direction to
produce a lower frequency ac voltage on the load. If TH1 and TH3 are fired as the supply
alternates from positive to negative then the voltage across the load is in positive direction
(these two thyristors acting like a single-phase centre tapped positive rectifier). If TH2 and
TH4 are fired as the supply alternates from positive to negative then the voltage on the load is
negative (TH2 and TH4 act like a single-phase centre tapped negative rectifier). It can be seen
that it is possible to obtain frequencies lower than the fundamental frequency of the supply,
however it is not possible to obtain frequencies higher than the supply frequency. It should be
noted the frequencies obtained are multiples of the periodic time of the fundamental.
1
  20ms
f
1


1
 50 Hz
20x10 3
0.5
0
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37
-0.5
-1
7
1
TH3
TH1
TH3
Phase control to give some
sort of semblance of a sine
wave.
0.5
0
-0.5
-1
TH4
TH2
TH2
1
0.5
0
  30ms
f
1

 33.33Hz
-0.5
-1
Burst Fire control.
1.5
1
0.5
0
  100ms
f
1
 10 Hz
100x10 3
-0.5
-1
-1.5
1.5
  60ms
1
0.5
f
1
 16.66 Hz
60x10 3
0
-0.5
-1
-1.5
8
1.5
  40ms
1
0.5
f
0
1
 25Hz
40x10 3
-0.5
-1
-1.5
Pulse Width Modulation (P.W.M) of A.C. Motors
The D.C. Link convertor.
The figure below illustrates the principle of a D.C.Link Convertor. The speed control signal
is applied to a pulse generator whose output is used to control a rectifier and invertor. The
output of the rectifier is smoothed before being applied to the invertor. The invertor gives a
three-phase output. The line-to-line output of the invertor is a square wave since it is derived
from a switched direct voltage.
Variable
D.C. Voltage
Three-phase
constant
frequency
supply
Controlled
Rectifier
Invertor
Three-phase
variable
frequency
supply
D.C. Link
Speed
Control
signal
A simplified single-phase p.w.m. circuit is shown below. Diagonally opposite pairs of
thyristors are triggered at any one time; TH1 and TH2 are triggered at one instant and (TH3
and TH4 are switched off). When TH3 and TH4 are triggered, TH1 and TH2 are switched off.
The voltage that appears across the ‘A.C.’ load depends on the length of time one pair of
thyristors conduct.
If the conduction period is short, the mean output voltage is high. The output voltage
waveform can be shaped by varying or modulating the width of the voltage pulses applied to
the load.
During the time interval PQ in thyristors TH1 and TH2 conduct, making point X in the
diagram positive with respect to Y; as the pulse is small, the mean output voltage produced
by the pulse is small. In the interval RS thyristors TH1 and TH2 conduct for a longer period
9
and of time, increasing the mean output, and so on. Thus the positive half-cycle of the
waveform is generated by firing TH1 and TH2 for short or long periods.
TH1
DC
supply
from rectifier
D
TH3
Load
TH4
TH2
The negative half cycle is generated by firing thyristors TH3 and TH4 for varying time
periods. In order to allow the control of alternating current at power factors other than unity
it is necessary to provide by pass circuits across reverse biased thyristors. This is achieved by
connecting a diode in inverse parallel across each thyristor.
1
0.5
0
PQ R S
-0.5
-1
10