INVERTERS

An inverter is a converter circuit which is used to convert dc power into ac power at desired output
voltage and frequency. In an inverter, input is dc voltage, but output is ac at desired voltage with
required frequency. The output voltage may be fixed or variable. Similarly, the frequency of output
voltage is also fixed or variable.

Most common applications of inverters are:

 Uninterruptible Power Supply(UPS)

 Battery operated vehicle drives
 Induction heating
 HVDC power transmission

Classification of inverters: Inverters can be classified depending upon the following factors:

 Input source
 Commutation
 Circuit configuration
 Wave shape of output voltage

1) Based on input source: Based on the nature of input source, inverters are classified as current
source inverter(CSI) and voltage source inverter(VSI).

a) Current Source Inverter(CSI): In this type of inverter, a current source with high internal
impedance is used as input of inverter. In CSI, the supply current does not change very rapidly, but
the load current can be controlled by varying dc input voltage of CSI.

b) Voltage Source Inverter(VSI): In voltage source inverter(VSI), a dc voltage source with very
small internal impedance is used as input of inverter. The dc side terminal voltage is constant but the
ac side output voltage may be constant or variable irrespective of load current. The VSI can be
classified as half- bridge VSI and full- bridge VSI.

2) Commutation: According to commutation method, inverters may be classified as line commutated

inverters and forced commutated inverters.
a) Line commutated inverters: Single- phase or three- phase fully controlled acts as an inverter
when the firing angle α is greater than 90o. This is a voltage source inverter and the used switching
devices such as thyristors are naturally commutated.

b) Forced commutated inverters: These type of inverters require additional circuits for commutation
of thyristors. Depending upon the commutation technique, these inverters are classified as auxiliary
commutated inverters and complementary inverters.

3) Circuit Configuration: According to the circuit topology or connection of semiconductor

switches, inverters can be classified as series inverters, parallel inverters, half bridge inverters and full
bridge inverters.

a) Series Inverters: In series inverters, inductor L and capacitor C are connected in series with the
load. In this inverter L and C are used as commutating elements and the performance of inverter
depends on the value of L and C.

b) Parallel Inverters: In case of parallel inverters, commutating elements are connected in parallel
with the conducting thyristor.

c) Half- Bridge Inverters and Full- Bridge Inverters: In half- bridge inverters, only one leg of
bridge exists. In case of full bridge inverters, either two legs or three legs are existing for single-
phase or three- phase inverters respectively.

4) Wave Shape of Output Voltage: In an ideal inverter, output voltage must be purely sinusoidal.
But due to switching of semiconductor devices as per requirement of inverter operation, output
voltage is non- sinusoidal and it contains harmonics. Depending upon the output voltage waveform,
inverters can be classified as square wave inverters and pulse width modulation inverters.

a) Square Wave Inverters: A square wave inverter generates a square wave ac output voltage of
constant amplitude. The amplitude of the output voltage can be controlled by varying the input dc
voltage.

b) Pulse Width Modulation Inverters: In pulse width modulation inverters, the output voltage
contains one or more pulses in each half cycle. By varying the width of these pulses, the amplitude of
output voltage can be controlled though the input dc voltage is constant.
Single- phase half bridge voltage source inverter:

The above figure shows the circuit configuration of a single phase half bridge voltage source inverter.
Here Q1 and Q2 are power semiconductor switches. The operation of a single- phase half bridge
voltage source inverter with resistive load can be divided into two different modes such as:

1. Mode 1(0 ≤t ≤ T/2): Switch Q1 conducts and Q2, D1 and D2 are in OFF.

2. Mode 2(T/2 ≤t ≤ T): Switch Q2 conducts and Q1, D1 and D2 are in OFF.

1. Mode 1: When the switch Q1 is closed for T/2 i.e. half of the time period, V/2 is applied across load
and current flows through load is V/2R.

2. Mode 2:
At t= T/2, switch Q1 is opened and switch Q2 is closed for T/2 duration. Then again -V/2 is applied
across load and -V/2R current will flow through load.

The output voltage and current waveforms are shown in the fig. below. Here the output voltage
waveform is a square wave. The current waveform is also similar to output voltage waveform. During
R load, diodes D1 and D2 are not conducting. The frequency of the output voltage is f= 1/T. The
output frequency can be controlled by varying the ON and OFF time of switches.

The average value of the output voltage is given by

The rms value of the output voltage is given by

Single- Phase Half - Bridge Voltage Source Inverter with RL Load: For RL load, the output
voltage waveform is similar to that with R load but the load current waveform is different from the
load current with resistive load. The operation of a single- phase half wave inverter with RL load can
be divided into four different modes such as

1. Mode I (0≤ t≤ t1): Diode D1 conducts

2. Mode II (t1≤ t≤ T/2): Switch Q1 conducts

3. Mode III (T/2≤ t≤ t2): Diode D2 conducts

4. Mode IV (t2≤ t≤ T): Diode Q2 conducts

1. Mode I: At t= 0, the gating signal is removed from switch Q2 and it becomes OFF. At this instant
the load current is i0 which is equal to its negative peak value (-I0). Due to inductive load, the load
current cannot be reversed instantly and then diode D1 starts to conduct at t= 0. Subsequently. the
output voltage across load is V/2 and the load current i0 increases from its negative peak value (-I0) as
the current cannot reverse instantaneously due to inductive load. Then the load current flows through
diode D1. In the time interval 0≤ t≤ t1, the voltage across load is positive, but load current is negative.
Hence the energy stored in inductance L during previous cycle must be fed back to dc supply through
D1 and the load current decreases slowly. At t= t1, the load current becomes zero.

2. Mode II : At the instant t= t1, diode D1 becomes OFF but switch Q1 is ON. The current starts to
flow in positive direction and it reaches its maximum positive peak value I0 at t= T/2. During this time
interval, both the output voltage as well as current is positive and energy stored in inductance L.

Fig.: Mode I(left) and Mode II(right) of single phase half bridge inverter with R- L load

3. Mode III: At t= T/2, switch Q1 becomes OFF. At this instant, the load current i0 is equal to its
positive peak value(I0). Due to inductive load, the load current cannot be reversed instantly and then
diode D2 starts conducting at t= T/2. After that the output voltage across load is -V/2 and the load
current i0 decreases from its positive peak value I0 as the current cannot reverse instantaneously due to
inductive load. This current flows through the diode D2. During the time T/2≤ t≤ t2, the output voltage
is negative but the load current is positive. It decreases slowly and reaches zero at t= t2. The energy
stored in inductance will be released and fed back to dc supply during this period.
4. Mode IV: At instant t= t2, diode D2 becomes OFF and switch S2 is ON. The load current starts to
flow in negative direction and it reaches maximum negative -I0 at t= T.

Fig.: Mode III(left) and Mode IV(right) of single phase half bridge inverter with R- L load

The output voltage and current waveforms are shown in the fig. below

Single- Phase Full - Bridge Voltage Source Inverter:

The above fig. shows single- phase full bridge inverter which consists of two pairs of controlled
switches (Q1, Q2 and Q3, Q4) and two pairs of diodes(D1, D2 and D3, D4). Among these devices, only
one pair of devices conducts simultaneously. When switches Q1 and Q2 are ON, the output voltage
across load is +V. Similarly when switches Q3 and Q4 are ON, the output voltage across load is -V.
Diodes D1, D2, D3 and D4 are used as feedback diodes.

Single- phase full bridge inverter with R load: Similar to single- phase half bridge inverter, the
operation of a single- phase full bridge inverter with R load can be divided into two different modes:

1. Mode 1(0 ≤t ≤ T/2): Switch Q1 and Q2 conducts.

2. Mode 2(T/2 ≤t ≤ T): Switch Q3 and Q4 conducts.

1. Mode 1: When the switch Q1 and Q2 are closed for T/2 i.e. half of the time period, voltage V is
applied across load and current flows through load is V/R.

Fig.: Mode 1 of single phase full bridge inverter with R load

2. Mode 2: At t= T/2, switch Q1 and Q2 are opened and switch Q3 and Q4 are closed for T/2 duration.
Then again -V is applied across load and -V/R current will flow through load.

Fig.: Mode 2 of single phase full bridge inverter with R load

The gating signals, output voltage and current waveforms are show in the fig. below.
Single- phase full bridge inverter with R- L load: For R- L load, the output voltage waveform is
similar to that with R load but the load current waveform is different from the load current with
resistive load. The operation of a single- phase half wave inverter with R- L load can be divided into
four different modes such as

1. Mode I (0≤ t≤ t1): Diodes D1 and D2 conduct

2. Mode II (t1≤ t≤ T/2): Switches Q1 and Q2 conduct

3. Mode III (T/2≤ t≤ t2): Diodes D3 and D4 conduct

4. Mode IV (t2≤ t≤ T): Switches Q3 and Q4 conduct

1. Mode I: At t= 0, the gating signal is removed from switch Q3 and Q4 and these switches will be
turned OFF. At this instant the load current is i0 which is equal to its negative peak value (-I0). Due to
inductive load, the load current cannot be reversed instantly and then diodes D1 and D2 starts to
conduct at t= 0. Subsequently. the output voltage across load is V and the load current i0 increases
from its negative peak value (-I0) as the current cannot reverse instantaneously due to inductive load.
Then the load current flows through diodes D1 and D2. In the time interval 0≤ t≤ t1, the voltage across
load is positive, but load current is negative. Hence the energy stored in inductance L during previous
cycle must be fed back to dc supply through D1 and D2 and the load current decreases slowly. At t= t1,
the load current becomes zero.
 Fig.: Mode I of single phase full bridge inverter with R- L load

2. Mode II : At the instant t= t1, feedback diodes D1 and D2 become OFF but switches Q1 and Q2 are
turned ON. The current starts to flow in positive direction and it reaches its maximum positive peak
value I0 at t= T/2. During this time interval, both the output voltage as well as current is positive and
energy stored in inductance L.

Fig.: Mode II of single phase full bridge inverter with R- L load

3. Mode III: At t= T/2, switches Q1 and Q2 become OFF. At this instant, the load current i0 is equal
to its positive peak value(I0). Due to inductive load, the load current cannot be reversed instantly and
then diodes D3 and D4 start conducting at t= T/2. After that the output voltage across load is -V and
the load current i0 decreases from its positive peak value I0 as the current cannot reverse
instantaneously due to inductive load. This current flows through the diodes D3 and D4. During the
time T/2≤ t≤ t2, the output voltage is negative but the load current is positive. It decreases slowly and
reaches zero at t= t2. The energy stored in inductance will be released and fed back to dc supply
during this period.

Fig.: Mode III of single phase full bridge inverter with R- L load
4. Mode IV: At instant t= t2, diodes D3 and D4 become OFF and switch S3 and S4 is ON. The load
current starts to flow in negative direction and it reaches maximum negative -I0 at t= T.

Fig.: Mode IV of single phase full bridge inverter with R- L load

The gating signal and output voltage and current waveforms are shown in the fig. below