1 UNIT – IV INVERTERS MODIFIED MCMURRAY INVERTER This inverter uses auxiliary commutation scheme to turn off a conducting thyristor. A single- phase modified McMurray half-bridge inverter is shown in Fig. 1. The main inverter circuit is similar to the half-bridge circuit, except that it uses thyristors T1 and T2 in place of commutated switches S1 and S2 .The commutation circuit consists of two auxiliary thyristors TA1 and TA2 along with anti-parallel diodes DA1 and DA2 commutating elements L and C, and damping resistance R. To transfer current from T1 to T2, TA1 is triggered and to transfer can from T2 to T1 TA21 is triggered. Figure 1: Circuit of a modified McMurray half-bridge inverter. For the sake of simplification, it is assumed that all the devices and circuit elements are Ideal. Moreover, it is assumed that the load is highly inductive so that the load current remains constant during commutation interval. The operation is divided into eight modes. The equivalent circuits for different modes are shown in Fig. 2, wherein, thick lines show conducting part of the circuit, and non-conducting parts are shown by dotted lines. The relevant waveforms are shown in Fig. 3.
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UNIT IV INVERTERS YEAR/Power Electronics...1 UNIT – IV INVERTERS MODIFIED MCMURRAY INVERTER This inverter uses auxiliary commutation scheme to turn off a conducting thyristor. A
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UNIT – IV
INVERTERS
MODIFIED MCMURRAY INVERTER
This inverter uses auxiliary commutation scheme to turn off a conducting thyristor. A single-
phase modified McMurray half-bridge inverter is shown in Fig. 1. The main inverter circuit is
similar to the half-bridge circuit, except that it uses thyristors T1 and T2 in place of commutated
switches S1 and S2 .The commutation circuit consists of two auxiliary thyristors TA1 and TA2
along with anti-parallel diodes DA1 and DA2 commutating elements L and C, and damping
resistance R. To transfer current from T1 to T2, TA1 is triggered and to transfer can from T2 to
T1 TA21 is triggered.
Figure 1: Circuit of a modified McMurray half-bridge inverter.
For the sake of simplification, it is assumed that all the devices and circuit elements are Ideal.
Moreover, it is assumed that the load is highly inductive so that the load current remains
constant during commutation interval. The operation is divided into eight modes. The
equivalent circuits for different modes are shown in Fig. 2, wherein, thick lines show
conducting part of the circuit, and non-conducting parts are shown by dotted lines. The relevant
waveforms are shown in Fig. 3.
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Figure 2 Different modes of operation of a modified McMurray half-bridge inverter.
Figure 3 Waveforms of a modified McMurray half-bridge inverter.
Mode I (t < 0) In this mode main thyristor T conducts and a constant positive load current (Io)
flows. The load voltage is positive and the capacitor is charged up to +V (left plate positive),
shown in Fig. 2a. The auxiliary thyristor TA1 is forward biased by the capacitor voltage.
Mode II (0 <t <t1) TA1 is triggered at t = 0 to commutate T1 .This results in flow of resonating
current i through L, C, T1 and TA1 as shown in Fig. 2b. This current increases while the net
current through T1 reduces. At t = t1, ic becomes equal to I0 while reduces
to zero. Then T1 turns-off.
Mode III (t1 <t <t2) As the resonating current becomes higher than the load current ( ), the
surplus current flows through the diode D1 as shown in Fig. 2c. The resonating current
reaches the peak value and the falls to at t2 as shown in Fig. 3. The current through D1 also
becomes zero at t2 which commutates D1 During this interval, the capacitor voltage reduces.
It becomes negative (right plate positive) after reaches its peak value.
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Mode IV (t2 <t <t3) When D1 turns off at the end of mode-III, the capacitor is charged in
reverse direction (right plate positive) by the constant load current During this interval, the
voltage across diode D2 ,VD2, is equal to . This may be verified by applying KVL to
the path form by the voltage source, TA1 ,C, L and D2 . At t3 the capacitor charges to a voltage
slightly higi than V. This results in forward biasing of D2 and D2 turns on.
Mode V (t3 <t <t4) When D2 turns on, I0 is shared by iD2 and ic as shown in Fig. 2e. As ic
decreases, iD2 increases, which is equal to . At = 0 and TA1 turns off. After
t3 trigger signal is given to T2. However, it will turn on only when D turns off.
Mode VI (t4 < t < t5) As tries to reverse at t4 , DA1 turns on. At this instant, the capacitor
voltage is more than the source voltage. Now the capacitor discharges through V, D2 ,L, DA1
and the damping resistance R. At t5 the capacitor current falls to zero and DA1 is commutated.
Moreover, the capacitor is charged up to -V (right plate positive).
Mode VII (t5<t < t6) During this interval, flows through D2 and the lower source, as shown
in Fig. 2g. The load current reduces under the influence of the negative source voltage. At t6,
becomes zero and D2 turns off.
Mode VIII (t > t6) When D2 turns off, T2 which is already receiving the gate signal, turns on.
It conducts and the negative load current (Fig. 2h), rises to become equal to at t = t7 . At this
instant, the capacitor voltage forward biases the auxiliary thyristor TA2 . To initiate the turn-
off process of T2, TA2 will be triggered.
In McMurray inverter, the trapped voltage across the capacitor, after the commutation
process, is higher than the input voltage (vc > V). In case of a modified McMurray inverter, a
branch consisting of DA1, DA2 and R is added in the basic McMurray inverter circuit. The
excess potential left across the capacitor is reduced to the level of source voltage. The capacitor
discharges through the oscillatory path consisting of L, C, R, DA1 and the dc source, till it
becomes equal to V. Thus the capacitor is charged up to a fixed level, V, irrespective of load
current.
The circuit of a single-phase modified McMurray full-bridge inverter is shown in Fig
4. The circuit uses two auxiliary commutation circuits, one for each leg. Triggering of thyristor
pair TA1-TA2 turns off the thyristor pair T1-T2 and triggering of thyristor pair TA3 – TA4 turns
off thyristor pair T3-T4
Figure 4 A modified McMurray full-bridge inverter.
VOLTAGE CONTROL OF INVERTERS
A inverter may require voltage control to:
• cope with the variations in the input dc voltage
• compensate the voltage regulation of the inverter switches and transformer
• provide variable or adjustable voltage to the load.
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Certain loads, such as variable frequency induction motor drive, require simultaneous
control of frequency and voltage. Controlling the conduction intervals of the inverter switches
can control frequency of the inverter output. Voltage control may be done by any of the
following techniques:
1. Control of input dc voltage
2. External control of inverter ac output voltage
3. Internal control of inverter.
CONTROL OF INPUT DC VOLTAGE
The output voltage of an inverter may be controlled by controlling the input dc voltage
supplied to the inverter. Figure 5 shows the various schemes used to control the input dc
voltage. If the basic source is dc, variable dc voltage may be obtained using a chopper or a dc-
to-dc converter, as shown in Fig. 5a. If the basic source is ac, variable dc voltage may be
obtained using any of the schemes shown in Fig. 5b-d.
Figure 5 Inverter voltage control by control of dc input voltage.
In the scheme of Fig. 5b, the input ac voltage is first converted into a variable ac voltage
using an ac voltage controller and then it is converted into dc with the help of an uncontrolled
rectifier. In this system, variable voltage, variable frequency ac is obtained after three
conversion stages. Obviously, efficiency of the system is poor. Moreover, the input power
factor becomes poor at low voltages.
Figure 5c shows an improved scheme. In this scheme, variable dc voltage is obtained
using a controlled rectifier. As only two conversion stages are required, the efficiency of system
is better than that for the previous scheme. At low output voltages, the input power factor is
poor. Another drawback of the scheme is that the output of the controlled rectifier contains
appreciable amount of low-frequency harmonics. Therefore, large size filter components are
required. This makes the system response sluggish.
Drawbacks of the system of Fig. 5c are removed in the system shown in Fig. 5d. This
system converts the input voltage into dc using an uncontrolled rectifier. The constant dc
voltage is then converted into a variable dc using a high-frequency (dc-to-dc converter). As the
chopper operates at a high frequency, its output contains harmonics at very high frequencies.
Thus the size of filter components is reduced. Moreover, the fundamental input power factor
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remains unity under all conditions of operation. However, losses in the system increase due to
use of an additional converter.
EXTERNAL CONTROL OF AC OUTPUT VOLTAGE
The constant ac output voltage (rms) from an inverter may be controlled using an ac
regulator (ac phase control). This method introduces a large harmonic content in the output
voltage. Moreover, the method can be used only for small power applications.
Figure 6 Series-connected inverters.
For high-power applications, two square-wave inverters may be connected in series to
variable ac voltage, as shown in Fig. 6. The output voltages of the inverter I and inverter II are
given to the primaries of the two transformers, whose secondary windings are connected in
series. The output voltages of the two transformers, v01 and v02 have same magnitude. The
phase angle between v01 and v02 , , can be controlled by controlling the phase angle between
the control signals of the two inverters. The resultant output voltage (v0) has a constant
magnitude 2V (double the peak of v01 and v02) and a variable pulse width, π-, as shown in Fig.
7. By varying from 0 to π, the width of output pulses may be varied from π to 0 and hence
the output voltage may be controlled from 2V to zero. At low voltages, it becomes a thin pulse,
the harmonic contents in the output voltage become large. Therefore, this method of voltage
control is used for voltage control (lower side) up to 25 to 30 per cent of the rated voltage.
Figure 7 Waveforms of series-connected inverters.
INTERNAL CONTROL OF INVERTERS
In this technique, the voltage control is obtained within the inverter. The output of the
inverter is in the form of a pulse width modulated wave. Controlling the width of output pulses,
controls the output voltage. This method not only provides variable output voltage but also
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eliminates certain low frequency harmonics, which are responsible for poor performance. This
method is therefore, the most popular method of voltage control of inverter. Depending on the
required range of voltage control and required performance, a suitable PWM technique may be
used.
PULSE-WIDTH MODULATED INVERTERS
The square-wave inverters, suffer from two major drawbacks:
1. For fixed-source voltage, the output voltage of the inverter cannot be controlled. To achieve
voltage control, the inverter must be fed either from a controlled ac-dc or dc-dc converter.
2. The output voltage contains appreciable harmonics (low-frequency range). THD is also very
high (48.34%).
To achieve voltage control within the inverter and to reduce the harmonic contents in
the output voltage, pulse-width modulated (PWM) inverters are used. In PWM inverters,
widths of output pulses are modulated to achieve the voltage control. Among the large number