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CLASS-11
2 PULSE CONVERTER - SINGLE PHASE FULL WAVE CONTROLLED CONVERTER
SINGLE PHASE FULLY CONTROLLED BRIDGE CONVERTER-R, RL, RLE LOAD, PERFORMANCE
PARAMETERS
Single Phase Fully Controlled Converters
Fig 10.3 (a) shows the circuit diagram of a single phase fully controlled bridge converter. It is one of the
most popular converter circuits and is widely used in the speed control of separately excited dc machines. Indeed,
the R–L–E load shown in this figure may represent the electrical equivalent circuit of a separately excited dc motor.
The single phase fully controlled bridge converter is obtained by replacing all the diode of the corresponding
uncontrolled converter by thyristors. Thyristors T and T are fired together while T and T are fired 180º after T
1
2
3
4
1
and T . From the circuit diagram of Fig 10.3(a) it is clear that for any load current to flow at least one thyristor from
2
the top group (T , T ) and one thyristor from the bottom group (T , T ) must conduct. It can also be argued that
1
3
2
4
neither T T nor T T can conduct simultaneously. For example whenever T and T are in the forward blocking state
1 3
2 4
3
4
and a gate pulse is applied to them, they turn ON and at the same time a negative voltage is applied across T and T
1
2
commutating them immediately. Similar argument holds for T and T .
1
2
For the same reason T T or T T can not conduct simultaneously. Therefore, the only possible conduction
1 4
2 3
modes when the current i can flow are T T and T T . Of coarse it is possible that at a given moment none of the
0
1 2
3 4
thyristors conduct. This situation will typically occur when the load current becomes zero in between the firings of
T T and T T . Once the load current becomes zero all thyristors remain off. In this mode the load current remains
1 2
3 4
zero. Consequently the converter is said to be operating in the discontinuous conduction mode.
Fig 10.3(b) shows the voltage across different devices and the dc output voltage during each of these
conduction modes. It is to be noted that whenever T and T conducts, the voltage across T and T becomes –v .
1
2
3
4
i
Therefore T and T can be fired only when v is negative i.e, over the negative half cycle of the input supply voltage.
3
4
i
Similarly T and T can be fired only over the positive half cycle of the input supply. The voltage across the devices
1
2
when none of the thyristors conduct depends on the off state impedance of each device. The values listed in Fig 10.3
(b) assume identical devices.
Under normal operating condition of the converter the load current may or may not remain zero over some
interval of the input voltage cycle. If i is always greater than zero then the converter is said to be operating in the
0
continuous conduction mode. In this mode of operation of the converter T T and T T conducts for alternate half
1 2
1
3 4
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cycle of the input supply. However, in the discontinuous conduction mode none of the thyristors conduct over some
portion of the input cycle. The load current remains zero during that period.
Operation in the continuous conduction mode
As has been explained earlier in the continuous conduction mode of operation i never becomes zero,
0
therefore, either T T or T T conducts. Fig 10.4 shows the waveforms of different variables in the steady state. The
1 2
3 4
firing angle of the converter is α. The angle θ is given by
1Esinθ=2V
-
It is assumed that at t = 0 T T was conducting. As T T are fired at ωt = α they turn on commutating T T
3 4
1 2
3 4
immediately. T T are again fired at ωt = π + α. Till this point T T conducts. The period of conduction of different
3 4
1 2
thyristors are pictorially depicted in the second waveform (also called the conduction diagram) of Fig 10.4.
The dc link voltage waveform shown next follows from this conduction diagram and the conduction table
shown in Fig 10.3(b). It is observed that the emf source E is greater than the dc link voltage till ωt = α. Therefore,
the load current i continues to fall till this point. However, as T T are fired at this point v becomes greater than E
0
1 2
0
and i starts increasing through R-L and E. At ωt = π – θ v again equals E. Depending upon the load circuit
0
0
2
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parameters i reaches its maximum at around this point and starts falling afterwards. Continuous conduction mode
o
will be possible only if i remains greater than zero till T T are fired at ωt = π + α where upon the same process
0
3 4
repeats. The resulting i waveform is shown below v . The input ac current waveform i is obtained from i by noting
0
0
i
0
that whenever T T conducts i = i and i = - i whenever T T conducts. The last waveform shows the typical voltage
1 2
i
0
i
0
3 4
waveform across the thyristor T . It is to be noted that when the thyristor turns off at ωt = π + α a negative voltage is
1
applied across it for a duration of π – α. The thyristor must turn off during this interval for successful operation of
the converter.
It is noted that the dc voltage waveform is periodic over half the input cycle. Therefore, it can be expressed in a
Fourier series as follows.
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Operation in the discontinuous conduction mode
So far we have assumed that the converter operates in continuous conduction mode without paying
attention to the load condition required for it. In figure 10.4 the voltage across the R and L component of the load is
negative in the region π - θ ≤ ωt ≤ π + α. Therefore i continues to decrease till a new pair of thyristor is fired at ωt =
0
π + α. Now if the value of R, L and E are such that i becomes zero before ωt = π + α the conduction becomes
0
discontinuous. Obviously then, at the boundary between continuous and discontinuous conduction the minimum
value of i which occurs at ωt = α and ωt = π + α will be zero.
0
4
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Fig 10.6 shows waveforms of different variables on the boundary
between continuous and discontinuous conduction modes and in the discontinuous conduction mode. It should be
stressed that on the boundary between continuous and discontinuous conduction modes the load current is still
continuous. Therefore, all the analysis of continuous conduction mode applies to this case as well. However in the
discontinuous conduction mode i remains zero for certain interval. During this interval none of the thyristors
0
conduct. These intervals are shown by hatched lines in the conduction diagram of Fig 10.6(b). In this conduction
mode i starts rising from zero as T T are fired at ωt = α. The load current continues to increase till ωt = π – θ. After
0
1 2
this, the output voltage v falls below the emf E and i decreases till ωt = β when it becomes zero. Since the thyristors
0
0
cannot conduct current in the reverse direction i remains at zero till ωt = π + α when T and T are fired. During the
0
3
4
period β ≤ ωt ≤ π + α none of the thyristors conduct. During this period v attains the value E.
0
Inverter Mode of operation
The expression for average dc voltage from a single phase fully
controlled converter in continuous conduction mode was
For α < π/2, V > 0. Since the thyristors conducts current only in one direction I > 0 always. Therefore
d
0
power flowing to the dc side P = V I > 0 for α < π/2. However for α > π/2, V < 0. Hence P < 0. This may be
0 0
0
interpreted as the load side giving power back to the ac side and the converter in this case operate as a line
commutated current source inverter. So it may be tempting to conclude that the same converter circuit may be
operated as an inverter by just increasing α beyond π/2. This might have been true had it been possible to maintain
continuous conduction for α < π/2 without making any modification to the converter or load connection. To supply
power, the load EMF source can be utilized. However the connection of this source in Fig 10.3 is such that it can
only absorb power but can not supply it. In fact, if an attempt is made to supply power to the ac side (by making α >
π/2) the energy stored in the load inductor will be exhausted and the current will become discontinuous as shown in
Fig 10.7 (a).
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Therefore for sustained inverter mode of operation the connection of E must be reversed as shown in Fig
10.7(b). Fig 10.8 (a) and (b) below shows the waveforms of the inverter operating in continuous conduction mode
and discontinuous conduction mode respectively. Analysis of the converter remains unaltered from the rectifier
mode of operation provided θ is defined as shown.
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