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How many thyristors are there in a three-phase full wave controller?
Three-Phase Full Converters:
If all diode is replaced by thyristor, a three-phase full converter bridge formed as shown below,
Voltage and Current Equation:
The average output voltage (V0) is given by,
Where Vm is the maximum value of line voltage.
Voltage and current waveform for α = 30° and for constant current can be drawn as,
From waveform RMS value of Thyristor current (IT = iT1) will be,
Where I0 is constant DC load current.
In a single Phase full converter with resistive load and firing angle α. The load current is non-zero and zero, respectively for-
Single phase full converter with resistive load:
Current flowing through a resistive load will be
I = Vo/R
We can see from the above graph that the current will be zero from '0 to α' and the current will be non-zero from α to π.
So, the current will be non-zero and zero from 'π - α' and 'α' respectively.
3 phase, 6 pulse converter shown in the figure, the load is taking ripple free constant current of 10 A. The average output voltage of the converter is 150 V at a firing angle α = 30º. Find the value of line resistance R in ohms. (Up to two decimal places)
The average output voltage of 3 phase 6 pulse converter is given by
here Vml is the maximum value of line-to-line voltage
The voltage drop in the resistance is given as
2RI0, here 2 resistance will come in to picture in every 60º conduction of two phases
so this voltage drop is subtracted from the final output of this converter.
The average output voltage is equal to
R = 4.19 Ω
In a dual converter, if the firing angle of one bridge is 30°, then the firing angle of second bridge is ______.
Modes of operation of Dual converter:
Non-circulating current mode:
Circulating current Mode:
Application of dual converter:
Given that, firing angle of one bridge is 30° (i.e. α1 = 30°)
We know that firing angles can never be greater than 180°.
i.e. α1 + α2 = 180°
∴ 30° + α2 = 180°
α2 = 180° - 30°
α2 = 150°
A delayed full-wave rectified sinusoidal current has an average value equal to half its maximum value. Find the delay angle θ.
The average output voltage of a full-wave controlled rectifier with R load is given by:
Where Vm is the maximum value of supply voltage
α is the firing angle or delay angle
Average load current:
Given that, the average value of the average load current is equal to half of its maximum value.
A delayed full-wave rectified sinusoidal current has an average value equal to one – third its maximum value. Find the delay angle.
Considered Vm is the maximum value of AC input voltage of converter and V0 is the average output voltage converter and α is delay angle.
For single-phase semi converter or delayed full-wave rectifier,
V0 = Vm/3
Hence, the equation becomes,
π/3 = (1 + cosα)
cos α = 0.047
α = cos-1(0.047)
Distortion factor (DF) and total harmonic distortion (THD) are related by.
Total harmonic distortion:
Vor = Total rms value
V01 = Fundamental rms value
Distortion factor (DF): It is the ratio of the fundamental component to r.m.s value of the waveform.
Distortion factor (DF) and total harmonic distortion (THD) are related as
A freewheeling diode in a phase-controlled rectifier:
A single-phase, full-bridge diode rectifier fed from a 230 V, 50 Hz sinusoidal source supplies a series combination of finite resistance, R, and a very large inductance, L. The two most dominant frequency components in the source current are:
Fourier series of source current in a full bridge rectifier is given by
For n = 1, 3, 5, ......
The most dominant frequency components in the above expression are f, 3f
A single-phase, full-bridge diode rectifier fed from a 230 V, 50 Hz sinusoidal source
Dominant frequencies = f, 3f
= 50 and 150 Hz
A six-pulse thyristor bridge rectifier is connected to a balanced three-phase, 50 Hz AC source. Assuming that the DC output current of the rectifier is constant, the lowest harmonic component in the AC input current is
In a six-pulse thyristor bridge rectifier, the harmonics present are = 6 k ± 1
So, the harmonics are = 5, 7, 11, 13, ...
lowest harmonic component = 5th harmonic supply frequency = 50 Hz
5th harmonic frequency = 5f = 250 Hz
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