Q3 Flashcards
1
Q
Q3. slide 1 main ideas
A
- six-pulse rectifier most common topology for 3 phase ac voltages
- simplify analysis, redraw circuit
- Consists of 2 group of diodes, top and bottom group
- Top group diode operation
- Bottom group diode operatioin
2
Q
Explain operation of 6-pulse rectifier
A
- In top group of diodes, cathode terminals at same potential
- diode with its anode at highest potential will conduct, other 2 will be reversed biased
- in bottom group, anode terminals at same potential
- diode with its cathode at the lowest potential will conduct, other 2 will be reversed biased
3
Q
Q3. slide 2 main ideas
A
- see voltage waveforms 3 phase voltages, output voltage of rectifier, phase a input I assuming constant output I
- Voltages Vpn and Vnn, are voltages at points p and n with respect to ac voltage neutral point
- Output voltage equal to difference of Vpn and Vnn
- Top group and bottom group operation
- figure of phase a
- calculate average output voltage
4
Q
Q3. slide 2 explain top and bottom group operation
A
- In top group diodes with its anode at highest potential will conduct
- Vpn will be equal to phase voltage with highest potential
- in bottom group same principle applies
- Vnn equal to phase voltage with lowest potential
5
Q
Q3. slide 2 explain phase a I figure
A
- phase a I equal to output I when D1 conducts, that is when phase a voltage is the highest
- phase a I equal to - output I when D4 conducts, that0s when phase a voltage is the lowest
6
Q
How to obtain average output voltage of six-pulse rectifier diode
A
- To obtain average output voltage, we consider one of 6 segments of output voltage
- each segment of output voltage equals L-L voltage
- L-L voltage can be expressed as a cosine function from -pi/6 to pi/6
- we integrate this expression and divide by pi/3
7
Q
Q3. slide 3 main ideas
A
- If we replace constant output I for and C and R we can have two cases: Ls small, Ls large
- Input I when Ls small
- Explain phase a I with figure
- Input I goes to zero
8
Q
slide 3. explain input I waveform when Ls small
A
- Input I in each half-cycle consists of 2 pulses occurring when L-L voltage is higher than output voltage
- for phase a I, first pulse occurs when Vab is higher than output voltage, and D1 and D6 are conducting
- second pulse occurs when Vac is higher than output voltage, and D1 and D2 are conducting.
9
Q
Q3. slide 4 explain effect of large Ls
A
- input I in each half-cycle does not go to zero
- it increases conduction time, reduces peak I, and reduces THD
- We obtain smoother current rise
- Impedance of Ls is usually selected as 2% or more than base impedance
10
Q
Explain general impact of grid-side inductance
A
- Increases conduction time and reduces peak I
- I waveform is improved imrpoving PF to an acceptable level
- THD is reduced
- Output voltage depends on output load, lower compared to case with no L (due to voltage drop in L and varying output I)
- L and output C form a LPF, reducing peak-peak voltage ripple at the output
11
Q
Q3. slide 6 main ideas
A
- Necessary to control large inrush I that occurs when rectifier is connected to voltage source
- Cause of inrush I
- Problem of inrush I
- Solutions
12
Q
Explain cause of inrush I and problems
A
- caused by the very large output C (to keep voltage nearly constant)
- Initially voltage across C is zero, thus act as a short circuit and large I flows through diode to charge dc-capacitor
- Large I might destroy the diode
13
Q
Describe solutions to limit inrush I
A
- current-limiting resistor or NTC allows to charge the C with a small controlled I. When C reaches certain voltage, R is bypassed by mechanical contactors or thyristor to reduce power dissipation.
- Replace diode by thyristors to control start of conduction. Thyristors turned on at instant close to the end of half-wave so that source voltage is small and I to charge capacitor is limited. When C is charge to 80% thyristors are turned as diodes.
14
Q
Main problems of passive rectifiers
A
- Input I are not pure sinusoidal, leading to high THD and low PF.
- Injection of harmonic I impacts quality of AC voltage waveforms.
- Harmonic distortion and low PF overloads circuit wiring (if PF is low, I to supply given load increases).
- There is no control on the output voltage.
15
Q
List and explain solution for problems of passive rectifiers
A
- Add regulator circuits to provide stable output voltage regardless of input voltage.
- THD, PF and I waveform can be improved by used large grid side L or filters.
- PFC converter to shape the input I
