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Consider the following statements associated with single phase full converters:
1. Midpoint converter configuration is used in case the terminals on dc side have to be grounded.
2. The transformer rating in midpoint converter is double the load rating.
3. SCRs are subjected to a peak inverse voltage of 2 V_{m} in single phase midpoint converter.
4. Bridge converter is preferred over midpoint converter.
Which of these statements are correct?
In midpoint converter, each secondary should be able to supply the load power. Due to this reason, the transformer rating in midpoint converter is double the load rating. Thus, statement2 is not correct.
Assertion (A): The commutation overlap is more predominant is semiconverters than in full converters.
Reason (R): The commutation period in seconds, when outgoing and incoming SCRs are conducting, is known as the overlap period.
The commutation overlap is more predominant in full converter than in semi converter. Hence, assertion is a false statement.
For a singlephase full converter having a full load current of I_{0} match List  I (Overlap angle) with Listll (Output voltage) and select the correct answer using the codes given below the lists (L_{s} = source inductance):
Codes:
• In a singlephase full converter, as long as μ < 180°, the output voltage will be given by
• When μ = 180°, the load will be permanently short circuited by SCRs and the output voltage will be zero because during the overlap angle, all SCRs will be conducting.
• Since,
Which of the following converters can feed power in any of the four quadrants?
A dual converter using two full converters can give four quadrant operation as shown below.
Consider the following statements:
1. In a full converter, direction of current cannot reverse.
2. Semiconverters are single quadrant converters.
3. A full converter can operate as a twoquadrant converter.
4. A full converter operates as a rectifier in first quadrant and as an inverter in the second quadrant.
5. In a full converter, direction of current cannot reverse but polarity of output voltage can be reversed.
Q. Which of the statements given above are correct?
A full converter operates as a rectifier in first quadrant (both V_{0}, I_{0} positive) from α = 0° to 90° and as an inverter (V_{0} positive but I_{0} negative) from α = 90° to 180° in the fourth quadrant as shown below.
Thus, statement4 is not correct. Statements 1, 2,3 and 5 are correct.
ACtoDC circulating current dual converters are operated with which of the following relationship between their triggering angles (α_{1} and α_{2})?
In ACtoDC circulating current dual converters, the firing pulses of the two converters are so adjusted that α_{1} + α_{2} = 180°.
A singlephase halfwave controlled converter with resistive load is operated from a 120 V, 50 Hz supply. If the average output voltage is 25% of the maximum possible output voltage, the firing angle of the thyristor would be equal to
The average output voltage is
The maximum output voltage is obtained when α = 0°
∴
According to question,
or,
or,
or,
A single phase fully controlled thyristor bridge converter supplies a load consisting of R, L and E has shown in figure below. The inductance L in the circuit is so large that the output current may be considered to be virtually constant. The firing angle α is
For a singlephase fully controlled thyristor bridge converter with R, L, E load, the average output voltage is
or,
= 198 cos α ....(i)
Now, V_{dc}  I_{dc }R+ E (10 x 0.5) + 135
or, V_{dc} = 140V ...(ii)
From equations (i) and (ii),we have:
198 cos α = 140
or,
or, Firing angle α = 45°
Consider the following statements:
1. Fullycontrolled and half controlled circuits both are capable of regeneration.
2. The higher the pulse number, the lower is the magnitude of ripple voltage and current in the loadcircuit and the a.c. supply.
3. Bridge circuits and halfwave circuits both require same number of thyristors to carry the same current.
4. Halfwave circuits tend to be used for lowvoltage, highcurrent use.Which of the statements given above is/are not correct?
• Only fully controlled circuits are capable of inversion and therefore regeneration, while halfcontrolled circuits are not. Thus, statement1 is not correct.
• Statement2 is correct.
• Bridge circuits require twice as many thyristors than halfwave circuits to carry the same current. Thus, statement3 is not correct.
• Statement4 is correct.
In a bridge converter the dc load current is 90 A, the commutating inductance per phase is 50 and the power supply is 2.3 kV, 50 Hz. The reduction in output voltage due to overlap is around
The reduction in output voltage due to overlap is
The holding current of SCRs in fullconverter is 200 mA and the delay time is 2.5 p.sec. Converter is supplied from a starconnected 208 V, 50 Hz supply, has a load of R = 2 Ω L = 8 mH and the firing angle is 60°. The minimum width of gate pulse t_{p} for L = 0 H is
Given,
∴ Phase voltage,
and peak voltage
Now,
At ωt = 60°, e = √3x169.7sin90° ≈ 293.93V
Rate of rise of anode current, di/dt at the instant of triggering is
∴
∴ Minimum width of gatepulse is
t_{p} = t + t_{d} = 0 + 2.5 = 2.5μs
The nature of load current, l.e., whether load is continuous or discontinuous in controlled rectifiers
The term continuous means that load current never ceases but continues to flow through SCR or their combination. The term discontinuous is applied to the condition when load current reaches zero during each half cycle before the next SCR in sequence is fired. In practice, the output current may become discontinuous at high values of firing angle or at low values of load current.
In a 3phase full converter, the output voltage during overlap is equal to
Output voltage with no overlap = internal voltage of the 3phase full converter
Output voltage with overlap
Average value of fall in output voltage due to overlap)
The peak inverse voltage in ac to dc converter system is highest in
PIV of bridge converter =
PIV of half wave converter
PIV of singlephase fullwave midpoint converter
=  2 V_{m }(Highest)
PIV of singlephase fullconverter =  V_{m}
A transformer is having source voltage of 400 volt and turns ratio of 2:1. The transformer is centretapped. If the secondary is connected to single phase fullwave converter, PIV per SCR will be
Secondary voltage =
∴ PIV = 200√2 V
22 docs274 tests

22 docs274 tests
