Test: Three Phase Converters - 2


20 Questions MCQ Test Power Electronics | Test: Three Phase Converters - 2


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QUESTION: 1

A three-phase full converter supplied from a 230 V source is working as a line commutated inverter. The load consists of RLE type with R = 5 Ω, E = 200 V and L = 1 mH. A continues current of 10 A is flowing through the load, find the value of the firing angle delay.

Solution:

Vo = 200 – 10×5 = 150V as the circuit is operating as an inverter Vo = -150V
Now, Vo = (3Vml/π) cos α
α = cos-1(-150π/3√2×230) = 118.88°.

QUESTION: 2

 A three-phase full converter is driving a DC motor. If a continues current of Im amperes is flowing through the motor load, then find the rms value of supply current drawn by the converter to drive the motor.

Solution:

The RMS value of the supply current IS over π radians would be
(IS)2 = (1/π) x (Im)2 x (2π/3) = Im√2/√3.

QUESTION: 3

Name the below given circuit.

Solution:

 It uses 3 SCRs and 3 diodes, hence it is a semi-converter. Option (b) and (d) make no sense, because there can be no full controlled semi-converter.

QUESTION: 4

In the below given circuit, each SCR and diode conduct for

Solution:

 At any given time, one SCR and one diode is conducting, each conduct for 120° per cycle.

QUESTION: 5

In the below given circuit, __ and __ conduct along with T2.

Solution:

When one SCR conducts, a diode conducts along with it at a time to provide the path of current flow. . For example, if T2 starts conducting at 90° it will conduct till 90+120 = 210°. But while T2 is conducting, half of the time i.e. from 90 to 150 D1 is conducting and another half of the time D3 is conducting. T2 and D2 cannot conduct together as it will cause a short circuit. Hence, T2-D1 conduct for 60° and then T2-D3 conduct for another 60°.

QUESTION: 6

 In a three-phase semi-converter, at a time one SCR and one diode conduct simultaneously. With SCR T1 conducting which diode(s) is most likely to conduct along with T1?

Solution:

T1 and D1 together will cause a S.C. D2 or D3 any of these two can conduct along with T1 depending on which phase voltage is currently active RB or RY.

QUESTION: 7

What is the value of voltage at the output terminal when the freewheeling diode (FD) is conducting?

Solution:

When FD is conducting it will short circuit the load terminal resulting in zero voltage. It won’t be E because the terminals are shorted. It can be E when none of the devices are conducting (This can happen only when α > 120°).

QUESTION: 8

A 3-phase full converter feeds power to an R load of 10 Ω. For a firing angle delay of 30° the load takes 5 kW. An inductor of large value is also connected to the load to make the current ripple free. Find the value of per phase input voltage.

Solution:

 Ior = Vo/R = (3Vml/Rπ) cos α
P = 5 kW = Ior2 x R = [(3Vml/π) cos α]2 x 1/R] Therefore, Vs (line) = √50000 x (π/√2 x 3 x cos30) = 191.22 V
Vs (phase) = 191/√3 V.

QUESTION: 9

A three-phase semi-converter circuit is given a supply of 400 V. It produces at the output terminals an average voltage of 381 V. Find the rectification efficiency of the converter circuit.

Solution:

Rectification efficiency = Pdc/Pac
Pdc = Vo x Io . . . (both average values of output current and voltage)
Pac = Vrms x Irms . . . (both rms values of input current and voltage)
For a semi-converter Irms = Io
Therefore, Rectification efficiency = 381/400 = 95.25 %.

QUESTION: 10

In a 3-phase semi-converter, for firing angle less than 60° the freewheeling diode conducts for

Solution:

 In case of a semi-converter operating with α < 60°, FD does not comes into play, as the voltage never falls to zero and gives no chance for the inductor to discharge.

QUESTION: 11

In a 3-phase full wave converter, if V is the maximum value of line voltage at the input, then each SCR is subjected to a peak negative voltage of

Solution:

PIV = √3 Vml in case of a semi-converter.

QUESTION: 12

In a 3-phase semi-converter, firing angle is less than 60°, as such each SCR and diode conduct respectively for __________ (in degrees)

Solution:

Each will conduct for 120° per cycle whatever the firing angle is.

QUESTION: 13

The effect of source inductance on the performance of a 3-phase controlled converter is to

Solution:

 It reduces the average value of the output voltage by introducing a overlap delay μ.

QUESTION: 14

Which of the following converter circuits would require a neutral point?

Solution:

Half wave converter would require delta-star transformer, the secondary winding star connection requires a neutral point.

QUESTION: 15

The range of firing angle for a 3-phase, 3-pulse converter feeding a resistive load is __________ (in degrees).

Solution:

Firing angle for a 3-phase, 3-pulse converter feeding a resistive load is 0 to 150 degrees.

QUESTION: 16

 A 3-phase bridge converter is given a three-phase supply in the phase sequence R-Y-B. Let the neutral to R phase voltage be Vm sinωt. The first SCR (connected to R phase) is fired at an angle of 15°. What is the maximum value at the output terminals at this instant?

Solution:

In case of a 3 phase bridge converter, the maximum value of voltage at the output terminal is always 1.5 Vm.

QUESTION: 17

The PIV experienced by each SCRs in M-3 converter is __________ times that in a 3-phase full converter having the same output voltage.

Solution:

 In case of M-3 type of connection, the devices have to handle more peak inverse voltage (PIV) than the 3-phase full converter which has 6 SCRs.

QUESTION: 18

Each SCR of a 3-phase 6-pulse converter conducts for

Solution:

A 3-phase 6-pulse converter is nothing but the 3-pulse full controlled converter using 6 devices each conducting for 120°.

QUESTION: 19

A 3-phase full converter has an average output voltage of 365 V for zero degree firing and resistive load. For a firing angle of 90 degree, the output voltage would be

Solution:

Cos 90 = 0.

QUESTION: 20

 Semi-converters are

Solution:

Semi-converters are single quadrant converters, because the voltage and current can only be both positive due to the diodes connected.