Power electronics and drives miscellaneous Easy Questions and Answers | Page - 22

Power electronics and drives miscellaneous

Match the switch arrangements on the top row to the steady-state V-I characteristics on the lower row. The steady state operating points are shown by large black dots.

1. A - I , B - II , C - III , D - IV
1. A - II , B - IV , C - I , D - III
1. A - IV , B - III , C - I , D - II
1. A - IV , B - III , C - II , D - I

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Negative voltage, current = 0

Correct Option: C

Negative voltage, current = 0

In the chopper circuit shown, the main thyristor (T_M) is operated at a duty ratio of 0.8 which is
much larger the commutation interval. If the maximum allowable reapplied dv on T_M
dt

is 50 V/μs, what should be the theoretical minimum value of C₁? Assume current ripple through L₀ to be negligible.

1. 0.2 μF
1. 0.02 μF
1. 2 μF
1. 20 μF

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I₀ = 100 = 12.5 amp
0.8

Duty ratio of T_M = 0.8
∴ Current through M, I_M = 12.5 × 0.8 = 10A
Now I_M = C dV_M
dt

∴ 10 = C × 50
10^-6

or C = 50 × 10^-6 = 0.2 μF
10

Correct Option: A

I₀ = 100 = 12.5 amp
0.8

Duty ratio of T_M = 0.8
∴ Current through M, I_M = 12.5 × 0.8 = 10A
Now I_M = C dV_M
dt

∴ 10 = C × 50
10^-6

or C = 50 × 10^-6 = 0.2 μF
10

The Current Source Inverter shown in the figure given below is operated by alternately turning on thyristor pairs (T₁, T₂) and (T₃, T₄). If load is purely resistive, then theoretical maximum output frequency obtainable will be

1. 125 kHz
1. 250 kHz
1. 500 kHz
1. 50 kHz

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Let T₃ and T₄ already conducting at t = 0
Now, trigger T₁ and T₂, so T₃, T₄ force commutated.

Now capacitor start to discharge, becomes zero. Now charge with opposite polarity
At t = T , trigger T₃, T₄
2

So T₂, T₁ force commutated.
Now capacitor start to discharge, becomes zero and now charge with positive polarity
This completes one cycle

Time constant, = τ = RC = 0.5μs
∴ Total τ = 4 × τ = 4 × 0.5 μs = 2 μs
∴ f = 1 = 1 = 500 kHz
T 2 × 10^-6

Correct Option: C

Let T₃ and T₄ already conducting at t = 0
Now, trigger T₁ and T₂, so T₃, T₄ force commutated.

Now capacitor start to discharge, becomes zero. Now charge with opposite polarity
At t = T , trigger T₃, T₄
2

So T₂, T₁ force commutated.
Now capacitor start to discharge, becomes zero and now charge with positive polarity
This completes one cycle

Time constant, = τ = RC = 0.5μs
∴ Total τ = 4 × τ = 4 × 0.5 μs = 2 μs
∴ f = 1 = 1 = 500 kHz
T 2 × 10^-6

An SCR is considered to be a semi-controlled device because

1. it can be turned OFF but not ON with a gate pulse
1. it conducts only during one half-cycle of an alternating current wave
1. it can be turned ON but not OFF with a gate pulse
1. it can be turned ON only during one half-cycle of an alternating voltage wave

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NA

Correct Option: C

NA

The following circuit has a source voltage V_s as shown in the graph. The current through the circuit is also shown.

The element connected between a and b could be

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When a forward biased diode is reverse biased, initially it conducts due to the flow of stored charges. After some ti me it goes int o nonconducting state.
Alternately
For 0 < t < 100 ms, V_s = 0,
i_s = 0; diode not conduct
For 100 < t < 200 ms, V_s = 10V,
i = 10 = 1 mA ; diode conducts
10K

For 200 < t < 300 ms, V_s = – 10V, i = 0; diode reverse bias
current is zero after small reverse recovery time.
For 300 < t < 400 ms, V_s = 0;
i = 0; diode not conduct

Correct Option: A

When a forward biased diode is reverse biased, initially it conducts due to the flow of stored charges. After some ti me it goes int o nonconducting state.
Alternately
For 0 < t < 100 ms, V_s = 0,
i_s = 0; diode not conduct
For 100 < t < 200 ms, V_s = 10V,
i = 10 = 1 mA ; diode conducts
10K

For 200 < t < 300 ms, V_s = – 10V, i = 0; diode reverse bias
current is zero after small reverse recovery time.
For 300 < t < 400 ms, V_s = 0;
i = 0; diode not conduct