Test: Phase Controlled Rectifiers- 2 - Electrical Engineering (EE) MCQ

In mid-point converter, each secondary should be able to supply the load power. Due to this reason, the transformer rating in mid-point converter is double the load rating. Thus, statement-2 is not correct.

The commutation overlap is more predominant in full converter than in semi converter. Hence, assertion is a false statement.

• In a single-phase 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, 

A dual converter using two full converters can give four quadrant operation as shown below.

A full converter operates as a rectifier in first quadrant (both V₀, I₀ positive) from α = 0° to 90° and as an inverter (V₀ positive but I₀ negative) from α = 90° to 180° in the fourth quadrant as shown below.

Thus, statement-4 is not correct. Statements 1, 2,3 and 5 are correct.

In AC-to-DC circulating current dual converters, the firing pulses of the two converters are so adjusted that α₁ + α₂ = 180°.

The average output voltage is

The maximum output voltage is obtained when α = 0°
∴ 
According to question,

or, 
or, 
or, 

For a single-phase 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°

• Only fully controlled circuits are capable of inversion and therefore regeneration, while half-controlled circuits are not. Thus, statement-1 is not correct.
• Statement-2 is correct.
• Bridge circuits require twice as many thyristors than half-wave circuits to carry the same current. Thus, statement-3 is not correct.
• Statement-4 is correct.

The reduction in output voltage due to overlap 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 gate-pulse is
t_p = t + t_d = 0 + 2.5 = 2.5μs

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.

Output voltage with no overlap = internal voltage of the 3-phase full converter

Output voltage with overlap

Average value of fall in output voltage due to overlap)

PIV of  bridge converter = 
PIV of  half wave converter

PIV of single-phase full-wave mid-point converter 
= - 2 V_m (Highest)
PIV of single-phase full-converter = - V_m

Secondary voltage = 
∴ PIV = 200√2 V