Test: Power Electronics - 1
25 Questions MCQ Test Power Electronics | Test: Power Electronics - 1
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The output frequency can be more or less than input frequency.
In the below figure the average load current is 15 A. The rms value of transformer secondary current is
RMS secondary current = 
= 15/1.414 = 10.608 A.
In a 3 phase fully controlled converter the firing frequency is
It is a 6 pulse converter.
Turn on and turn off times of similar SCRs may not be same.
The amount of dielectric heating is inversely proportional to frequency.
It is proportional to frequency.
Semiconvertor operates only in first quadrant.
The phenomenon of overlap in converter operation due to
Source inductance delays turn off and hence causes overlap.
A thyristor has a maximum allowable junction temperature of 120ºC and the ambient temperature is 40ºC. If thermal resistance is 1.6º C/W, the maximum allowable internal power dissipation is
.
For a 12 pulse operation, the two 6 pulse units are fed by
30� phase difference is necessary.
In a single phase full wave converter (M - 2 connection) feeding a highly inductive load, the firing angle for each thyristor is a in the respective half cycle. The period of conduction of each thyristor is
Since conduction is from a to p + a, period of conduction is p in each half cycle.
An SCR is triggered at 40ºin the positive half cycle only. The average anode current is 50 A. If the firing angle is changed to 80º, the average anode current is likely to be
.
The characteristics of a non-linear resistance is i = kv4. If i becomes 100 times, v becomes
v ∝ (i)1/4 ∝ (100)1/4 ≈ 3.
In a step down chopper using pulse width modulation, Ton = 3 x 10-3 and Toff = 1 x 10-3 s. The chopping frequency is
.
Thyristors are suitable for dc circuit breakers but not for ac circuit breakers.
They are suitable for both dc and ac circuit breakers.
Assertion (A): Inverters and choppers use fast switching thyristors
Reason (R): Fast switching SCR has low turn off time.
Low turn off time enables fast switching.
The value of capacitor C for dynamic equalising circuit of series connected thyristors is determined by
Value of C is proportional to maximum difference between reverse charge of SCRs in series.
A chopper is a dc-ac converter. It converts a fixed dc voltage into a variable dc voltage.
Assertion (A): If the duty cycle of chopper is low the current would not become discontinuous
Reason (R): If the time constant L/R of the load in chopper is low the current may become discontinuous.
If duty cycle is low, current may become discontinuous.
In a single phase full wave regulator, the firing angles in the positive and negative half cycles are generally
Firing angles are kept equal to get symmetrical output voltage.
An electric heater is controlled by thyristors through on-off control. If a = 0.4, the heating is
Vl = V a. Therefore Vl2 - V2 (a). Heating is proportional to a.
A two winding transformer is feeding a single phase half wave rectifier circuit. The load is purely resistive. The rms value of transformer secondary current is Is and rms value of load current in Irms. Then
.
In a BJT terminals are emitter, base and collector irrespective of rating.
In single phase half wave regulator, the average current over one full cycle
The input current during positive half cycle is less than that during negative half cycle.
The dynamic equalising circuit consists of a series combination of capacitor C and resistor RC across each thyristor. This resistance RC (along with parallel connected diode)
Discharge current flows through Re .
A single phase semiconverter is feeding a highly inductive load and has freewheeling diode across the load. The waveshapes of output voltage and output current
When freewheeling diode conducts output current exists but output voltage is zero.
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