Electrical Engineering - Power Electronics and Drives
Exam Duration: 45 Mins Total Questions : 30
At a room temperature of 300C, minimum voltage and current required to fire a SCR is
- (a)
3V,40mA
- (b)
0.6V,40mA
- (c)
no limit
- (d)
3V, 100mA
The maximum \(\frac { dl }{ dt } \) in a SCR is
- (a)
directly proportional to Vm of supply voltage
- (b)
inversely proportional to Vm of supply voltage
- (c)
inversely proportional to L in the circuit
- (d)
directly proportional to L in the circuit
A SCR fires at a minimum current of 80mA and -650C. The minimum current required to fire at 250C is
- (a)
>80mA
- (b)
equal to 80mA
- (c)
<80mA
- (d)
half of 80mA
In order to obtain static voltage equalization in series connected SCRs, connections are made of
- (a)
one resistor across the string
- (b)
resistors of different values across each SCR
- (c)
resistors of the same values across each SCR
- (d)
one resistor in series with the string
Which semiconductor power device out of the following is not a current triggered device?
- (a)
Thyristor
- (b)
GTO
- (c)
Triac
- (d)
MOSFET
A single phase diode bridge rectifier supplies a highly inductive load.The load current can be assumed to be ripple free.The AC supply side current waveform will be
- (a)
sinusoidal
- (b)
constant DC
- (c)
square
- (d)
triangular
In DC chopper if T is the chopping period,then the output voltage can be controlled by PWM by varying
- (a)
T keeping \(T_{ On }\) constant
- (b)
\(T_{ On }\) keeping T constant
- (c)
T keeping \(T_{ Off }\) constant
- (d)
\(T_{ Off }\) keeping T constant
In DC choppers,for periodic time T,the output voltage can be controlled by FM by varying
- (a)
T keeping \(T_{ On }\) constant
- (b)
\(T_{ On }\) keeping T constant
- (c)
\(T_{ Off }\) keeping T constant
- (d)
T keeping \(T_{ Off }\) constant
In DC choppers if \(T_{ On }\) is the On period and f is the chopping frequency ,then output voltage in terms of the input voltage \(V_{ I }\) is given by
- (a)
\(V_{ s }T_{ On }/f\)
- (b)
\(V_{ s }f/T_{ On }\)
- (c)
\(V_{ s }/(fT_{ On })\)
- (d)
\(V_{ s }fT_{ On }\)
In DC choppers the waveform for input and output voltages are respectively
- (a)
discontinuous and continuous
- (b)
continuous and discontinuous
- (c)
both continuous
- (d)
both discontinuous
Single-pulse modulation used in PWM inverter \(V_{ s }\) is the input DC voltage For eliminating third harmonic the magnitude o rms value o fundamental component of output voltage and pulse width are respectively
- (a)
\(\frac { 2\sqrt { 2 } }{ \pi } V_{ s }and120^{ 0 }\)
- (b)
\(\frac { 0.4V_{ s } }{ \pi } V_{ s }and60^{ 0 }\)
- (c)
\(\frac { 2\sqrt { 2 } }{ \pi } V_{ s }and60^{ 0 }\)
- (d)
\(\frac { 4V_{ s } }{ \pi } and120^{ 0 }\)
An Ac Induction motor is used fir a speed control application R it is driven from an intverter with a constant V/f control.The motor name plate details are as follows :
V=415 V,Ph=3,f=50 Hz,N=2850 rpm
TAhe motor is run with the inverter ouitput frequency set at 40 Hz and with half the rated slip.The running speed of the motor is
- (a)
2400 rpm
- (b)
2280 rpm
- (c)
2340 rpm
- (d)
2790 rpm
In case of armature controlled sepearetly excited DC motor drive with closed-loop control an inner current loop is useful because it
- (a)
limits the speed o the motor to a sfe value
- (b)
helps in improving the drive energy efficiency
- (c)
limits the peak current o the motor to the permissible value
- (d)
reduces the steady state speed error.
The frequency o ripple in the output voltage o a three-phase half controlled bridge rectifier depends on the
- (a)
firing angle
- (b)
load inductance
- (c)
load resistance
- (d)
supply frequency
In cycloconverter speed control of induction motor cycloconverter frequency
- (a)
is equal to that supply frequency o 50 Hz
- (b)
is higher than the supply frequency
- (c)
can be higher or lower than supply frequency
- (d)
is always less and about one third of supply frequency
The MOSFET switch in its On state my be conidered equivalent to
- (a)
resistor
- (b)
inductor
- (c)
capacitor
- (d)
battery
A Bipolar Junction Translator (BJT) is used s a power control switch by biasing it in the cut-off region (Off state) or in the saturation region (On state), In the On state ).In the On state for the BJT
- (a)
both the base emitter and base collector junctions re reverse biased
- (b)
the base emitter junction is reverse biased and the base collector junction is forward biased
- (c)
the base emitter juntion is forward biased and the base collector junction is reverse biased
- (d)
both the base emitter and base collector junctions are forward biased
If a diode is connected in antiparallel with a thyristor then
- (a)
Both turn-o power loss and turn-off time decrease
- (b)
Turn-off power loss decreases but turn-off time increases
- (c)
Turn-off power loss increases but turn-off time decreases
- (d)
None of the above
When a line commutated converter operates in the inverter mode
- (a)
it draws both real and reactive powers from the AC supply
- (b)
it delivers both real and reactive power to the AC supply
- (c)
it delivers real power to the AC supply
- (d)
it draws reactive power from the AC supply
Wjhat is the rms value of the voltage waveform shown in figure?
- (a)
\(\frac { 200 }{ \pi } V\)
- (b)
\(\frac { 100 }{ \pi } V\)
- (c)
200 V
- (d)
100 V
A single phase full wave controlled bridge converter feed in inductive load.The two SCRs in the converter are connected to a common DC bus, The converter has to have a freewheeling diodes.
- (a)
because the converter inherently does not provide for freewheeling
- (b)
because the converter does not provide for freewheeling for high value of triggering angles
- (c)
or else the freewheeling action of the converter will cause shorting o the AC supply
- (d)
or else if a gate pulse to one of the SCRs id missed it will subsequently cause a high load current in the other SCR.
Figure below shows two thyristors each rated 500 A (continuous) sharing a load current through thyristor y is 120 A The current through thyristor a will be nearly
- (a)
120 A
- (b)
100 A
- (c)
90 A
- (d)
110 A
When the firing angle \(\alpha \) of a single-phase fully controlled rectifier feeding constant DC current into a load is \(30^{ 0 }\) the displacement power factor o the rectifier is
- (a)
1
- (b)
0.5
- (c)
\(\frac { 1 }{ \sqrt { 3 } } \)
- (d)
\(\frac { \sqrt { 3 } }{ 2 } \)
Consider a phase controlled converter shown in figure The thyritor is fired at an angle \(\alpha \) in every positive half cycle of the input voltage If the peak value of the instaneous output equals 230 V.The firing angle is close to
- (a)
\(45^{ 0 }\)
- (b)
\(135^{ 0 }\)
- (c)
\(90^{ 0 }\)
- (d)
\(83.6^{ 0 }\)
Match list I with List II and select the correct answer using the codes given below the lists.
List I | List II |
P.\(I_{ m }\) | 1.800 mA |
Q \(I_{ rm }\) | 2.100 mA |
R. \(I_{ DC }\) | 3.81.2 mA |
4. 50 mA | |
5.31.8 mA |
- (a)
P Q R 1 3 5 - (b)
P Q R 2 4 5 - (c)
P Q R 1 2 5 - (d)
P Q R 2 3 5
The chopping frequency will be
- (a)
25 Hz
- (b)
50 Hz
- (c)
25 KHz
- (d)
50 KHz
The current source inverter shown in figure is operated by alternately turning-on thyristor pairs \((T_{ 1 },T_{ 2 })\) and \((T_{ 3 },T_{ 4 })\) if the load is purely resistive the theoretical maximum output frequency obtained will be
- (a)
125 KHz
- (b)
250 KHz
- (c)
500 KHz
- (d)
50 KHz
Consider the chopper circuit of figure below The chopper operates at 400 Hz and 50% duty cycle The load current remains almost ripple free at 10 A. Assuming the input voltage to be 200 V and the devices to be ideal the turn-off time available to the thyristor \(Th_{ m }\) is
- (a)
30 \(\mu s\)
- (b)
40 \(\mu s\)
- (c)
50 \(\mu s\)
- (d)
60 \(\mu s\)
In the chopper circuit shown the main thyristor \((T_{ m })\) is operated at a duty ratio of 0.8 which is much larger than the commutation interval.If the maximum allowable reapplied dV/dt on \((T_{ m })\) is 50V/\(\mu s\) what should be the theoratical minimum value of \(C_{ 1 }\)Assume current ripple through \(L_{ 0 }\) to be negligible.
- (a)
0.2 \(\mu F\)
- (b)
0.02 \(\mu F\)
- (c)
2 \(\mu F\)
- (d)
20 \(\mu F\)
An AC induction motor is used for a speed control application .It is driven an inverter with a constant V?F control The motor nameplate details are as follows
V=415 V,Ph=3,f=50 Hz,N=2850 rpm
The motor is run with the inverter output frequency set at 40 Hz and with half the rated slip.The running speed of the motor is
- (a)
2400 rpm
- (b)
2280 rpm
- (c)
2340 rpm
- (d)
2790 rpm