Test Your Concept - 01

Test Your Concept :

                                                           Subject : MACHINES

                                                      Topic : Synchronous Machines

Q : A synchronous generator connected to an infinite bus is overexcited. Considering only the  reactive power, from the point of view of the system, the machine acts as

A. a capacitor

B. an inductor

C. a resistor

D. as RL circuit


Solution : An overexcited synchronous generator delivers reactive power to load and operates at lagging power factor.

Q : A 3-phase synchronous machine is synchronized with infinite bus. Now the prime-mover is disconnected from synchronous machine. with this, the synchronous machine would.

A. Work as a synchronous motor at a leading p.f.

B. Work as a synchronous motor at a lagging p.f.

C. Work as a 3-phase induction motor at a lagging p.f.

D. stop


Solution : When prime-mover is removed. Then synchronous alternator will works as a synchronous motor and operate at a leading p.f to operating in magnetizing mode.



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MCQs on Power Electronics 1.4 | All Student World

1) Compared to transistor,__________ have lower on state conduction losses and high power handling capability
a)    Triacs
b)    semi conductor diodes
c)    MOSFETs
d)    Thyristors

2). BTC is used for
a)    High power phase control
b)    High power current control
c)    Low power current control   
d)    Low power phase control  

3). A thyristor can termed as
a)    DC Switch    
b)    AC Switch
c)    Both A abd B
d)    Square wave Switch

4). Which of the following device has highest di/dt and dv/dt capability
a)    SIT   
b)    SITH
c)    GTO
d)    SCR

5).Which of the following is disadvantage of fast recovery diodes?
a)    Doping is carried out  
b)    Recovery is only 5μs
c)    Recovery is only 50μ
d)    Non of these

6). If the anode current is 800 A,then the amount of current required to turn off the GTO is about
a)    20 A
b)    600 A
c)    400 A
d)    200 A

7).The latching current of GTO should be of order ?
a)    100 mA  
b)    500 mA
c)    1 A
d)    2 A

8).The GTO can be turned off
a)    By a positive gate pulse
b)    By a negative gate pulse
c)    By a negative anode- cathode voltage
d)    By removing gate pulse

9).Switching frequency of SITH is
a)    5 KHz
b)    10 KHz
c)    60 KHz    
d)    100 KHz

10). The function of snubber circuit connected across the SCR is to
a)    Suppress dv/dt
b)    Increase dv/dt
c)    Decrease dv/dt
d)    Decrease di/dt

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MCQs on Power Electronics | All Student World

1). what is used to protect the SCR from over current ?
a)    CB and fuse
b)    Heat sink
c)    Snubber circuit
d)    Voltage clamping device

2). CB used for over current protection of thyristor operates when the fault current is
a)    of long period
b)    of short period
c)    both (a) and (b)   
d)    neither (a) nor (b)  

3). when a large surge current of very short duration flows through a thyristor then which one of the following device will operate to protect the thyristor ?
a)    CB    
b)    Snubber circuit
c)    voltage clamping device
d)    Fast acting current limiting device (FACL fuse)

4). what is used to protect a thyristor from high di / dt condition ?
a)    fuse   
b)    Snubber circuit
c)    Inductor
d)    Voltage clamping device

5). What happen due to high di / dt ?
a)    Breakdown of junction  
b)    Local hot spot
c)    Insulation failure
d)    Non of these

6). Which of the following is used in SCR to protect from high dv / dt ?
a)    Snubber circuit
b)    fuse
c)    Equalizing circuit
d)    circuit breaker

7). What may happen high dv / dt ?
a)    Unwanted turn ON  
b)    Breakdown of J2 junction
c)    both a and b
d)    Anyone of these

8). why resistor is used in Snubber circuit ?
a)    To minimize the loss
b)    To minimize the charging current
c)    To minimize the discharging current
d)    All of these

9). Delay time is defined by the interval when
a)    gate current increased from 90 % to 100 % of its final value
b)    anode current reaches 10 % from forward leakage current
c)    anode voltage drop from 100 % to 90 % of its actual value    
d)    all of these.

10). Snubber circuit is used with SCR
a)    In series
b)    in parallel
c)    either series or parallel
d)    anti parallel


MCQs on Power Electronics | All student world

1). The typical value of SCR formodern alternator is
a)    1.5
b)    0.5
c)    1.0
d)    1.2

2). An SCR is considered to be a semi controlled device because
a)    it can be turned OFF but not ON with a gate pulse
b)    it conducts only during one half cycle of an alternating current wave
c)    it can be turned ON but NOT with a gate pulse   
d)    it can be turned ON only during one half cycle of an AC  

3). An SCR has half cycle surge current rating of 3000 A for 50 Hz supply. one cycle surge current will be
a)    1500 A    
b)    6000 A
c)    2121.32 A
d)    4242.64

4). A single phase one pulse controlled circuit has a resistance R and counter EMF E load 400 sin(314 t) as the source voltage . For a load counter emf of 200 v, the range of firing angle control is
a)    30° to 150°   
b)    30° to 180°
c)    60° to 120°
d)    60° to 180°

5). A single phase full bridge Inverter can operated in load communication mode in case load consist of
a)    RL  
b)    RLC underdamped
c)    RLC overdamped
d)    RLC critically damped.

6). The latching current of SCR is 20 mA its holding current will be
a)    23mA
b)    40mA
c)    10mA
d)    60mA

4). Thyristor can be protected from over voltages by using
a)    Voltage clamping device  
b)    fuse
c)    heat sink
d)    snubber circuit

8). Example of a voltage clamping device
a)    fast acting fude
b)    snubber circuit
c)    metal oxide varistor
d)    aluminium block

9). Under over voltage condition impedance offered by the voltage clamping device is
a)    high
b)    low
c)    moderate    
d)    infinity

10). Under normal operating condition voltage clamping device offers impedance of
a)    high value
b)    low value
c)    zero value
d)    moderate value