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The two windings of a transformer is
not linked at all. (D)
Ans : B
salient pole synchronous motor is running at no load. Its field current is switched off.
come to stop.
to run at synchronous speed.
continue to run at a speed slightly more
than the synchronous speed.
continue to run at a speed slightly less
than the synchronous speed.
d.c. series motor should always be started with load because
no load, it will rotate at dangerously high speed.
will fail to start.
will not develop high starting torque.
frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at
full load speed is about
In a stepper motor the angular displacement
be precisely controlled.
cannot be readily interfaced with micro computer based controller.
angular displacement cannot be precisely controlled.
cannot be used for positioning of work tables and tools in NC machines.
power factor of a squirrel cage induction motor is
low at light load only.
at heavy load only.
low at light and heavy load both.
low at rated load only.
generation voltage is usually
11 KV and 33 KV. (B) between
132 KV and 400 KV.
400 KV and 700 KV. (D) None
of the above.
a synchronous motor is running at synchronous speed, the damper winding produces
eddy current torque.
torque aiding the developed torque.
a transformer primary is energised from a square wave voltage source, its
output voltage will be
A triangular wave. (D) A
a transformer the voltage regulation will be zero when it operates at
maximum power in cylindrical and salient pole machines is obtained
respectively at load angles of
90○ . (D) 90○ , < 90○ .
primary winding of a 220/6 V, 50 Hz transformer is energised from 110 V, 60 Hz
supply. The secondary output voltage
emf induced in the primary of a transformer
in phase with the flux. (B) lags
behind the flux by 90 degree.
the flux by 90 degree. (D) is
in phase opposition to that of flux.
relative speed between the magnetic fields of stator and rotor under steady
state operation is zero for a
phase induction machine.
phase induction machine.
current from the stator of an alternator is taken out to the external load
motor which can conveniently be operated at lagging as well as leading power
factors is the
cage induction motor. (B) wound
rotor induction motor.
A hysteresis motor
(A) is not a
(B) is a constant
(C) needs dc excitation.
(D) can not be run in
most suitable servomotor for low power applications is
dc series motor.
a dc shunt
an ac two-phase induction motor.
an ac series motor.
size of a conductor used in power cables depends on the
to be carried. (D) type
of insulation used
of the following methods of heating the one which is independent of supply
two-winding single phase transformer has a voltage regulation of 4.5% at
full-load and unity power-factor. At full-load and 0.80 power-factor lagging
load the voltage regulation will be
or more than 4.5%.
+ Vx sin
kVA = kW & kVAR =0
± Vx sin
+ Vx sin
p.f 0.8 lagging and %R = Vr at unity p.f
halved while the torque is kept constant.
w is doubled while Ia is halved. (D) w
is constant but Ia is
applied to a 3 phase, 4 pole,
induction motor. When the motor is
delivering rated output, the slip is found to be 0.05. The speed of the rotor
m.m.f. relative to the rotor structure is
r.p.m. (D) 75 r.p.m.
1500 (1-0.05) = 1425
speed = 1500 – 1425 = 75 rpm
alternator is delivering rated current at rated voltage and 0.8 power-factor
lagging case. If it is required to deliver rated current at rated voltage and
0.8 power-factor leading, the required excitation will
power factor and under excitation gives lagging p.f .
ceiling fan uses
start and capacitor run motor.
single-phase ac motor.
two phase motor.
1-phase-ON or 2-phase –ON modes of
The ‘sheath’ is used in cable to
strength to the cable.
the moisture from entering the cable.
chances of rust on strands.
is provided to give mechanical
drive motor used in a mixer-grinder is a
for AC & DC both supply systems.
the primary and 600 ohms resistance across the secondary. Assuming 100% efficiency, the primary
Amps. (D) 20 Amps.
V1 /R1 =
120/600 = 0.2 (h = 100%, losses are zero V1 = VR = I1R1)
when delivering 20 A to the load at the terminal voltage of 220V.
If the same machine is run as a motor it takes a line current of 20A from 220V supply. The speed of the machine as a motor will be
than 800 rpm.
than 800 rpm. (D) both
higher or lower than 800 rpm.
/ Fpz) Eg = V + Ia Ra ;
= V – Ia Ra ; in motor Eg
> E b for same terminal voltage
A 50 Hz, 3-phase induction motor has a full
load speed of 1440 r.p.m. The number of poles of the motor are
will be closer to N i.e 1440
When P=4 ; Ns = 1500 rpm ,
it is closer to N Therefore P =4 for N=1440
36 In a 3-phase synchronous motor
speed of stator MMF is always more than that of rotor MMF.
speed of stator MMF is always less than that of rotor MMF.
speed of stator MMF is synchronous speed while that of rotor MMF is zero.
and stator MMF are stationary with respect to each other.
rotor poles are engaged with stator poles and both run synchronously in same
direction Therefore, rotor & stator
mmf are stationary w.r.t each other.
a capacitor start single-phase induction motor, the capacitor is connected
series with main winding.
series with auxiliary winding.
series with both the windings.
parallel with auxiliary winding.
phase motor self start. We split the phases at 90 degree. Hence, motor behaves
like a two phase motor.
3-phase winding on rotor and a single-phase winding on stator.
3-phase winding on stator and a commutator winding on rotor.
3-phase winding on stator and a single-phase winding on rotor.
single-phase winding on stator and a commutator winding on rotor.
basic synchro unit called a synchro transmitter. It’s
construction similar to that of a
Three phase alternator.
the voltage of transmission increases, the volume of conductor
to skin effect.
size of the feeder is determined primarily by
current it is required to carry.
percent variation of voltage in the feeder.
voltage across the feeder.
distance of transmission.
amount of current flow.
41 The boundary of the protective zone is
determined by the
of relay used
the protective zone is determined by the
sensitivity of relay used. If the relay
is more sensitive, the protective zone will be
a three phase transformer, if the primary side is connected in star and
secondary side is connected in delta, what is the angle difference between
phase voltage in the two cases.
side lags by -30°. (B) star side lags by -30°.
side leads by -30°.
leads by +30°.
achieve low PT error, the burden value should
transformer, burden should be in permissible range to maintain errorless
of the induction machine is 0.02 and the stator supply frequency is 50 Hz. What
will be the frequency of the rotor induced emf?
Hz. (D) 2500 Hz.
induced emf = s f
0.02 x 50 = 1.0 Hz
4 pole lap wound dc shunt motor rotates at the speed of 1500 rpm, has a flux of
0.4 mWb and the total number of conductors are 1000. What is the value of emf?
Volts. (D) 10 Volts.
= 0.4 mWb, Z = 1000, P = 4, &
/ 60 A
synchronous reactance of the synchronous machine is .
between open circuit voltage and short circuit current at constant field
between short circuit voltage and open circuit current at constant field
between open circuit voltage and short circuit current at different field current
between short circuit voltage and open circuit current at different field
reactance of a synchronous machine is a total steady state reactance, presented
to applied voltage, when rotor is running synchronously without excitation.
case of a universal motor, torque pulsation is minimized by
universal motor, torque pulsation is minimized by rotor and load inertia.
cable has a working stress of _ kV/mm
defined by dielectric strength of mineral oil i.e. 15 kV/mm.
definite minimum time lag relay is also called
minimum time is fixed. The operating time is inversely proportional to the
magnitude of actuating quantity.
heat of nickel –chrome is _
Ans: None of these
of Nickel-Chrome is 440 J/kg°C to
speed-torque characteristics of a DC series motor are approximately similar
to those of the _motor.
shunt (D) two-phase
has same characteristics as DC series motor and also known as an a.c series
a 3 phase 1000
RPM 6 pole induction motor
with a slip of
f = N×P/ 120
an a.c. operated
universal motor has
be started under no-load condition.
inverse, can (B) nearly inverse, can
zero then speed may exceed up to infinite, that is dangerous for machine and
machine can be damaged.
a 1/ T
the heating process of the type
a simple method of temperature control is possible
by means of a special alloy which loses its magnetic properties at a particular
high temperature and regains them when cooled to a temperature below this value.
Indirect induction over (B) core
type induction furnace
furnace (D) high
frequency eddy current
property of alloy changes with change of the temperature and Heat is produced due to eddy current = i2R and i a
transmission system _ are provided.
tachometer is just a with one
phase excited from the carrier frequency.
two-phase induction motor
A.C. operated universal motor (D) hybrid stepper motor.
special purpose machine whose stator coil can be energized by electronically
torque, in a is
proportional to the square of the armature current
therefore Ta a Ia2
synchronous speed for a 3 phase 6-pole induction motor is 1200 rpm.
If the number of poles is now
reduced to 4 with the frequency remaining constant, the rotor speed with a slip
of 5% will be .
0.05 ×60 = 3.0
The eddy current loss in an a-c electric
motor is 100 watts at 50 Hz. Its loss at 100 Hz will be
maximum power for a given excitation in a synchronous motor is developed when
the power angle is equal to
commutator in a d.c. machine
Reduces power loss in armature.
Reduces power loss in field circuit.
Converts the induced a.c armature voltage
into direct voltage.
Is not necessary.
speed is less.
over other losses
parallel, they will share the load a under:
inversely proportional to their impedances.
squirrel cage induction motor with a
rated slip of 2% will have a rotor speed of
1500 (1-0.02) =1470 rpm
The voltage at the two ends of a transmission line are 132 KV and its
reactance is 40 ohm. The Capacity of
the line is
determined by power of line P = (V2/R)
or (V2/Z) when cos F =1
220/440 V, 50 Hz, 5 KVA, single phase transformer operates on
220V, 40Hz supply with secondary
winding open circuited. Then
eddy current and hysteresis losses decreases.
eddy current and hysteresis losses increases.
Eddy current loss remains the same but
hysteresis loss increases.
current loss increases but hysteresis loss remains the same.
khfBm1.6 and We = kef2Bm2.k
hysteresis and eddy current losses will be decreased when frequency decreases.
at unity power factor. If the field current is increased, power factor will become
Leading & current will decrease
Lagging & current will increase.
Lagging & current will decrease.
Leading & current will increase.
unity power factor. When field current increases, the excitation
will increase. Therefore, p.f will be leading and current will be I CosF < I
d.c. shunt motor runs at no load speed of 1140 r.p.m. At full load, armature
reaction weakens the main flux by 5% whereas the armature circuit voltage drops
by 10%. The motor full load speed in r.p.m. is
The introduction of interpoles in between
the main pole improves the performance of
The interpole produces additional flux to
augment the developed torque.
The flux waveform is improved with reduction
bottom halves of armature is removed.
A counter e.m.f. is induced in the coil
rotor power output of a 3-phase induction motor is 15 KW and corresponding
slip is 4%. The rotor copper loss will be
losses = rotor input- rotor output and output = (1-s) input
Input = output/(1-s) = 15000 /1-0.04 =
loss = 15625 -1500 = 625 watt
motor is reversed by
Shift shaded pole with respect to main pole
Reversing supply lead
Either A or
Neither A nor B
single phase, 50Hz supply and stator has two windings. These are connected
continuously from starting to running.
total of 40 teeth on 8 pole of stator. The number of rotor teeth for their
rotor will be
Step angle ‘þ’ = NS –
-40 + Nr/40 Nr x 3600
Nr = 50
Low head plants generally use
(A) Pelton Turbines
(B) Francis Turbine
or Francis Turbine
(D) Kaplan Turbines
shifting the shaded pole region with respect to main pole. But not by changing
supply lead because it has ac supply.
charging reactance of 50 Km length of line is 1500fi. The charging reactance
for 100Km length of line will be
= 1500/50 = 30 ohms
Characteristic reactance per 100km = 30
x 100 = 3000 ohms
temperature upto 3000°C.
produced actually by
current in armature gives rise to armature mmf which react with main field mmf.
share the load depending upon their
rating will share more load.
compared to shunt and compound DC motors, the series DC motor will have the
highest torque because of its comparatively at the start.
resistance. (B) Stronger
T a Ia 2
400kW, 3-phase, 440V, 50Hz induction motor has a speed of 950 r.p.m. on full- load. The machine has 6 poles. The slip of
the machine will be
capacitor-start motor, results in reduced
Noise. (B) Speed.
the capacitance reduces starting voltage, which results in reduced starting
Can be used for stopping a motor.
Cannot be easily applied to DC series motors.
Can be easily applied to DC shunt motors
Cannot be used when motor load has
also mean reversal of field and hence of
following method of generating electric power from sea is most advantageous?
thermal energy conversion
level of technology, tidal power for generating electric power from sea is most advantageous because of constant
availability of tidal power.
the field circuits of an unloaded salient pole synchronous motor gets suddenly
open circuited, then
runs at the slower speed.
runs at a very high speed.
continues to run at the same speed because synchronous motor speed does not
depend upon load, Na f.
for electric resistance seam welding.
cables are used.
core cables. (D) Oil filled.
single core cables are suitable.
the applied voltage by 90°
the applied voltage by somewhat less than 90°
the applied voltage by 90°
the applied voltage by somewhat less than 90°
conditions has to supply (i) iron
losses in the core i.e hysteresis loss
and eddy current loss (ii) a very small amount of Cu loss in the primary (there being no Cu loss in
secondary as it is open)
loss (B) Hysteresis loss
and windage losses
e.m.f., smaller the current and
hence lesser the I2R loss in the core.
A single phase Hysteresis motor
run at synchronous speed only
run at sub synchronous speed only
run at synchronous and super synchronous
run at synchronous and sub synchronous speed
revolves synchronously because the rotor poles magnetically lock up with the revolving stator poles of opposite polarity
temperature of resistance furnaces can be controlled by changing the:
number of heating elements
of the above
resistance furnaces can be controlled by changing either applied voltage or by
number of heating elements or by circuit
offers high impedance to 50 Hz power
high impedance to carrier frequency signal
offers low impedance to carrier frequency signal
(A) & (C)
unit employed in carrier current relaying
offers high impedance
to carrier frequency signal.
frequency range is 35 km – 500 kHz XL =