Q.1 Ans : B
Explanation:
Whenever alternating voltage is applied to the
primary of a electrical transformer, there will be a alternating flux through
the core of transformer. As this flux will link with secondary winding there
will be an induced emf in the secondary winding. The phenomenon of inducing voltage by changing flux is
called electrical induction, hence it can be concluded that the windings of a
transformer are linked inductively.
Q.2 Ans: B
Explanation:
Synchronous motors is called so because the speed of the rotor of this
motor is same as the rotating magnetic field. It is basically a fixed speed motor
because it has only one speed, which is synchronous speed and therefore no
intermediate speed is there or in other words it’s in synchronism with the
supply frequency.
Q.3 Ans: A
Explanation:
In a DC series motor, at starting time there is only residual flux in
the series field winding because the current is zero. We know that the speed is
inversely proportional to flux. Hence, if the residual flux is very low then
the speed is enormously high. It damages the internal parts of the motor. So,
the DC series motor should not be started on no load.
Q.4 Ans: C
Explanation:
Frequency of rotor current, fr = s × f
Q.5 Ans: A
Explanation:
A stepper
motor is an actuator that converts electrical pulse signals into angular
diaplacements. Once the stepper motor receives an electrical pulse signal, it
moves a fixed step angle in a predefined direction. Thus by controlling the
pulse number ,the angular displacement can be precisely controlled.
Q.6 Ans: A
Explanation:
The power
factor at no load is low because the magnetizing component of input current is
a large part of the total input current of the motor. When the load on the
motor is increased, the in-phase current supplied to the motor increases, but
the magnetizing component of current remains practically the same. This means
that the resultant line current is more nearly in phase with the voltage and
the power factor is improved when the motor is loaded, compared with an
unloaded motor which has its magnetizing current as a major component of the
input current.
Q.7 Ans: A
Explanation:
the
voltage at generation is usually between
11 kV and 33 kV becuse of
insulation limitation of the generator, as the generator voltage increases we will
need to provide extra insulation
across the windings & that will make the whole machine bulkier. Despite of that
voltage is steped up after
generation at suitable level.
Q.8 Ans: D
Explanation:
In case, synchronous motor
is of salient pole type, additional winding is placed in rotor pole face.
Initially when rotor is standstill, relative speed between damper winding and
rotating air gap flux in large and an emf is induced in it which produces the
required starting torque. As speed approaches synchronous speed , emf and
torque is reduced and finally when magnetic locking takes place, torque also
reduces to zero.
Q.9 Ans: A
Explanation:
Transformer
doesn't change the wave shape it just changes its value and also frequency
remains same so obviously output would be square wave with changed value
depends on transformation ratio
Q.10 Ans: B
Explanation:
Ta α Φ.Ia and
Φ α Ia ;
therefore Ta α Ia2
