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Design of Electrical Machines - Synchronous Machines - Important Short Questions and Answers: Design of Electrical Machines - Synchronous Machines

__SYNCHRONOUS MACHINES__

**1.
****Advantages of stationary armature
and rotating field type machine.**

^{Ø
}Since
armature winding is stationary the load circuit can be directly connected to
it.^{}

^{ }^{Ø
}As
the armature winding is fixed it is easy to provide insulation for high
Voltages.

^{ }^{Ø
}Weight
of field system is less as compared to armature so that higher speed can^{ }be achieved.

^{Ø
}Since
the exciter supplies low voltage d.c. it requires less amount of insulation.^{}

^{ }

**2.
****Define critical speed?**

The rotor of an alternator rotates
with prime mover speed. The rotor core is structure which has certain mass and
property of elasticity. The rotor core is designed corresponding to natural
frequency is called critical speed.

**3.
****Give the importance of compensating
winding in dc machine.**

^{Ø
}It
is provided in pole shoe.^{}

^{ }

^{Ø
}It
is connected in series with armature winding.^{}

^{ }

^{Ø
}It
is used to reduce armature reaction.^{}

^{ }

^{Ø
}Due
to this winding full range of speed variation can be obtained.^{}

^{ }

**4.
****Mention superiority of hydrogen over
air as coolant?**

^{Ø
}Heat
transfer co-efficient of hydrogen is 1.5 times that of air.^{}

^{ }

^{Ø
}Thermal
conductivity of hydrogen is 7 times that of air.^{}

^{ }

^{Ø
}Density
of hydrogen is 0.07 times that of air.^{}

^{ }

**5.
****Why deep bar rotor construction is
preferred in squirrel cage induction motor?**

It is preferable when high starting
torque is required. Because loose bars can be damaged quickly by mechanical
vibration and thermal cycling.

**6.** **What is varnish impregnation?**

The dipping of insulating material
into varnish to improve the resistance to moisture and creeping discharge is
called varnish impregnation.

**7.
****How to reduce the harmonic effects?**

^{Ø
}Short
pitch winding^{}

^{ }

^{Ø
}Distributed
winding^{}

^{ }

^{Ø
}Fractional
slot winding^{}

^{ }

^{Ø
}Large
air gap length^{}

^{ }

**8.
****Define heating time constant of the
machine.**

The time taken by the machine to
rise its temperature 63.2% of its final steady value.

**9.
****What are the types of stator
winding?**

^{Ø
}Single
layer winding^{}

^{ }

^{Ø
}Double
layer winding^{}

^{ }

**10.
****Why is it necessary to eliminate
voids or air packets in high voltage multi lunch coils?**

Since the voids carry air and air
has poor thermal conductivity heat transfer will be poor. Hence voids should be
eliminated.

**11.
****Classify synchronous machines.**

^{Ø
}Salient
pole machine^{}

^{ }

^{Ø
}Cylindrical
rotor machine^{}

^{ }

**12.
****List the advantages of revolving
field system.**

The advantages are

^{Ø
}It
permits the use of a stationary armature on which the windings can be easily
braced and insulated for high voltage.^{}

^{ }

^{Ø
}The
operation of slip rings on account of their sliding contact is under liable
with large currents at high potential difference. The use of slip ring carrying
large currents at high voltage is therefore avoided in the stationary armature
construction.^{}

^{ }

**13.
****Write the output equation of
synchronous machine.**

The output equation of an
synchronous machine is given by

KVA output Q = C_{0} D^{2}
L n_{s}

Where C_{0} = output
co-efficient

= 1.11 Π^{2} B_{av}
ac K_{ws} 10^{-3}

Q = KVA output for alternator and
KVA input for synchronous motor.

D = Diameter of stator core, m

L = Length of stator core, m n_{s}
= Synchronous speed, rps

B_{av} = Specific magnetic loading, wb/m^{2}

ac = Specific electric loading, amp.cond/m
K_{ws}= stator winding factor

**14.
****Mention the factors to be considered
for the selection of number of armature slots?**

^{Ø
}Balanced
windings^{}

^{ }

^{Ø
}Cost^{}

^{ }

^{Ø
}Host
spot temperature in winding^{}

^{ }

^{Ø
}Leakage
reaction^{}

^{ }

^{Ø
}Tooth
losses^{}

^{ }

^{Ø
}Tooth
flux density^{}

^{ }

**15.
****What are the types of coils employed
by the salient pole machines?**

The armature windings of salient
pole machines employ two types of coils:

^{Ø
}Single
turn bar^{}

^{ }

^{Ø
}Multi
turn^{}

^{ }

**16.
****How are iron and friction losses of
an alternator measured?**

Iron and friction losses of an
alternator can be measured by coupling the alternator to a suitable calibrated
d.c. motor and driving it at synchronous speed with normal excitation. Then,

Iron and Friction Losses = Output of
motor in Watts.

**17.
Is the efficiency of an alternator determined by direct loading?**

As with d.c. machines, the
efficiency of an alternator is not determined by direct loading owing to the
difficulty in finding a suitable load. The efficiency is generally determined
from losses.

**18.
Draw a block representing the analysis method of design.**

**19.
Mention the advantages of analysis method**.

The advantages are

^{Ø
}It
is fairly easy to program, to use and to understand^{}

^{ }

^{Ø
}Results
in considerable time saving thereby giving quick returns of the investments
made.^{}

^{ }

^{Ø
}The
programs based upon analysis methods are simple but they become the foundations
for later day larger and sophisticated programs.^{}

^{Ø
}The
results of analysis method are highly acceptable by designers.^{}

^{ }

**20.
****What is the length of mean turn of
the armature?**

The length of mean turn of the
armature is

**L _{mt}
= 2 L + 2.5 τ + 0.06 KV +0.2 **in metre

Where 2L is the length of turn
embedded in the slots

**L _{mt} = 2.5 τ + 0.06 KV +0.2 **in metre

Is the length in the overhang.

**21.
What is the limiting factor for the diameter of synchronous machine?**

The limiting factor for the diameter
of synchronous machine is the peripheral speed. The limiting value of
peripheral speed is 175 m/sec for cylindrical rotor machines and 80 m/sec for
salient pole machines.

**22.
Write the expression to calculate the height of field winding.**

**23.
What is the total space required for field winding?**

**24.
Give the expression to calculate the area of pole bodies**.

**25.
How is the copper area of field winding calculated?**

**26.
What are the advantages of synthesis method?**

The greatest advantage of synthesis
method is the savings in time in lapsed time and in engineering man hours on
account of the decision making left to the computer itself.

**27.
What are the disadvantages of synthesis method?**

The disadvantages are

^{Ø
}The
synthesis method involves too much of logic since the logical decisions are
taken by the computer. Now, the logical decisions have to be incorporated in
the program and before they are incorporated in the program, the teams of
engineers have to agree upon them. Firstly the logical decisions to arrive at a
optimum design are too many and then there are too many people with too many
ways to suggest to produce an optimum design and it becomes really hard to formulate
a logic that really produces an optimum design.^{}

^{ }

^{Ø
}The
formulation of a synthesis program taking into account the factor listed above
would make it too complex. The complex program formulated at high cost would
require the use of a large computer and also large running time involving huge
expenditure.^{}

^{ }

**28.
****How is the efficiency of an
alternator affected by load power factor?**

The efficiency of an alternator
depends not only on KVA output but also on power factor of the load. For a
given load, efficiency is maximum at unity power factor and decreases as the
power factor falls.

**29.
Name the two acceptable approaches to machine design.**

The two commonly acceptable
approaches to machines are

^{Ø
}Analysis
method^{}

^{ }

^{Ø
}Synthesis
method^{}

^{ }

**30.
****List few advantages of using a digital
computer for the design of electrical machines**.

The advantages are

^{Ø
}It
has capabilities to store amount of data, count integers, round off results
down to integers and refers to tables, graphs and other data in advance.^{}

^{ }

^{Ø
}It
makes it possible to select an optimized design with a reduction in cost and
improvement in performance.^{}

^{ }

**31.
****Give the purpose of providing damper
windings in synchronous machines.**

The purpose of damper winding is

^{Ø
}In
synchronous generators, it is provided to suppress the negative sequence field
and to damp the oscillations when the machine starts hunting.^{}

^{ }

^{Ø
}In
synchronous motor, its function is to provide starting torque and to develop
damping power when the machine starts hunting.^{}

**32.
****What is the range of rotor current
density?**

Rotor current density ranges from
about 2.5 A/mm^{2} for conventionally cooled machines. However, in
modern direct cooled generators, the rotor current density may be as high as
9.5– 14 A/mm^{2}.

**33.
Write the expression for air gap length in cylindrical rotor machine.**

**34.
Mention the factors that govern the design of field system of alternator.**

The following factors to be
considered for the design of field system in alternator:

^{Ø
}Number
of poles and voltage across each field coil^{}

^{ }

^{Ø
}Amp-turn
per pole^{}

^{ }

^{Ø
}Copper
loss in field coil^{}

^{ }

^{Ø
}Dissipating
surface of field coil^{}

^{ }

Ø
Specific
loss dissipation and allowable temperature rise

**35.
What is runway speed?**

The runway speed is defined as the
speed which the prime mover would have, if it is suddenly unloaded, when
working at its rated load.

**36.
****State the important features of
turbo-alternators.**

^{Ø
}The
rotors of turbo-alternators have large axial length and small diameters.^{}

^{ }

^{Ø
}Damping
torque is provided by the rotor itself and so there is no necessity for
additional damper winding.^{}

^{ }

^{Ø
}They
are suitable for high speed operations and so number of poles is usually 2 or
4.^{}

^{ }

**37.
****What are the prime movers used for
a) salient pole alternator b) Non-salient pole alternator?**

The prime movers used for salient
pole alternators are water wheels like Kaplan turbine, Francis turbine, pelton
wheel, etc., and diesel or petrol engines.

The prime movers used for
Non-salient pole alternators are steam turbines and gas turbines.

**38.
****Distinguish between cylindrical pole
and salient pole construction**.

In cylindrical pole construction the
rotor is made of solid cylinder and slots are cut on the outer periphery of the
cylinder to accommodate field conductors.

In salient pole construction, the
circular or rectangular poles are mounted on the outer surface of a cylinder.
The field coils are fixed on the pole.

The cylindrical pole construction is
suitable for high speed operations, whereas the salient pole construction is
suitable for the slow speed operations.

**39.
Salient pole alternators are not suitable for high speeds. Why?**

The salient pole rotors cannot
withstand the mechanical stresses developed at high speeds. The projecting
poles may be damaged due to mechanical stresses.

**40.
State the factors for separation of D and L for cylindrical rotor machine.**

The separation of D and L in
cylindrical rotor machine depends on the following factors:

^{Ø
}Peripheral
speed^{}

^{ }

^{Ø
}Number
of poles^{}

^{ }

^{Ø
}Short
circuit ratio (SCR)^{}

^{ }

**41.
Define pitch factor.**

**42.
Define distribution factor.**

**43.
****Mention the advantages of fractional
slot winding.**

^{Ø
}In
low speed machines with large number of poles, the fractional slot winding will
reduce tooth harmonics.^{}

^{Ø
}A
range of machines with different speeds can be designed with a single
lamination.^{}

^{ }

^{Ø
}The
fractional slot winding reduces the harmonics in mf and the leakage reactance
of the windings.^{}

^{Ø
}The
fractional slot winding allows only short chorded winding. Therefore the length
of mean turn of a coil reduces which results in shorter end connections and so
saving in copper.^{}

**44.
****What is short circuit ratio (SCR)?**

The Short circuit ratio (SCR) is
defined as the ratio of field current required to produce rated voltage on open
circuit to field current required to circulate rated current at short circuit.

It is also given by the reciprocal
of synchronous reactance, X_{d} in p.u.

^{Ø
}For
turbo-alternators SCR is between 0.5 to 0.7.^{}

^{ }

^{Ø
}For
salient pole alternator SCR varies from 1.0 to 1.5^{}

^{ }

**45.
****List the factors to be considered
for the choice of specific electric loading in synchronous machines.**

^{Ø
}Copper
loss^{}

^{ }

^{Ø
}Temperature
rise^{}

^{ }

^{Ø
}Synchronous
reactance^{}

^{ }

^{Ø
}Stray
load losses^{}

^{ }

^{Ø
}Voltage
rating^{}

^{ }

**46.
****Determine the total number of slots
in the stator of an alternator having 4 poles, 3 phase, 6 slots per pole for
each phase?**

Total number of slots =
slots/pole/phase * number of poles * Number of phases

=
6
* 4* 3

=
72
slots.

**47.
****How the value of SCR affects the
design of alternator.**

For high stability and low
regulation, the value of SCR should be high, which requires large air gap. When
the length of air gap is large, the mmf requirement will be high and so the
field system will be large. Hence the machine will be costlier.

**48.
****What are the advantages of large air
gap in synchronous machines?**

The advantages of large air gap are:

^{Ø
}Reduction
in armature reaction^{}

^{ }

^{Ø
}Small
value of regulation^{}

^{ }

^{Ø
}Higher
value of stability^{}

^{ }

^{Ø
}Better
cooling^{}

^{ }

^{Ø
}Lower
tooth pulsation losses^{}

^{ }

^{Ø
}Smaller
unbalanced magnetic pull^{}

^{ }

**49.
****Write the expression for the length
of air gap in salient pole synchronous machine.**

**50.
Why alternators are rated in KVA?**

The KVA rating of ac machine depends
on the power factor of the load. The power factor in turn depends on the
operating conditions. The operating conditions differ from place to place.
Therefore the KVA rating is specified for all ac machines.

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