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

__INTRODUCTION__

**1.What
are the types of electrical engineering materials?**

Basically there are three types of
materials used in electrical machines.

^{Ø
}Magnetic
materials^{}

^{ }

^{Ø
}Conducting
materials^{}

^{ }

^{Ø
}Insulating
materials^{}

**2.What
is meant by critical magnetic field?**

If the temperature of a material is
raised above its critical temperature its superconductivity disappears. The
field at which superconductivity vanishes is called critical magnetic field.

**3.** **What is askarel?**

An askarel is a synthetic non
flammable insulating liquid. The commonest askarel is hexa choloro diphynyl
tricholoro benzene.

**4.
****Name the magnetic materials used for
Yoke, Transformer Stampings and permanent magnet.**

Yoke of a dc machine, transformer
stamping, permanent magnet Yoke of a dc machine – cast steel

Transformer stamping – silicon steel

Permanent magnet – hard magnetic
material (Al, Ni, Co)

**5.
****Comment on the use of CRGOS strip
wound transformer core.**

CRGOS means cold rolled grain
oriented steel. This steel is manufactured by series of cold reductions and
intermediate annealings. This could reduce the material has strong directional
of highest permeability which results less hysteresis loss. This type of
material is suitable for use in transformers.

**6.** **What is meant by heating?**

The temperature of a machine rises when
it runs under load condition starting from cold condition. The temperature
raises is directly proportional to the power wasted. The heat dissipation may
be due to conduction, convection or radiation.

**7.** **What is meant by cooling?**

The cooling medium like air, water
or gas is provided to absorb the heat energy to save the machine. The cooling
medium is also called coo lent. The cooling is of two types like, direct and
indirect cooling.

**8.** **What is meant by radiation?**

If the heat energy is transferred from
one place to other with air of gas it is called radiation.

**9.
****What are the electrical properties
of insulating materials?**

^{Ø
}high
dielectric strength^{}

^{ }

^{Ø
}high
resistivity^{}

^{ }

^{Ø
}low
dielectric hysteresis^{}

^{ }

^{Ø
}good
thermal conductivity^{}

^{ }

^{Ø
}high
thermal stability^{}

^{ }

**10.
****Classification of insulating
materials.**

There are seven classes of
insulating materials used in electrical machines according to their thermal
stability in service.

**11.
What are the constructional elements of a transformer?**

The constructional elements of a
transformer are core, high and low voltage windings, cooling tubes or radiators
and tank.

**12.
****How the design problems of an
electrical machine can be classified?**

The design problems of electrical
machine can be classified as:

^{Ø
}Electromagnetic
design^{}

^{ }

^{Ø
}Mechanical
design^{}

^{ }

^{Ø
}Thermal
design^{}

^{ }

^{Ø
}Dielectric
design^{}

^{ }

**13.
****List the constructional elements of
a d.c. machine?**

The major constructional elements of
a d.c.machine are stator, rotor, brushes and brush holders. The various parts
of stator and rotor are listed below:

**Stator
**-

Yoke (or) Frame

Field pole

Pole shoe

Field winding

Interpole

**Rotor
**-

Armature core

Armature winding

Commutator

**14.
How is total m.m.f. calculated?**

The total mmf required to establish
the flux in the magnetic circuit is calculated using the knowledge of
dimensions and configuration of the magnetic circuit. The magnetic circuit is
split up into conventional parts, which may be connected in series or parallel.
The flux density is calculated in every part and m.m.f. per unit length; ‘at’
is found by consulting ‘B-H’ curves. The summation of m.m.fs in series gives
the total m.m.f.

**15.
****List the methods used for
determining the motor rating for variable load drives.**

^{Ø
}Method
of average losses^{}

^{ }

^{Ø
}Equivalent
current methods^{}

^{ }

^{Ø
}Equivalent
torque method^{}

^{ }

^{Ø
}Equivalent
power method^{}

^{ }

**16.
****Define rating.**

Rating of a motor is the power
output or the designated operating power limit based upon certain definite
conditions assigned to it by the manufacturer.

**17.
What are the problems that arise during the calculation of m.m.f. for air gap?**

The iron surfaces around the air gap
are not smooth and so the calculation of m.m.f. for the airgap by ordinary
methods gives wrong results. The problem is complicated by the fact that

^{Ø
}One
or both of the iron surfaces around the air gap may be slotted so that the flux
tends to concentrated on the teeth rather than distributing itself uniformly
over the air gap.^{}

^{ }

^{Ø
}There
are radial ventilating ducts in the machine for cooling purposes which effect
in a similar manner as above.^{}

^{ }

^{Ø
}In
salient pole machine, the gap dimensions are not constant over whole of the
pole pitch.^{}

^{ }

**18.
****Mention the methods used for
calculating the mmf for tapered teeth.**

^{Ø
}Graphical
method^{}

^{ }

Ø
Three
ordinate method ( Simpson’s Rule)

^{Ø
}B_{t
1/3} Method^{}

^{ }

**19.
****What is Carter’s gap co-efficient?**

The Carter’s gap co-efficient ( K_{cs})
is the ratio of slot width to gap length. The formula which gives the value of
K_{cs} directly is

**K _{cs} = 1 / [1 + (5 l_{g}/W_{s} )]**

Where l_{g} = gap length; W_{s}
= Width of slot

The other formula for Carter’s gap
co-efficient ( K_{cs}) for parallel sided open slot is

**K _{cs} = 2/Π [tan^{-1} y – 1/ Π log SQRT (1+
Π^{2})]**

Where **y = W _{s}**

**20.
****Write down the expression for
calculation of reluctance of air gap with slotted Armature.**

Reluctance of air gap with slotted armature

S_{g} = l_{g} / µ y_{s}
L = l_{g} /µ_{0} L (y_{s} - K_{cs} W_{s}
)

Where l_{g} = gap length; y_{s}
= slot pitch; K_{cs} = Carter’s co-efficient; W_{s} = Width of
slot.

**21.
Define field form factor.**

**22.
****Define stacking factor.**

Stacking factor is defined as the
ratio of actual length of iron in a stack of assembled core plates to total
axial length of stack.

**23.
What is gap contraction factor for slots?**

The ratio of reluctance of air gap
of slotted armature to reluctance of air gap of smooth armature is called gap contraction
factor for slots.

**24.
What is gap contraction factor for ducts?**

The ratio of reluctance of air gap
with ducts to reluctance of air gap without ducts is known as gap contraction
factor for ducts.

Where

L = Length of core;

K_{cd} = Carter’s
co-efficient for ducts ;

N_{d} = number of radial
ducts;

W_{d} = Width of each duct

**25.
Write the expression for mmf of air gap with smooth and slotted armatures.**

M.M.F. for air gap with smooth
armature is **AT _{g}**

M.M.F. for air gap with slotted
armature is **AT _{g}**

Where **Kg** is gap expansion factor; **B**
is flux density; **l _{g}** is
gap length.

**26.
Mention the problems encountered while calculating the mmf for teeth.**

The calculation of mmf necessary to maintain
the flux in the teeth is difficult during to the following problems:

^{Ø
}The
teeth are wedge-shaped or tapered when parallel sided slots are used. This
means that the area presented to the path of flux is not constant and this
gives different values of flux density over the length of teeth.^{}

^{ }

^{Ø
}The
slot provides another parallel path for flux, shunting the tooth. The teeth are
normally worked in the saturation region and therefore their permeability is
low and as a result an appreciable portion of the flux goes down the depth of
slots.^{}

^{ }

**27.
****Explain why real flux density in the
teeth is less than the apparent flux density.**

The slot provides an alternate path
for the flux to pass, although the flux entering an armature from the air gap
follows paths principally. If the teeth density is high, the mmf acting across
the teeth is very large and as the slots are in parallel with the teeth, this
mmf acts, across the slots also. Thus at saturation density, the flux passing
through the slots become large and cannot be neglected and calculation based on
‘no slot flux’ leads to wrong results. This shows that the real flux passing
through the teeth is always less than the total or apparent flux. As a result,
the real flux density in the teeth is always less than the apparent flux
density.

**28.
What is meant by apparent and real flux density?**

**29.
What is meant by rating of a machine?**

The rating of machine refer to the
whole of the numerical values of electrical and mechanical quantities with
their duration and sequence assigned to the machines by the manufacturer and
stated on the rating plate, the machine complying with the specified
conditions.

**30.
Mention the different types of duties of a machine.**

The following are the types of duty
as per IS : 4722 – 1968 “ Specification for rotating electric machinery”

^{Ø
}S_{1}
= Continuous duty^{}

^{ }

^{Ø
}S_{2}
= Short time duty^{}

^{ }

^{Ø
}S_{3}
= Intermittent periodic duty^{}

^{ }

^{Ø
}S_{4}
= Intermittent periodic duty with
starting^{}

^{ }

^{Ø
}S_{5}
= Intermittent periodic duty with
starting and braking^{}

^{ }

^{Ø
}S_{6}
= Continuous duty with intermittent periodic loading^{}

^{ }

^{Ø
}S_{7}
= Continuous duty with starting and braking^{}

^{ }

^{Ø
}S_{8}
= Continuous duty with periodic speed changes^{}

^{ }

**31.
****Define continuous rating.**

The continuous rating of a motor is
defined as the load that may be carried by the machine for an indefinite time
without the temperature rise of any part exceeding the maximum permissible
value.

**32.
Define short time rating.**

The short time rating of a motor may
be defined as its output at which it may be operated for a certain specified
time without exceeding the maximum permissible value of temperature rise.

**33.
Define duty factor.**

The duty factor ( also called load
factor or cyclic duration factor) is defined as the ratio of the heating period
to the period of whole cycle.

**34.
****What is meant by intermittent
rating?**

The intermittent rating of a motor
applies to an operating condition during which short time. Load periods
alternate with period of rest or no load without the motor reaching the thermal
equilibrium and without the maximum temperature rising above the maximum
permissible value.

**35.
****What are the major considerations to
evolve a good design of electrical machine?**

The major considerations to achieve
a good electrical machine are

^{Ø
}Specific
electric loading^{}

^{ }

^{Ø
}Specific
magnetic loading^{}

^{ }

^{Ø
}Temperature
rise^{}

^{ }

^{Ø
}Efficiency^{}

^{ }

^{Ø
}Length
of air gap^{}

^{ }

^{Ø
}Power
factor^{}

^{ }

**36.
****List the standard specifications for
transformer.**

^{Ø
}IS
1180 – 1989 : Specifications for out door 3-phase distribution transformer Upto 100 KVA.

^{Ø
}IS
2026 – 1972 : Specifications of power transformers^{}

^{ }

**37.
****What is magnetic circuit?**

The magnetic circuit is the path of
magnetic flux. The mmf of the circuit creates flux in the path against the
reluctance of the path. The equation which relates flux, mmf and reluctance is
given by

**38.
****Write any two essential differences
between magnetic and electric circuits.**

^{Ø
}When
the current flows in the electric circuit the energy is spent continuously,
whereas in magnetic circuit the energy is needed only to create the flux but
not to maintain it.^{}

^{ }

^{Ø
}Current
actually flows in the electric circuit, whereas the flux does not flow in a
magnetic circuit but it is only assumed to flow.^{}

^{ }

**39.
****What is magnetization curve?**

The magnetization curve is a graph
showing the relation between the magnetic field intensity, H and the flux
density, B of a magnetic circuit. It is used to estimate the mmf required for
flux path in the magnetic material and it is supplied by the manufacturers of
stampings and laminations.

**40.
What is loss curve?**

The loss curve is a graph showing
the relation between iron loss and magnetic field intensity, H. It is used to
estimate the iron loss of the magnetic materials and it is supplied by the
manufacturers of stampings and laminations.

**41.
****What is the difference in
permeability of magnetic and non-magnetic materials?**

In magnetic materials the
permeability is not constant and it depends on the saturation of the magnetic
material. But in non-magnetic materials the permeability is constant.

**42.
****How to find total mmf in a series
circuit?**

The various steps in estimation of
mmf of a section of magnetic circuit are:

^{Ø
}Determine
the flux in the concerned section.^{}

^{ }

^{Ø
}Calculate
the area of cross-section of the section.^{}

^{ }

^{Ø
}Calculate
the flux density in the section^{}

^{ }

^{Ø
}From
B – at curve of the magnetic material, determine the mmf per meter(at) for the
calculated flux density^{}

^{ }

Ø
The
mmf of the section is given by the product of length of the section and mmf per
metre.

**43.
****List the different types of slots
that are used in rotating machines.**

The different types of slots are

^{Ø
}Parallel
sided slots with flat bottom^{}

^{ }

^{Ø
}Parallel
sided slots with circular bottom^{}

^{ }

^{Ø
}Tapered
slots with flat bottom^{}

^{ }

^{Ø
}Tapered
slots with circular bottom^{}

^{ }

^{Ø
}Circular
slots^{}

^{ }

44. **Mention
the undesirable effects of unbalanced magnetic pull**.

The undesirable effects of unbalanced
magnetic pull are

^{Ø
}Saturation
of magnetic materials due to reduction in air gap.^{}

^{ }

^{Ø
}Excessive
vibration and noise due to unbalanced radial forces^{}

^{ }

**45.
****Mention the importance of conductor
dimensions.**

The dimension of the conductors
directly affects the following factors in rotating machines:

^{Ø
}Allowable
temperature rise^{}

^{ }

^{Ø
}Resistivity^{}

^{ }

^{Ø
}Current
density^{}

^{ }

^{Ø
}Specific
electric loading^{}

^{ }

**46.
****Define slot space factor or slot
insulation factor.**

The slot space factor is defined as
the ratio of conductor area to slot area.

**47.
What do you understand by slot pitch?**

The slot pitch is defined as the
distance between centers of two adjacent slots measured in linear scale.

Where D = Diameter of armature

S_{s} = Number of slots in armature

**48.
****State the parameters governing slot
utilization factor or slot space factor.**

The following factors decide the
slot utilization factor:

^{Ø
}Voltage
rating^{}

^{ }

^{Ø
}Thickness
of insulation^{}

^{ }

^{Ø
}Number
of conductors per slot^{}

^{ }

^{Ø
}Area
of cross-section of the conductor^{}

^{ }

^{Ø
}Dimensions
of the conductor^{}

^{ }

**49.
****Define specific permeance of a slot.**

Specific permeance of a slot is
defined as the permeance per unit length of slot or depth of field.

**50.
What is unbalanced magnetic pull?**

The unbalanced magnetic pull is the
radial force acting on the rotor due to non-uniform air-gap around armature
periphery.

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