[Q:-01]
What is an electric drive?
Ans
:
The combination of the prime mover as an
electric motor and the transmission system comprising shaft work, pulleys,
belts, gears etc. through which the necessary motion is transmitted to operate
the given working machines is called the electric drive.
[Q:-02]
What are the advantages of electric drive?
Ans
:
The various advantages are as follows :-
(i) It is
clear and economical.
(ii) As it
requires no fuel gases, there is no need of storing fuel or transportation.
(iii) The
electrical energy can be easily transmitted by using transmission and
distribution lines over long distances.
(iv) As
electric motors can be started and stopped easily by using suitable controlling
methods it enables easy starting and stopping of equipments.
(v) It is easy
to maintenance.
(vi) The
electrical braking system is much superior and economical.
[Q:-03]
What is the main disadvantages of electric drive?
Ans
:
In case of a failure of electric supply
the electric drive comes to a rest and so affects production.
[Q:-04]
What are the classifications of electric drives?
Ans
:
Electric Drives may be classified into
three categories :
(i) Group
drive,
(ii) Individual
drive , and
(iii) Multi-motor
drive.
[Q:-05]
What do you mean by group drive? Where is it used?
Ans
:
Group drive is the name used in connection
with a drive where a single motor imparts the energy for working several
machanism or machines by means of one or more line shafts supported on
bearings. It is also called a line shaft drive. Multi stepped pulleys are
fitted on the line shaft and power is transmitted to the driven machines with
belt conections at variable speed. It is used in small workshop.
[Q:-06]
What are the advantages and disadvantages of group drive?
Ans
:
Advantages :—
(1) It is
economical in consideration of the first cost of the motors and control gears.
(2) The cost
of a single motor of large capacity is less than the total cost of a number of
small motors of the same total capacity. Since all machines are not required to
run on full load at the same time the rating of the motor for group drive can
often be less than the aggregate of the output ratings of all the individual
motors which causes further reduction in cost.
(3) The
efficiency and power factor of a large group drive motor will be comparatively
higher.
(4) Short time
overload in any driven machine does not effect on the performance of group
drive motor.
Disadvantages :—
(1) If any
fault is occurred in the driving motor all the driven equipments will be idle.
(2)
Considerable power loss takes place in the energy transmission mechanism.
(3) Since the
driven machines have to be located as to suit the layout of the line shaft
flexibility of layout of the various machines is lost. In any case it is
impossible to install any machine at a distant place.
(4) The speed
control of the individual machine is very cumbersome using stepped pulleys,
belts etc.
(5) The use of
line shaft, pulleys and belts does not give good appearance and is also less
safe to operate.
(6) In any
existing workshop the possibility of installations of additional machines is
limited.
(7) When a
good number of machines are not in operation the motor will work at low
capacity causing low operational efficiency.
(8) The level
of noise produced at the worksite is quite high.
[Q:-07]
What do you mean by individual drive? Give some examples.
Ans
:
Individual drive is the name used in
connection with a drive where a single motor imparts the energy for working a
single machine.
A drilling machine, machine tools and hand
driven electrical tools are the examples of individual drive.
[Q:-08]
What are the advantages and disadvantages of individual drive ?
Ans:
Advantages :-
(i) Each
operator has complete control of his machine which enables him to vary its
speed if necessary and stop while not in use thus eliminating no load losses.
(ii) The
machines can be placed in any desired position and can be moved very easily.
(iii) The
motor and its control unit can be built as an integral part of the machine
which results in good appearance, cleanliness and safety.
(iv) For
driving heavy machines it is advantageous considering the flexibility of
control system.
(v) It reduces
the cost of superstructure.
Disadvantages :—
(i) It costs
more than group drive.
(ii) As the
energy is transmitted to the different parts of the same mechanism by means of
mechanical parts like gears, pulleys etc. there occurs some power loss .
[Q:-09]
What do you mean by multi-motor drive? Where is it used ?
Ans:
Multi-motor drive means a drive comprised
of a number of individual drives each of which operate one of many working
machines in a production unit.
[Q:-10]
What are the points to be considered before selecting a drive ?
Ans
:
The following
points are to be considered :
(i) Load
direction considering overhual.
(ii) Drive
direction considering reversal of motor rotation.
(iii) Nature
of mechanical load ensuring the matching of speed-torque characteristics for
both the motor and load.
(iv) Load
torque characteristics.
(v) Stability
considering the load and motor characteristics.
(vi) Duty
cycle considering the variation of the load speed versus time and/or torque
versus time requirement.
[Q:-11]
What do you mean by the duty cycle?
Ans
:
When the load and/or torque versus time
variation is periodic and repetitive then one cycle of variation of load with
time is called the duty cycle.
[Q:-12]
Name the different types of mechanical loads with categorical examples.
Ans
:
(i) Constant
and continuous loads : Centrifugal pumps, fans and paper making machines
operating for a long time come under this category.
(ii)
Continuous variable loads : Hoist winches, conveyors, metal cutting lathes etc.
operating for a long time with fluctuation of loads come under this category.
(iii)
Pulsating loads : Machines with crank shaft such as textile looms,
reciprocating pumps and compressors, frame saws etc. come under this category.
(iv) Impact
loads : Machines used with drives and having heavy flywheel which have regular
and repetitive load peaks or pulses such as rolling mills, presses, shearing
machines, forging hammers etc. come under this category.
(v) Short time
intermittent loads : Hoisting mechanism, excavators, roller trains, cranes etc.
come under this category.
(vi) Short
time Loads : Servo motors used for remote control of clamping rods of drilling
machine, motor generator for charging batteries come under this category.
[Q:-13]
On what factors the selection of an electric motor depends?
Ans
:
The selection of an electric motor for a
particular drive depends on the following factors :-
(a) Type of
electric supply whether A.C. or D.C.
(b) Nature of
drive whether group or individual drive.
(c) Nature of
load whether the load demands less or high starting torque.
(d) Electrical
characteristics of the motor —
(i) Starting characteristics
(ii) Running characteristics
(iii) Speed control and
(iv) Braking.
(e) Size and
rating of motors
(i) whether the motor is for continuous,
intermittent or variable load cycle
(ii) whether the overload capacity or pull
out torque meet the requirements.
(f) Mechanical
characteristics—
(i) Type of enclosure and nature of
bearing to suit working conditions.
(ii) The type of transmission or drive
i.e. by director or through gears, a chain or belt &
(iii) Noise
(g) Economic
consideration
(i) Capital expenditure, and
(ii) Running expenditure.
[Q:-14]
What is meant by the rating of the motors ?
Ans
:
The rating of an electric motor is the
amount of power which it can deliver without becoming unduly hot.
[Q:-15]
What are the different types of duties of motor power ratings ?
Ans
:
The duties are—
(i) Continuous
duty (S1).
(ii) Short
time duty (S2).
(iii)
Intermittent periodic duty (S3).
(iv)
Intermittent periodic duty with starting (S4) .
(v)
Intermittent periodic duty with starting and braking (S5).
(vi)
Continuous duty with intermittent periodic loading (S6).
(vii)
Continuous duty with starting and braking (S7).
(viii)
Continuous duty with periodic speed changes (S8).
[Q:-16]
What do you mean by continuous rating of a motor ?
Ans
:
Continuous rating is that amount of power
which it can deliver continuously for sufficiently long interval of time so
that it reaches the final steady state temperature.
[Q:-17]
What do you mean by short time rating of a motor ?
Ans
:
Short time rating is the maximum power
which it can deliver at a constant load for a short period of time followed by
a long period of rest so as to cool down to the initial temperature.
[Q:-18]
What do you mean by intermittent periodic rating of a Motor?
Ans
:
Intermittent periodic rating is the
maximum power which it can deliver with a constant load for a fixed short
period of time (1 hour or half an hour or 15 minutes) followed by a short
period of rest according to the duty cycle so that the motor does not reach a
steady temperature during Working periods and cools down to ambient temperature
within short period of rests.
[Q:-19]
What are the different types of electric braking of a loaded induction motor ?
Ans
:
There are
three types of electric braking of loaded induction motor e.g.
(i)
Regenerative braking—in this method the motor is used as induction generator by
over driving at hyper synchronous speed with negative slip.
(ii)
Plugging—In this method braking action is achieved very quickly by
disconnecting and then reversing the two connections for reversing its rotation
until it stops. A zero speed switch must be used to disconnect the plugging
contactor so that the motor does not reverse. (iii) Dynamic braking—In this
method the motor is disconnected from the a.c. supply and a d.c. current of
approximately 1.3 times the rated current of the machine is injected to one of
the stator phases by applying a suitable low d.c. voltage. Once the motor halts
the d.c. supply is disconnected.
[Q:-20]
Why an enclosure is used in an electric motor?
Ans
:
An enclosure is used to protect a motor
from outside materials and persons interfering with the working though it
decreases the output by hindering the free flow of cooling air.
[Q:-21]
What are the various types of enclosures in which electric motors are
available?
Ans
:
(i)
Mechanically Protected (P),
(ii) Screen
Protected (SP),
(iii) Screen
Protected Drip Proof (SPDP),
(iv) Splash
Proof (SPLP),
(v) Screen and
Filters (S&F),
(vi) Screen
and Filters with Blowers (S & FB),
(vii) Double
Pipe Ventilated (DPV),
(viii) Totally
Enclosed (TE),
(ix) Totally
Enclosed Fan Cooled (TEFC),
(x) Closed Air
Circuit Air Cooled (CACA),
(xi) Closed
Air Circuit Water Cooled (CACW),
(xii) Totally
Enclosed Separately Air Cooled through pipe (TESAC),
(xiii) Totally
Enclosed Tube Ventilated (TETV),
(xiv) Weather
Proof (WP) &
(xv) Flame
Proof (FP) enclosure.
[Q:-22]
Where screen is protected drip proof type enclosure used?
Ans
:
In damp situation like water pumping
station screen protected type with drip proof enclosures are used to prevent
moisture and water from entering the motor.
[Q:-23]
Where is Screen and Filter protected type enclosure used ?
Ans
:
In case of large rating motors screen and
fitter protected type enclosure consisting of filters at the air inlet and the
fine wire-mesh at the exit is used to provide higher degree of protection
against dust particles.
[Q:-24]
Where is Screen and filter with Blower protected type enclosure used ?
Ans
:
In case of very large rating slow speed
motors this type of enclosure consisting of blower mounted on the ventilation
box in addition to the inlet filter and outlet screen is used not only to
protect dust particles but also to increase the air flow.
[Q:-25]
Where are totally enclosed motors used?
Ans
:
In dusty places such as textile plant,
flour mill, coal mines, coal handling plant, saw mill etc. totally enclosed
motors are used to prevent the foreign matter entering the machine.
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