Drive question ans

1. ShortAnswerType QuestionsWith Answers
0. 1. What is an electric drive ?
Ans. An electricdrive is defined as a form of machine equipment
designed to convert electrical energy into mechanicalenergy
and provide electrical control of this process.
The basic elements of an electric drive are the electric
motor, the transmission, and the electrical control system.
Electric drives often include conversion equipment
(frequency changers, rectifier units, motor-generator sets
etc.)to attain smoother and more versatile control and make
the motor operate on specific speed-torque characteristic.
Q. 2. Why electricdrive is preferred over mechanical drive ?
[Gorakhpur Univ. 1982]
Ans. Electric drive is preferred over mechanical drive because
of its simplicity, cleanliness, easy and s1nooth control,
flexibility in layout,compactness, reliability, long life, easy
starting, low cost (initial as well as maintenance cost),
facility for remote control etc. Modern electric drive with

2. than imparting motion to the working machine So as to
asystem of automatic control fulfills several
functions other
carry out the production process involved in
the most
efficient manner. Using automatically controlled
electric
drives, machinery and production units may be
operated
with high productivity, ensuring at the same
time high
quality.
Q. 3. Enumerate the factors governing selection Of
electric
Ans. The various factors that govern the selection of
electric
motors.
motor for a particular service are :
1. Nature of electric supply available-dc or ac
2 Tvpes of
drivesindividual, group or multimotor
3. Nature of load-active or passive.
4 Electrical characteristics (operating, starting, speed
control and braking).
S. Mechanical considerations (Types of enclosures. tvnes
of bearings, types of transmission for drive, noise level
etc.)
6. Service capacity rating
7. Appearance and
8. Cost
0. 4. Write classification of electric drives. (PT.U. December 261
TRANSMISSION
Tracion
SYSTEM
MOTOR-THE PRIME MOVER
Q. 5. What is group drive ?
Fig. 1
Ans. The various types of electric drives used in industry may
be divided intothree types :The group drive, the individual
drive and multimotor drive.
DRIVEN
MACHINE
Ans. Group drive, also sometimes called the line shaft drive,
0. 6. What is individual drive ?
means a drive in which a single electric motor drives a
line shaft by means of which an entire group of working
machines may be operated.
Ans. In an individual drive, a single motor is used to drive one
individual machine. Example of such drives are single
spindle drilling machines, various types of electrical hand
0. 7. What is multimotor drive ?
tools and simple types of metal working machine tools au
mechanisms.
nowadays ?
Ans. Multimotor drive consists of several individual drives ca
of which serves to operate one of many working members
or mechanisms in some production unit.
Q. 8. Why individual drive is preferred over group drive
(Gorakhpur Univ. 1981)
Ans. Though the individual drive costs more than group drive.
but each operator has complete control of his machine,
which enables him to vary its speed if necessary annod stop
while not is use, thus eliminating no-load losses. The
machines can be placed in any desired position and can be
built as an integral part of the machine which results in
moved very easily. The motor and its Control unit can
be

3. good appearance.
cleanliness and safety. For driving heavy
A
chines such as for ifts, cranes, shapers, lathes etc. and
for the purposes where constancy of speed and flexibility
control is required, such as in paper mills and textile
industry. individual drive is essential.
Eor new factories individual drive is preferred, as it causes
eame saving in the cost of superstructure hecause of heing
much lighter and less expensive.
0. 9. How does active torque differ from passive torque ?
Active torques are due to either gravitational force or
deformation in elastic bodies whereas passive torques are
due to friction or due to shear and deformation in inelastic
bodies. Active torque continues toact in the same direction
even after the direction of drive has been reversed while
passive torque always opposes the motion, retarding the
rotation of the driven machine.
0 10. What is meant by two-quadrant operation of a dc motor ?
Ans. The operation of an electric motor in forward motoring
mode and in regenerating (or braking mode) is called the
two-quadrant operation of the motor.
0.11. What factors govern selection of motor for adrive ?
(PT.U. December z007]
Ans. For selection of reliable and efficient mnotor it is essential
that the conditions of service are well known. It is not
sufficient to simply specify the output power in kW and
the speed but it is also necessary to know the following
additional particulars :
() Torqueat the shaft during running, starting and at
different loads.
(ii) Accelerating torque and braking torque.
(iüi) Switching frequency.
(iv) Efficiency of motor at different loads.
(v) Other working requirements.
In studying the behaviour of a motor selected for a
particular driven unit, one of the first problems involved is
to determine whether the speed-torque characteristic of the
motor suits the requirements imposed by the speed-torque
characteristic of the driven unit. Drive behaviour during
the transient period of a start-up, braking, or speed change
Over also depends upon how the speed-torque characteristics
of the motor and the driven unit vary with speed.
12. Whyspeed-torque (not torque-speed) characteristic curves
are commonly used in utilization studies ?
Ats. In application engineering, it is usually the load torque
Which is known and the speed at which the motor can drive
this load is unknown and is to be determined. Thus the torque
Is independent variable and speed is dependent variable.
Consequently speed-torque characteristic curves with torque
4 2OsCissa are commonly used in utilization studies.
Q. 13. What will be the effect on direction of rotation of a de
motor if the supply terminals are reversed ?
An. No effect. Motor will rotate inthe same direction, as before
he reversal of the line terminals.
9 4. Why should a dc series motor not be run without load ?
A. On no fond de series motor would attain tremendously high
speed and the motor may get damaged due to heavy
Centritugal forçes set up in rotating parts.
15. Why is series motor preferred for traction purposes ?
Am. Because dc series m
motor develops large starting torque and
ows down with the increase in load and so gets
automatically relieved from heavy excessive load.

4. 0. 16. Why the de motors are mostly gear motors ?
Ans. Because the low-speed motors are much more expensive
than high-speed motors of the same kW output rating.
0. 17. Why are induction motors called asynchronous motors ?
Ans. Because the rotor does not turn in synchronous with the
rotating field developed by the stator currents.
0. 18. Is the maximum torque of a3-phase induction motor
dependent on the rotor resistance ?
Ans. No, it is independent of rotor resistance.
107
Q. 19. Why the power factor of an induction motor is low at
starting ?
Ans. The rotor frequency and rotor reactance are high under
starting conditions and, therefore, rotor currents lag the
rotor emf by alarge angle. This results in low power factor
at starting.
Q. 20. The starting torque of a squirel cage induction motor
cannot be altered, when the applied voltage is constant,
why ? [M.K. Univ. Nov. 1996]
Ans. The starting torque of a squirrel cage induction motor
cannot be increased as there is no provision for inserting
resistance in the rotor circuit.
Q. 21. How will you improve the starting torque of a3-phase induc
tion motor ? [Madras Univ. Elec. Machines II October 1995]
Ans. The starting torque of a 3-phase induction motor can be
improved by inserting external resistance in each phase of
the rotor circuit, which is possible only in case of wound
rotor motors.
0. 22. What are the main advantages of a cage motor?
[M.S. Univ. April 1996]
Ans. The main advantages of a cage motor are simple and rugged
construction, low initial as well as maintenance cost, better
efficiency and power factor, better cooling, more pull-out
torque, greater maximum power output, nearly constant
speed and simple starting arrangement.
0.23. What are the main disadvantages of a cage motor
(M.S. Univ. April 1996]
Ans. The main drawbacks of a cage motor are low starting torque
with large starting current and no possibility of a speed
control.
0. 24. What are the advantages and disadvantages of wound rotor
over squirrel cage rotor in case of induction motor ?
[PT.U. December 2002]
Ans. In wound rotor external resistance can be inserted and.
therefore,
(i) high starting torque can be obtained by inserting an
external resistance of proper value in each phase of
the rotor circuit and
(ii) adjustable speed can be obtained simply by adding
external resistors in the rotor circuit.
The drawback of wound rotor over squirrel cage rotor are
initialand maintenance costs are more because wound rotor
needs slip rings, brushes, short-circuiting devices etc.
provided?
Q. 25.Why in some induction motors double cage rotors are
[A.M.I.E. Sec. B. Summer 1994]
Ans. In some induction motors double cage rotors are provided
so as to get high starting torque with a low starting current
as well as high operating efficiency. Such motors operate
with the advantages of high-resistance rotor circuit during
starting and low-resistance rotor circuit under running
conditions.

5. Q. 26. State the advantages of squirrel cage induction motors over
dc motors. (Rajasthan Univ. 2008]
Ans. The important advantages of squirrelcage induction motors
over de motors are: (i)Ruggedness (ii) Lower maintenance
(iii) Better reliability. (iv) Lower cost, weight, volume and
interia, (v) Higher efficiency, (vi) Ability to operate
satisfactorily with sharp voltage fluctuations and in dirty
Q. 27. Why the VIf is kept constant while controlling the speed
of a 3-phase induction motor ?
Ans. In order to maintain flux level constant.
Q. 28. How the power factor of a synchronous motor is changed
keeping the shaft load undisturbed ?
Ans. By varying the field excitation.
Q. 29. For which type of loads, induction motor with voltage
controller is suitable and why ?
[A.M.I.E. Sec. B. Summer 1996]
Ans. Induction motor with voltage controller is suitable only for
loads where the torque required drops off considerably with
the drop in speed such as for driving fans because a large
change in voltage is required for a relatively small change
in speed and developed torque reduces greatly with the
reduction in supply voltage.
Q. 30. How can the speed of a linear induction motor be
controlled ? [Bharathiyar Univ. Nov. 1995]
Ans. By arying both frequency and voltage simultaneously.
Q. 31. Is the torque developed by a single phase induction motor
at synchronous speed zero ?
Ans. No, it is negative.
Q. 32. Why single phase induction motors are usually set on rubber
spring mounts?
Ans. In a single phase induction motor, pulsating torque is
developed as a direct consequence of the pulsating power
in a single phase circuit.
Q. 33. Does a shaded-pole motor have large starting torque ?
Ans. No, it has very little starting torque.
Q. 34. What is a shaded-pole motor ?
Ans. Shaded-pole motor is a salient-pole, split-phase type,
squirrel cage rotor single-phase induction motor provided
with one or more auxiliary short-circuited stator windings
displaced in magnetic position from the main winding.
Q. 35. State the applications of shaded-pole single phase induction
[Madras Univ. April 1996]
Ans. Shaded-pole motor develops very little starting torque and,
therefore, it is used only for small fans, electric clocks,
hair dryers, time phonographs, advertising displays, toys,
instruments and such other applications.
motors.
Q. 36. Why is the normal full-load slip of a single phase induction
motor higher than that of a three-phase motor ?
[A.M.I.E. Sec. B. Summer 1993]
Ans. The normal full-load slip of a single phase induction motor
is higher than that of a 3- motor, owing to development
of backward rotating field. The power is to be delivered to
the backward field from the power converted into
inechanical power by the forward field.
Q. 37. What are the applications of electric drives ?
[P.T.U, December 2009]
Ans. Squirrel cage induction motor is suitable for constant speed
industrial drives of small power where speed control is not
required and where starting torque requirements are of
environment.

6. medium or low value, such as for printing
machinery, flour
mills and other shaft drives of small power.
Wound rotor (or slip-ring) induction motorS are
used for
loads requiring severe starting conditions or for loads requir.
ing speed control such as for driving lineshaafts,
ifts, pumps,
compressors, small electricexcavators, printing presses, turn
generators, winding machines. cranes, hoists,
elevatorS,
tables, strokers, large ventilating fans, crushers ete
Single phase induction motors find wide use in fans,
refrigerators, air conditioners, vacuum
cleaners,
washing
machines, other kitchen equipment, record players, small
farming appliances etc.
0. 38. State whether repulsion motor has series speed-torque
characteristics, or shunt speed-torque characteristics
Ans. Series speed-torque characteristics.
0. 39. What is a stepper motor ?
Ans. A motor which turns in discrete movement is called .
stepper motor.
Q. 40. For stopping the motor, the line switch should alwavs
opened rather than throwing back the starting arm. Why 2
Ans. If the starter arm is thrown back the field circuit is broken
at the last contact button and owing to inductive nature of
field, this will cause a hot arc and burn the contact.
Q. 41. Why do you require starters for starting a 3-phase induction
motor ?
Ans. Starter is required to limit the initial rush of current toa
predetermined value at the starting instant.
0. 42. Which is the cheapest method of starting a 3-phase
induction motor?
Ans. Direct-on-line (D-0-L) starting.
Q. 43. What type of protection is provided in the starters meant
for 3-phase induction motors ?
Ans. Overload and undervoltage protection.
be
Q. 44. Which method we must adopt to control the speed of a dc
shunt mnotor above the base speed ?
Ans. Field control method.
Q. 45. While controlling the speed of a de shunt motor what should
be done to achieve a constant torque drive ?
[A.M.I.E. Sec. B. Summer 1995)
Ans. Applied voltage should be maintained constant so as to
maintain field strength constant.
Q. 46. If the number of poles on a motor is increased to lower the
speed, how will the power factor be affected ?
Ans. Power factor will be reduced.
Q. 47. How is speed control affected in a 3-phase ac series co
ImutatOr motor ?
Ans. In a 3-phase ac series commutator motor, the speed conuo
is affected by moving all brushes simultaneously round u
commutator by means of a hand-wheel.
Q. 48. How is speed control affected in a 3-phase ac shunt Co
mutator motor?
AS. ln a s-phase ac shunt commutator motor, the spee0
adjusted by changing the tapping points on atransformer
inserted between the supply mains and the brushes. W
are in fixed position.
Q. 49. How speed variation is affected in a repulsion moto
Ans. Speed variation in a repulsion motor is affected elt
varying the impressed voltage or by changing the position
of brushes.

7. Electric Drives
0. 50. What is meant by plugging?
Ans. Plugging means stopping of an electric motor by
instantaneously reversing its torque till it stops.
0. 51. How plugging of a dc motor is done ?
Ans. Plugging is obtained ina de motor by reversing the supply
terminals to the armature of the motor.
. $2. What is the most economical method of electric braking ?
Ans. Regenerative braking.
.53. Electric braking is accomplished in an induction motor by
interchanging two stator terminals. Name the method.
[A.M.I.E. Sec. B. Winter 1995]
Ans. Plugging or counter-current braking.
0. 54. When it is required to control the induction motor speed
during deceleration what kind of braking is preferable to
employ ? [A.M.I.E. Sec. B. Winter 1996]
Ans. DC dynamic braking is highly suitable for controlling the
speed of an induction motor during deceleration by
controlling the dc excitation.
0. 55. How is choice of drive governed ?
Ans. Choice of drive is governed by the speed of driving and
driven machines, convenience, space available, including
clutching arrangement required and cost.
0. 56. What is meant by "rating of motor" ? (P.TU.December 2009]
Ans. A name plate fixed to the outside frame of an electrical
machine records the data pertaining to its rating. Amachine
rating specifies the voltage, current, speed, excitation, power
factor, efficiency, power output etc.
The rating of a machine should give all information
necessary so that, if the machine is operated within the
limits of all factors specified in its rating, the machine will
operate satisfactorily and safely and will give reasonable
length of service. Therefore, the rating of a machine must
give the necessary information to safeguard the application
of the machine from conditions of operation which (i) would
result in unsafe mechanical or electrical strains upon any
part of its structure or (ii) would result in excessive
deterioration of the mechanical or electrical characteristics
of the mnaterials of which the machine is constructed, To
give this information, the rating of an electric machine
Should include the output, voltage, speed, and any other
Information that may necessary for the proper operation of
the machine.
2 57, On what factors does the rate of rise of temperature depend ?
AnS. The rate of temperature rise depends upon () amount of
neat produced and (ii) the amount of heat dissipated per
°C rise of the surface of the machine.
Q58. Why the cooling time constant of a rotating machine 1S
Usually larger than its heating time constant ?
[A.M.I.E. Sec. B. Winter 1996]
Ans. The cooling time constant of a rotating machine is usually
larger than its heating time constant owing to poor
Ventilation çonditions when the machine cools.
Q..59, What is meant by load equalization?
Ans. The process of smoothing out of fluctuations in load is
known as load equalization.
Q. 60. How is load equalization achieved ?
Ans. The most Common method of load equalization is by use
of flywheel. During the light load period the flywheel
acçelerates and stores the excessive energy drawn from the
supply and during peak load period the flywheel decelerates
and supplies some of its stored energy to the load in