Part - A
UNIT - 1
1. Write down the fundamental torque equation
of motor load system?
TM=TL+Jdw/dt
where TM - motor torque in N-m ; TL - load torque in N-m ;
J - moment of inertia of drive system in Kg-m2 w -
angular velocity in mechanical radians/sec
2.
Write the
differential equation which describes the motor load system?
3. What are the disadvantages of DC motor as
compared to AC motor?
Due to presence
of commutator and brushes dc motor have a number of
Disadvantages
such as
i) High cost,
weight and volume
ii) Need for frequent maintenance
iii) Unsuitable
for explosive and contaminated environments
iv) Restrictions on maximum voltage speed and
power ratings.
4. What is dynamic torque?
TM=TL+Jdw/dt
Toque component
J (dωm/dt) is called the dynamic torque because it is present only
during the transient operations.
5. Draw
the load torque characteristics of constant power loads.
6. What is active and passive load torques?
Give examples.
Active
torques: Load torque which have the potential to drive the motor under
equilibrium condition are called active load torques. Such load torques usually
retains their sign when the direction of the drive rotation is changed. Torques
due to either gravitational force or deformation in elastic bodies, come under
this category.
Ex: Hoists, lifts or elevators and
railway locomotives
Passive
torques: Load torques which always oppose the motion and change their sign
on the reversal of motion are called passive load torques. Such torques is due to friction, windage,
cutting etc.
Ex: lathes, fan, pumps etc.
7. What are the three modes of operation of an
electric drive?
a)
Steady state b) Acceleration including starting c)
Deceleration including stopping
8.
List the applications of electrical drives.
i) Electric propulsion ii)
Pumps, fans and compressors iii)
Automotive applications iv) Cement, paper, pulp and textile mills v) Conveyors,
elevators, escalators and lifts
9. State the conditions for steady state
stability of motor load system?
TM<TL
i.e., dw/dt =0 i.e., the motor will continue to run at the same speed, if
it were running or will continue to be at rest, if it were not running.
10. What is
regenerative braking
In regenerative braking, generated energy is supplied to
the source. For this to happen the following condition should be satisfied
E>V and negative Ia
11. Sketch the speed torque characteristics
curve of a fan type load.
The torque
produced by the fan is directly proportional to the square of the speed
throughout the range of usable fan speeds. Centrifugal pumps, propellers in
ships or aeroplane also have the same type of speed torque characteristic.
12. When does an equilibrium point be stable in speed torque
plane of motor load system?
13. Define electrical drive.
Systems employed
for motion control are called drives and may employ any one of the prime mover such as diesel, gas, hydraulic
and electric motor for supplying mechanical energy for motion control. Drive
employing electric motor is known as electric drives.
14. Draw the block diagram of electrical drive.
15. List out the advantages of electric drive
i)
Flexible control characteristics
ii)
Wide range of torque, speed and power
iii)
High efficiency
iv)
Can operate in all four quadrant of speed torque plane.
v)
Adaptability to almost any type of environmental or operating conditions
16. What are the functions of a power
modulator?
i) Modulates flow of power from the source to the motor
ii) During transient operations, such as starting, braking
and speed reversal, it restricts source and motor currents within permissible
values
iii) Converts electrical energy of the source in the form
suitable to the motor.
iv) Selects the mode of operation of the motor, i.e.
motoring or braking.
17. List out the classification of electrical
drive
Load
and environment
|
Loads
|
Continuous or intermittent, positive,
negative, constant or variable in magnitude.
|
Environment
|
Temperature, humidity, vibration and shocks
|
|
Modes
of operation
|
Continuous
duty
Short
time duty
Intermittent
duty
|
|
Mode
of control
|
Manual
Semiautomatic
Automatic
|
|
Number
of machines and interrelations
|
Individual drive
Group drive
Multi motor drive
|
|
Dynamics
and transients
|
Uncontrolled transient period
Controlled
transient period
|
|
Methods
of speed control
|
Reversible and non reversible smooth speed control
Reversible and non reversible step
speed control
|
18. Explain group drive with its merits and
demerits.
Group Drive
consists of a single motor which actuates several machines by means of one or more line shafts it is most
economical compare to other drive.
Disadvantages:
A.
The efficiency of the drive is low, because of the
losses occurring in several transmitting mechanisms.
B.
Any fault that occurs in the driving motor renders all
the driven equipment idle.
C.
Power loss take place in energy transmitting mechanism
D.
The level of noise produced at the work site is quite
high
In brief describe
about the Individual drive.
In the individual
drive, an electric motor is used for transmitting motion to various parts or
mechanisms belonging to single equipment.
Ex: Lathe machine
In the case of individual drive too, the
energy is transmitted to the different parts of the same mechanism by means of
gears, pulley etc. Hence there occurs some power loss. This disadvantage is
removed in the multi motor drive.
19. In brief describe about the Multi motor
drive.
In multi motor
drives, separate motors are provided for actuating different parts of the
driven mechanism.
Example: Travelling cranes, Paper mills
& rolling mills
20. Compare Individual Drive and Group Drive
Individual Drive
|
Group Drive
|
|
Initial cost
|
Low
|
High
|
Machine attached
|
One
|
More than one
|
More likely used
|
For job production
|
For mass production
|
Efficiency
|
High
|
Less
|
21. Write a brief note on the motors employed
in variable speed drives.
i) AC motors (Induction and Synchronous) are employed in
variable speed drive due to the development of semiconductor converter.
ii) Due to presence of commutator and brushes dc motors are
not employed in variable speed drive.
iii) Special machines – Switched reluctance motor
22. List out the load torques that varies with
time.
Continuous constant loads - Centrifugal pumps
or fan operating for a long time
Under the same conditions
Continuous variable loads - Metal cutting
lathes, conveyors etc.
Pulsating
loads - Reciprocating
pumps and compressors & textile looms
Impact loads - Rolling mills, presses
and forging hammers
Short time
intermittent loads - Excavators and cranes
Short time
load - Servo Motors and
motor generator sets for charging batteries.
23. What are the main factors which decide the
choice of electrical drive for a given application?
i) steady state
operation requirements ii) Transient operation requirements iii) Requirements
related to the source iv) Capital and running cost v) Space and weight
restrictions
24. List out the disadvantages of mechanical
braking
Mechanical
braking has number of disadvantages: Frequent maintenance and replacement of
brake shoes, lower life, braking power is always wasted as heat.
25. State and explain the functions of various
converters.
i) AC to DC converters
ii) AC regulators (to get variable ac voltage of the same frequency from
a source of fixed ac voltage)
iii) Choppers (to get variable dc voltage from a source of fixed dc
voltage)
iv) Inverters
v) Cycloconverter (fixed voltage and frequency ac to variable voltage
and variable frequency ac)
26. Explain i) Friction torque ii) Windage
torque
Friction torque Tf: Friction will be
present at the motor shaft and also in various parts of the load. Tf is
equivalent value of various friction torques referred to the motor shaft.
Windage torque: When a motor runs, wind
generates a torque opposing the motion. This is known as windage torque.
27. What is the current status of dc and ac
drives?
In the past induction and synchronous motor drives were
mainly used in fixed speed applications. Variable speed applications were
dominated by dc drives. Emergence of thyristors leads to the development of
variable speed induction motor drive.
Although the induction motor was cheaper than dc motor, the
converter and control circuit of an induction motor was very expensive.
28. Explain what do you understand by the
steady state stability? What is the main assumption?
Concept of steady state stability has been developed to
readily evaluate the stability of an equilibrium point from steady state speed
torque curves of the motor and load, thus avoiding solution of differential
equations valid for transient operation of the drive. During transient
operation, motor can be assumed to be in electrical equilibrium implying that
steady state sped torque curves are also applicable to the transient operation.
29. Explain how the following speed transitions
are carried out
i) Increase in the
speed in same direction.
ii) Decrease in the
speed.
iii) Speed reversal.
i) Time taken for a
given change in speed depends on inertia of motor load system and the amount by
which motor torque exceeds the load torque. Increase in motor torque is
accompanied by an increase in motor current.
ii) Deceleration occurs when load torque exceeds the motor
torque. Reducing the motor torque to zero alone does not provide enough
deceleration, mechanical or electric braking may be employed.
30. What do you understand by constant torque?
A variable speed drive is called constant torque drive if
the drive‘s maximum torque capability does not change with a change in speed
setting. The corresponding mode (or region) of operation is called constant
torque mode.
31. State and explain the disadvantages of
using a motor of wrong rating.
Use of a motor having insufficient rating, either fails to
drive the load or lowers the productivity and reliability through frequent
damages and shut downs due to overloading of the motor.
32. List out the drawbacks of electric drive
They require a continuous power supply, particularly in
vehicle propulsion. The power supply equipment needs to be carried on board,
requires lot space and it’s bulky. Problems of saturation of iron and cooling
make the electric motor have a lower power /weight ratio.
33. State the conditions for acceleration and
decelerating steady state stability of motor load system?
TM>TL
i.e., dw/dt>0 i.e., the drive will be accelerating, in particular,
picking up speed to reach rated speed.
TM<TL i.e., dw/dt<0 i.e., the drive will
be decelerating and particularly coming to rest.
Part - B
UNIT – 1
1. State the essential parts of
electrical drive. What are the functions of power modulator? 8
2. What are the main factors which
decide the choice of electrical drive for a given application? 8
3. Derive the fundamental torque equations for a
motor load system. 6
4. Explain in detail about steady state
stability equilibrium point in
electrical drive 10
5. List the common factors to be
considered for selecting a motor 8
A number of factors go into the
selection of a machine for a particular application:
a.
Cost
b.
Thermal capacity
c.
Efficiency
d.
Torque – speed profile
e.
Acceleration
f.
Power density, volume of the motor
g.
Ripple, cogging torques
h.
Robustness
i.
Peak torque capability
6. Discuss the different modes of
operation of an electric drive. 8
7. A motor is coupled to a load having
the following characteristics
Motor: T m = 15-0.5 Wm
Load T l = 0.5 W m2.
Find out the stable operating point
for this condition.
8. Explain about electrical and
mechanical characteristics of commonly used electric motors? 16
9.
Derive an equation
to find out equivalent load torque in motor load system with translational and
rotational motion? 8
10.
A drive has
following equations for motor and load torque T=(1+2Wm) and Tl=3(Wm)½. Obtain
the equilibrium points and determine their steady state stability. 6
11.
Draw and explain the
speed torque characteristics of various types of loads 8
12.
Explain in detail
about the multi quadrant
dynamics of electric drives 8
13.
A motor drives two
loads. One has rotational motion. It is coupled to the motor through a
reduction gear with a = 0.1 and efficiency of 90%. The load has a moment of
inertia of 10 kg-m2 and a torque of 10 N-m. Other load has translational motion
and consists of 1000kg weight to be lifted up at a uniform speed of 1.5 m/s.
coupling between this load and the motor has an efficiency of 85%. Motor has
inertia of 0.2 kg-m2 and runs at a constant speed of 1420 rpm. Determine
equivalent inertia referred to the motor shaft and power developed by the
motor.
14.
Explain how the following speed transients are carried out: i) increase in speed in same direction ii) decrease speed in same
direction iii) speed reversal
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