1. What is meant by electrical
Systems employed for motion control are called
drives and they employ any of the prime movers such as diesel or petrol
engines, gas or steam turbines, hydraulic motors and electric motors for
supplying mathematical energy for motion control. Drives employing electric
motion are called electric drives.
2. What are the requirements of
an electric drive?
operation should be assured.
should have good transient response
3. Specify the functions of power
modulator performs one or more of the following four functions.
flow of power form the source to the motor in such a manner that motor is
imparted speed-torque characteristics required by the load.
transient operations, such as starting, braking and speed reversal, it
restricts source and motor currents within permissible values; excessive
current drawn from source may overload it or may cause a voltage dip.
4. Mention the different types of
2) Individual drive
different types of electrical drives.
1) dc drives
2) ac drives
6. What are the advantages of
They have flexible control characteristics. the
steady state and dynamic characteristics of electrical drives can be shaped to
satisfy load requirements.
can be provided with automatic fault detection systems, programmable logic
controllers and computers can be employed to automatically ctrl the drive
operations in a desired sequence.
2) They are
available in which range of torque, speed and power.
3) It can
operate in all the four quadrants of speed-torque plane. Electric braking gives
smooth deceleration and increases life of the equipment compared to other forms
gear required for speed control, starting and braking is usually simple and
easy to operate.
the functions performed by electric drives?
functions performed by electric drives include the following.
fans, ventilators, compressors and pumps etc.
goods by hoists and cranes
motion to conveyors in factories, mines and warehouses and
excavators and escalators, electric locomotives, trains, cars, trolley buses,
lifts and drums winders etc.
the disadvantages of electric drives?
disadvantages of electric drives are
drives system is tied only up to the electrified area.
condition arising under the short circuits, leakage from conductors and
breakdown of overhead conductor may lead to fatal accidents.
in supply for a few minutes may paralyses the whole system.
9. What are the advantages of
group drive over individual drive?
advantages of group drive over individual drive are
Initial cost: Initial cost of group drive is less as compared to that of the
of operation: Group drive system is useful because all the operations are
requirement: Less space is required in group drive as compared to individual
maintenance cost: It requires little maintenance as compared to individual
group drive is not used extensively.
the initial cost of group drive is less but yet this system is not used
extensively because of following disadvantages.
factor: Group drive has low power factor
Group drive system when used and if all the machines are not working together
the main motor shall work at very much reduced load.
In group drive if the main motor fails whole industry will come to stand still.
Such arrangement is not possible in group drive i.e., this arrangement is not
suitable for the place where flexibility is the prime factor.
Group drive does not provide constant speed.
f. Types of
machines: Group drive is not suitable fro driving heavy machines such as
cranes, lifts and hoists etc.
11. Write short notes on
individual electric drives.
In individual drive, each individual machine is
driven by a separate motor. This motor also imparts motion to various other
parts of the machine. Examples of such machines are single spindle drilling
machines (Universal motor is used) and lathes. In a lathe, the motor rotates
the spindle, moves the feed and also with the help of gears, transmits motion
to lubricating and cooling pumps. A three phase squirrel cage induction motor
is used as the drive. In many such applications the electric motor forms an
integral part of the machine.
the different factors for the selection of electric drives?
state operation requirements.
related to the source.
and running cost, maintenance needs life.
5) Space and
the parts of electrical drives.
motors and load.
14. Mention the applications of
· Paper mills
· Electric traction Cement mills
· Steel mills
the types of enclosures
16. Mention the different types
of classes of duty
duty, Discontinuous duty, Short time duty, intermittent duty.
17. What is meant by regenerative
Regenerative braking occurs when the motor speed
exceeds the synchronous speed. In this case the IM runs as the induction m\c is
converting the mechanical power into electrical power which is delivered back
to the electrical system. This method of braking is known as regenerative
18. What is meant by dynamic
Dynamic braking of electric motors occurs when the
energy stored in the rotating mass is dissipated in an electrical resistance.
This requires a motor to operate as a gen. to convert the stored energy into
19. What is meant by plugging?
It is one method of braking of IM. When phase
sequence of supply of the motor running at the speed is reversed by
interchanging connections of any two phases of stator with respect to supply
terminals, operation shifts from motoring to plugging region.
20. What is critical speed?
It is the
speed that separates continuous conduction from discontinuous conduction mode.
braking is suitable for reversing the motor?
is suitable for reversing the motor.
equivalent current method
The motor selected should have a current rating
more than or equal to the current. It is also necessary to check the overload
of the motor. This method of determining the power rating of the motor is known
as equivalent current method.
23. Define cooling time constant
It is defined as the ratio between C and A. Cooling
time constant is denoted as Tau. Tau = C/A
C=amount of heat required to raise the temp of the motor body by 1 degree
Celsius A=amount of heat dissipated by the motor per unit time per degree
24. What are the methods of
operation of electric drives?
Acceleration including starting Deceleration
25. Define four quadrant
The motor operates in two mode: motoring and
braking. In motoring, it converts electrical energy into mechanical energy
which supports its motion. In braking, it works as a generator, converting
mathematical energy into electrical energy and thus opposes the motion. Motor
can provide motoring and braking operations for both forward and reverse directions.
26. What is meant by mechanical
The curve is drawn between speed and torque. This
characteristic is called mechanical characteristics.
27. Mention the types of braking
braking Dynamic braking
28. What are the advantage and
disadvantages of D.C. drives?
advantages of D.C. drives are,
starting, braking and reversing.
The disadvantage of D.C. drives is the presence of
a mechanical commutator which limits the maximum power rating and the speed.
29. Give some applications of D.C. drives.
applications of D.C. drives are,
30. Why the variable speed
applications are dominated by D.C. drives?
The variable speed applications are dominated by
D.C. drives because of lower cost, reliability and simple control.