Introduction to Drives

1. ELECTRIC DRIVES AND CONTROL
Introduction to Drives
Ms. J. Glory Priyadharshini,
Assistant Professor/EEE,
Sri Ramakrishna Institute of Technology,
Coimbatore.

2. Module 1- Introduction
Many Industrial equipments driving a load need a prime mover.
Prime Mover: InitialSource of motive power, Converts energy from energy source to mechanical
energy. ex) Engines, Motors etc.,
Traditional method of driving load can be employed using engines/Motor as prime mover.
Most Commonly used prime mover is ElectricMotor.
DISADVANTAGES OF TRADITIONAL METHOD:
 Starting and Braking of Motoris difficult
 Speed Control is difficult
 Control Characteristicsarefixed
 Startersareneeded
WHYELECTRICMOTORS ARE COMMONLY USED AS PRIME MOVER?
 Performance Characteristics, Operating conditions canbe adjusted as per the requirement
of load using suitable control equipment.
 Additional Protectioncircuits, precise speed control can also be achieved using suitable
control equipment with electricmotor.
 As a result, better efficiency of motor over a wide rangeof speed can be improved.
ADVANTAGES OF ELECTRIC DRIVES
 The control (steady stateand dynamic) characteristicscanbe shaped to satisfy load
requirements.
 Electricbraking can be employed in easy manner.
 Optimal control strategiescan be implemented to achieve high dynamic performance and
high efficiency.
 Drives can also be provided with automaticfault detection systems.
 They areavailable in wide rangeof torque, speed and power.
 Compared to other prime movers, electricaldrives have longer life, lower noise, lower
maintenance requirements and cleaner operation.
 They areadaptable to almost any operating conditions.
 Do not pollute theenvironment.
 Most of themotors used in electricaldrives are self-starting and can be startedinstantly and
loaded immediately.
CHOICE OF ELECTRICDRIVES:
 Steady stateoperationrequirements:Natureofspeed torque characteristics, speed
regulation, efficiency, duty cycle, quadrants of operation, speed fluctuations
 Transient operationrequirements: valuesof accelerationand deceleration, starting,
braking, reversing performance
 Requirements related to thesource: typesof sources, magnitudeof voltage, voltage
fluctuations, power factor, harmonics and their effect on other loads, ability to accept
regeneratedpower.

3.  Capital and running cost, maintenanceneeds,life
 Spaceand weight restrictions,
 Environment and location, Reliability
APPLICATION OF ELECTRICDRIVE
It is used in a largenumber of industrial and domestic applications like transportationsystems,
rolling mills, paper machines, textilemills, machine tools, fans, pumps, robots and washing, etc.
PARTS OF ELECTRICDRIVE
A system which employs ElectricMotor as its prime mover is known as ElectricDrive.
1. Source:
Source provides theenergy required for the operation the system.
Source may be DC supply or Single phase AC Supply or threephase AC supply based on the
availability.
Most Drives arepowered from acsource either directly or through converter link.
For Low power drives 1-Phase Source is used and for HighPower Drives 3-Phase source is
used.
Ex) in underground traction500 to 750V DcSupply is used.
Some drives are powered from battery, depending on thesize batteryvoltagemay change
as 6V,12V,24V,48Vand110Vdc.
2. Power Modulator:
It performs one or more of the following functions:
i. Modulates the flow of power from source to motor such that themotor imparts
required speed torque characteristicsby the load.
ii. During transient operations, such as starting, braking, speed reversal power
modulator restricts thesource and motor currents within the permissible values.
iii. Converts electricalenergyof thesource in the form of suitable to motor.
iv. Selects the mode of operation of motor, i.e., motoring or braking.
When power modulator is employed mainly to perform function (iii), it is termedas power
converter. Power modulators can be classified into threetypes,
1. Converters, 2. Variableimpedance circuits, 3. Switching circuits.
a. Converters
They provide adjustable voltage/current/frequency to control speed, torque output
power of themotor. The various type of converters are,
o AC to DC rectifiers
o DC to DC choppers

4. o AC voltage controllers (voltagelevel is controlled)
o Cyclo converter (Frequency is controlled)
o DC to AC inverters
Rectifying circuits:
AC Voltage controller:

5. Inverter:
Chopper:
Cycloconverter:
b. Switching circuits:
Switching circuits areneeded to achieveany one of the following.
 Changing motor connection to change its quadrant of operation.
 Changing motor circuits parametersin discrete steps for automaticstarting and
braking control.
 For operating motors and drives according to a predetermined sequence
 To provide inter locking their by preventing maloperation
 To Disconnect motor under up normal condition Eg: electromagnetic contactors,
PLC in sequencing and inter locking operation, solid staterelays etc.
c. Variable impedance
 Variableresisters arecommonly used for AC and DCdrives and also needed for
dynamic braking of drives.
 Semiconductors switch in parallelwith a fixed resistance is used where stepless
variation is needed.
 Inductors areemployed to limit starting current of ac motors.
 In high power applications liquid rheostats, known as slip regulatorsareemployed
to get stepless variation of resistance.
3. Motor
 Commonly used motors aredc Motors-shunt, series, compound and permanent
magnet
 Induction Motors: Squirrel cage, wound rotor and linear
 Synchronous motors: wound field and permanent magnet

6.  Brushless DC motors, Stepper motors and Switched reluctancemotors canbe used.
Induction motors arepreferred overdc motorsdueto thefollowing reasons:
o Presence of commutatorand brushes
o Higher cost, weight, volume and inertia
o Needs frequent maintenance
o Unsuitable for explosive and contaminatedenvironments
o Restrictions on maximum voltage, speed and power ratings.
4. Controland Sensing Unit
Controls for the power modulator are provided in the control unit. Nature of the control unit
for a particulardrivedependson thepower modulatorthat is used. It generatescontrolsignals
required by power modulator and sensing unit senses the status of motor and load. Control
and sensing unit in electric drive provides the feedback closed loop and plays a vital role in
improving the overall performance of drive system.
CLASSIFICATION OF ELECTRICDRIVE
ElectricDrives arenormally classified into three groups based on their development or arrangement
as:
• Individual drive
• Group drive
• Multimotor drive
1. IndividualDrives
 If a single motor is used to drive or actuatea given mechanism and it does all the jobs
connected with the load.
 As one motor is used, transmitting devices are required.
 Ex: Lathe
 All the operations connected withoperating a lathe is performed by a single machine.
 One motor used in latherotatesthe spindle, moves feed with thehelp of gearsand imparts
motion to thelubricating and cooling pumps.
Advantages:
• Machines can be located at convenient places.
• Continuity in theproduction of the processing industry is ensured to a high level of
reliability.
• If thereis a fault in one motor, the effect on theproduction or output of the industry will not
be appreciable.
Disadvantages:
• Initial cost is very high.
2. Group drives
• If several groups of mechanism or machines are organizedon one shaft and driven or
actuatedby one motor, the system is called group drive or shaft drive.
• The various mechanisms connected may have different speeds. Hencethe shaft is equipped
with multistepped pulleys and belts for connection to individual loads.
Advantages:
• Group drive is most economical because, the rating of the motor used may be comparatively
less than theaggregateofthe individual motors required todrive eachequipment, because
all may not be working simultaneously.
• Group drive reduces the initial cost of installing a particularindustry.
• Cost is less because of investment in one motor which is lesser in HP rating.

7. Disadvantages:
• The use of this kind of drive is restricteddue tothe following reasons:
• It is not possible to install any machine as per our wish. so, flexibility of lay out is lost.
• The possibility of installation of additional machines in an existing industry is limited.
• In case of any fault to the main driving motor, all theother motors will be stopped
immediately. So, all systems will remainidle and is not advisable for any industry.
• Level of noise produced at the site is high.
• Becauseof the restrictions in placing other motors, this kind of drive will result in untidy
appearance, andit is also less safe to operate.
• Since all the motors has to be connected through belts and pulleys, largeamount of energy
is wastedin transmitting mechanisms. Therefore, power loss is high.
3. MultimotorDrives
 In this type of drive, eachoperation of the mechanism is taken careof by a separatemotor
or separatemotors are provided for actuating different partsof the driven mechanism.
 The system contains several individual drives, each of which is used to operateits own
mechanism.
 This type of drive finds its application in complicatedmachine tools, travelling cranes, drives
used in paper mills, rolling mills etc.,
 In cranes, separatemotors are used for hoisting, long travelmotion and cross travelmotion.