16-01-2013, 02:33 PM
ELECTRIC DRIVES
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INTRODUCTION
• An electric motor is a better prime move for driving mechanical load than hydraulic, steam or diesel engines as it is possible to control the performance of an electric motor is quite easy.
• For obtaining electric drives, both A.C and D.C motors are used.
• The utilization of electric energy is always advantageous as it is cheaper.
• It is easy to maintain the voltage at consumer premises within the prescribed limits and it is possible to increase or decrease the voltage without appreciable loss of power.
• In spite of the advantages of A.C system sometimes it becomes essential to use D.C energy as industrial drive
The general meaning of a Drive is the system, which is driven by some energy. The source of energy may be anything like wind, water, oil, steam, solar or electricity etc. When the source of energy is electricity, the drive is called Electric Drive. In any drive system, we take some output in terms of energy or work done.
Many industrial applications requiring rotating electric drives are normally capable of speed control and often require an equipment to attain a versatile and smooth speed control and make the motor to operate at a desired specific speed torque characteristic. These drives are characterized by the nature of speed torque characteristic such as constant torque drives and constant power drives. These are sometimes characterized by the type of motor used in the drive i.e. dc and ac drives making use of dc and ac motors respectively.
Types of electric drives:
The various types of electric drives used in industries may be divided into three types:
1. Individual motor drive 2. Group drive 3. Multi-motor drive.
Individual motor drive:
In individual drive, a single electric motor is used to drive one individual machine.
The machines can be placed in any desired position and can be moved very easily. The machines can be built as an integral part of the complete system, which results in a good appearance, cleanliness and safety. For the purposes where constancy of speed and flexibility of control is required, such as in paper mills and textile industry, individual drive is essential.
Individual drive is used where complete control of the machine is necessary, so that the machine can be operated at required speed and can be stopped while not in use. Hence, the no load losses in an individual drive are eliminated. Individual drive is preferred in case of new factories because of being much lighter and less expensive and hence saving cost of super structure is possible
Group drive:
A group drive is meant 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.
Or
If in a system, a single electric motor is used to drive two or more than two machines from line shafting by means of belts then this electric motor comes under the category of group drive.
It is also sometimes called the line shaft drive. The line shaft is fitted with multi-stepped pulleys and belts that connect these pulleys and the shafts of the driven machines serve to vary their speed. This drive is economical in consideration of the first cost of the motors and control gear. A single motor of large capacity costs less than the total cost of a number of small motors of the same total capacity The efficiency and power factor of a large group drive motor will be higher, provided it is operated fairly 10% overload when being driven by group drive.
Multi-motor Drive:
It consists of several individual drives each of which serves to operate one of many working members or mechanisms in some production unit.
Such drive is essential in complicated metal-cutting machine tools, paper making machines, rolling mills, and similar types of machinery. The use of multi-motor drive is continuously expanding in modern industry as their advantages outweigh the increase in capital cost as compared to the group drives.
Travelling crane is a example, which employs three motors for performing three different operations i.e., hoisting, long travel motion and cross travel.
• The use of individual and multi-motor drives has enable the introduction of automation in production processes, which in turn has considerably increased the productivity of various industrial organizations. Complete or partial automation helps to operate various mechanisms at optimum conditions and to increase reliability and safety of operation.
Disadvantages of electric drive:
1. A squirrel cage Induction motor is lesser costlier than electrical drive because of using converters and control circuits. 2. Technology for dc drives is improved compared to ac drive.
3. It is not reliable. 4. In event of failure of supply, the electric drive comes to rest which may paralyze the whole system. 5. On occurrence of faults such as short circuits, leakage from conductors and break down of over head conductors may lead to fatal accidents