Types of Electric Drives:
The various types of electric drives used in industry may be divided into three types, namely
- Group Drive,
- Individual Drive and
- Multi-motor drives.
1. Group Drive:
By 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. This types of electric Drives 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 and 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 viz a single 100 kW motor costs much less than that of ten motors of 10 kW each. Since all the machines may not be used on full load at the same time, therefore, the kW rating of the motor for group drive can often be less than the aggregate of kW output ratings of all the individual motors, and further causes reduction in cost. The efficiency and power factor of a large group drive motor will be higher, provided it is operated fairly near its rated load. If the machines are liable to short but sharp overloads, group drive is again advantageous, because 100% overload on an individual machine will cause hardly 10% overload when being driven by group drive.
Owing to its following drawbacks and objectionable features, this form of drive has become obsolete nowadays and the modern trend is to employ individual and multimotor drives:
- In group drive, speed control of individual machine is very cumbersome using stepped pulleys, belts etc.
- Owing to use of line shafting, pulleys and belts group drive does not provide good appearance and is also less safe to operate.
- In group drive since machines have to be installed to suit the layout of the line shafting, as such flexibility of layout of the various machines is lost. Also, it is not possible to instal any machine at a distant place.
- The possibility of installation of additional machines in an existing industry is limited.
- If at a certain instance all the machines are not in operation, the motor will operate at low capacity and, therefore, operation efficiency will be low.
- The breakdown of large single motor causes all the operations to be stopped. In some processes, where stoppage of one operation causes stoppage of whole sequence such as in textile industry or flour mills, this drawback is not effective and group drive, therefore, can be used.
- Considerable power loss takes place in the energy transmitting mechanism.
- The level of the noise produced at the work site is quite large.
Group drive is adopted, when existing factories are changed over from engine drive to electric motor drive simply by replacing the oil or steam engine by an electric motor of corresponding output retaining all the old shafts and belts.
2. Individual Drive:
In this types of electric Drives, a single electric motor is used to drive one individual machine. Example of such a drive are single-spindle drilling machines, various types of electrical hand tools and simple types of metal working machine tools and mechanisms. Though it costs more than group drive, but each operator has complete control of his machine, which enables him to vary its speed if necessary and stop while not is use, thus eliminating no-load losses. The machines can be placed in any desired position and can be moved very easily. The motor and its control unit can be built as an integral part of the machine which results in good appearance, cleanliness and safety. For driving heavy machines such as for lifts, cranes, shapers, lathes etc. and 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.
For new factories individual drive is preferred, as it causes some saving in the cost of superstructure because of being much lighter and less expensive.
In this types of individual drive too, the energy is transmitted to the different parts of the same mechanism by means of mechanical parts (such as gears, pulleys etc.). Thus some power loss occurs in the energy transmission mechanism. This drawback is overcome in the case of multimotor drives.
3. Multimotor Drive:
Multimotor Drive consists of several individual drives each of which serves to operate one of many working members or mechanisms in some production unit. For example, in travelling cranes, there are three motors : one for hoisting, another for long travel motion and the third for cross travel motors. Such drive is essential in complicated metal-cutting machine tools, paper making machines, rolling mills, rotary printing machines and similar types of other machinery. The use of multimotor drive is continuously expanding in modern industry as their advantages outweigh the increase in capital cost as compared to the group drives.
The use of individual and multimotor drives has enabled introduction of automation in production processes, which in turn has considerably increased the productivity of various industrial organisations. Complete or partial automation helps to operate various mechanisms at optimum conditions and to increase reliability and safety of operations.