Applications of Electric Drives:
The industrial Applications of Electric Drives are numerous. To arrive at a particular motor of suitable size for a given application, a knowledge of the following is very much necessary:
- The torque-speed curve of the industrial load is required. This fixes up the type of motor required to drive the load.
- The environmental conditions in the industry where the motor is located. This also decides the ambient temperature at which the motor operates.
- Duty cycle of the load, and the frequency of starting and braking. The kW rating of the motor is decided by the load cycle, taking into consideration both the thermal as well as mechanical overload capacity of the motor. The starting and braking of the motor, if employed very frequently alters the kW rating of the motor because of heating of the motor due to extra losses occurring during these conditions. The methods of starting and braking must also be known, The motor rating must be so chosen that it is capable of driving the highest loading in the duty cycle without getting thermally overloaded, providing the necessary torque at all conditions,
- The speed control needed also decides the type of motor. Smooth speed control over a wide range is not a problem with dc motors but had been a problem with ac motors till the advent of thyristor power converters. With the recent developments in the area of these power converters, the ac motors also are becoming popular as variable speed drive motors. The economics involved in the methods of speed control employed also need due consideration.
- The necessary information is required to decide between the individual and group drive. In a group drive, as has already been mentioned, one single machine is used to do several jobs. It is therefore economical to employ a group drive. In a group drive there are serious disadvantages, viz., servicing or repair of the motor requires complete stoppage. Transmission losses occur and efficiency is poor. Line shaft makes the place untidy. In an individual drive on the other hand, separate motors are employed to perform separate jobs. Basically it is very costly. But it has facility for overtime working, complete shut down or stoppage is not required, in case of repair and absence of line shaft makes the place clear. Hence taking into consideration all these factors, a proper choice between these two types may be made.
- To meet certain specific duties in a particular application special designs of the motors may be required. Special controls and protection circuits may be needed. The motors may require protection against single phasing; overloading, overheating, etc. The rating of the motor must be suitable to do the job under the worst conditions available. Based on the above information it is possible to decide the requirements of a drive motor to suit a given application. Of the numerous industrial applications in which electric motors are employed as drive motors, only a few are discussed here:
1. Textile mills
2. Steel rolling mills
3. Cranes and hoists
4. Cement mills
5. Paper mills
6. Sugar mills
7. Machine tool applications
8. Coal mining
9. Centrifugal pumps
10. Turbo compressors
Some industrial Applications of Electric Drives may have several stages by the time the end product is ready from the basic raw material. While selecting motors for operations in these stages, one has to give due consideration to the above factors, such as environmental conditions of the stage of operation, electrical features of the motor, i.e., starting torque, braking, speed control, etc., and the ambient temperature. The requirements of the drive must be very well defined to properly select the motor.