Choice of Electrical Drives:
Choice of Electrical Drives depends on a number of factors. Some of the important factors are:
- Steady state operation requirements: Nature of speed torque characteristics, speed regulation, speed range, efficiency, duty cycle, quadrants of operation, speed fluctuations if any, ratings.
- Transient operation requirements: Values of acceleration and deceleration, starting, braking and reversing performance.
- Requirements related to the source: Type of source, and its capacity, magnitude of voltage, voltage fluctuations, power factor, harmonics and their effect on other loads, ability to accept regenerated power.
- Capital and running cost, maintenance needs, life.
- Space and weight restrictions if any.
- Environment and location.
- Reliability.
Status of DC and AC Drives
In the past induction and synchronous motor drives were mainly used in fixed speed applications. Variable speed applications were dominated by dc motor drives. Emergence of thyristors in 1957 lead to the development of variable speed induction motor drives in late sixties which were efficient and could match the performance of dc drives. Consequently, because of the advantages of squirrel-cage induction motors over dc motors (Sec. 1.3.1), it was predicted that induction motor drives will replace dc drives in variable speed applications. However, following hurdles forbided for the prediction to come true:
- Although squirrel-cage induction motor was cheaper than dc motor, the converter and control circuit of an induction motor drive was very expensive compared to those for a dc drive. Therefore, total cost of an induction motor drive was significantly higher than that of a dc drive.
- While the technology of dc drives was well established, that of ac was new.
- ac drives were not as reliable as dc.
- Developments in linear and digital ICs, and VLSIs were helpful in improving the performance and reliability of ac drives. But then these developments also led to similar improvements in dc drives.
Improvement in thyristor capabilities, availability of power transistors in early seventies and that of GTOs and IGBTs in late seventies and late eighties respectively; reduction in cost of thyristors, power transistors and GTOs; developments of VLSIs and microprocessors; and improvement in control techniques of converters have resulted into reduction in cost, simple controllers, and improvement in performance and reliability for ac drives. Although even now majority of variable speed applications employ dc drives, the ac drives are preferred over dc drives in a number of applications with the result, ac drive applications are growing. Induction motor drives find applications in low to high power applications and synchronous motor drives are employed in very high power (megawatts) and medium power drives. The permanent magnet-synchronous motor and brushless dc motor drives are being considered for replacing dc servo motors for fractional hp range. As the trend exists, applications of ac drives will continue to grow. However, dc drives will also continue to be used for quite some time.