With the development of power electronics technology, and industrial automatic control technology, AC variable speed system has been widely applied in the field of industrial motor drag.

The application of PLC system design, the following basic principles should be followed in order to ensure the stability of the system work.

1.Maximize meet the the object control requirements of the accused;

2.System structure and strive to be simple;

3. System to be stable, reliable work;

4.Control system convenient function expansion and upgrades;

5 friendly interface.

PLC's powerful, easy to use, high reliability, is often used as an on-site data collection and control of equipment. The design is the use of the frequency converter and PLC control the pool water level.