Operation principle and design issues
Waste water pumping stations are very common application examples of three phase induction motor. There are numerous varieties of power supply and control logic, depending on exact conditions in the place of installation, the role of pumping station inside a sewer system, and of course, applied design logic.
Similar to some previous articles, a real example of pump control will be described within this article as well. Attention will be given to the description of the operation principle, with some comments about specific design issues included.
Unlike previous examples, more than one pump will be subject of control logic.
The waste water pumping station is placed in the vicinity of one small town on the Adriatic coast, near the local road. This is one of six main pumping stations that are connected in a row (cascade connection) in that way, serving to supply a waste water treatment plant that cleans the water prior to outfall into the Adriatic Sea.
Pumping station is consisting of two submersible pumps, with motor rated power of 21 kW each. Regular operation mode of this pumping station is 1 working + 1 spare pump, which means there is not more than one pump active at any time, i.e. if one pump is turned on, the other cannot be turned on as long the first pump is active.
During normal operation, pumping station is supplied from local LV distribution network. In the case of power supply disturbances or outage (which are not rare in this particular case) there is a standby diesel genset, serving as backup power supply source.
Pumps, level meter and level switches are placed inside pump manhole, below the ground. Electrical cabinet is placed inside metal enclosed container, while diesel genset is equipped with prefabricated enclosure for outside installation and placed next to container.