Cavitation is a destructive condition caused by the presence of bubbles that are formed when a centrifugal pump or vertical turbine pump operates with low liquid levels. The bubbles form and then burst, resulting in pitting and destruction of the impeller. A current-sensing relay in the circuit can prevent this from occurring (Fig. 2 below).

Fig. 2. The current-sensing relay in this water pump electrical circuit prevents the pump from running when the water level is too low.

When a pump is operated with a flooded suction and liquid completely covers its inlet, the pump’s motor will draw normal operating current. On the other hand, if the liquid level falls below the inlet, the pump’s motor will draw less current. The circuit in Fig. 2 operates as follows:

  • Start button is depressed, causing starter M to energize.
  • Simultaneously, time delay TD begins to time.
  • Because CR is an undercurrent relay, its contact will not be closed when the motor is initially started.
  • TD is used to short circuit CR’s normally open contact during starting.
  • Current relay CR energizes when the motor’s current exceeds the low-current setting.
  • TD’s normally closed contact opens after TD times out, enabling CR to protect the pump from a low-liquid situation.
  • When the motor’s current falls below the setpoint, CR’s contacts open and de-energize M.

Note that the pump will not restart automatically, as the operator will have to assure that sufficient liquid is available before restarting.

This circuit can be used for pumps in fixed locations, such as high-service pumps used to fill a water tower, or where pit-pumps are used to pump water from pits in coal mines or quarries. In the latter case, the pumps are normally unattended. When the water level in the pit falls from the pumping action, the pump will shut off. An employee inspects the pumps periodically to check their status.