Motor drive circuit for pressure control of a pumping system

A liquid supply system comprising

• a) a motor-pump unit including a variable speed electric motor;
• b) a pipe connected to said unit for conveying liquid pumped by said unit;
• c) a pressure sensor connected to said pipe for providing a pressure signal representative of the liquid pressure in said pipe;
• and
• d) unit operating apparatus responsive to said pressure signal and connected to said unit for operating said unit, said apparatus comprising
• 1) speed control means for operating said unit in a first mode wherein the speed of said unit is adjusted to maintain a substantially constant pressure in said pipe, said speed control means being connected to said pressure sensor and to said unit;
• 2) speed change means for operating said unit in a second mode wherein the speed of said unit is reduced;
• 3) pressure monitoring means responsive to said pressure sensor for providing an indication of a drop of pressure in said pipe;
• and
• 4) intermittent means connected to said monitoring means for normally operating said unit in said first mode, for intermittently operating said unit in said second mode, and for turning off said unit when said monitoring means fails to indicate a drop of pressure when said unit is operating in said second mode.