A Glossary of Electrical Pump Terms for Contractors

electric pump

Have you ever wondered what the difference is between Soft Start and Star-Delta or perhaps been unsure of what an inverter is? Bookmark this page and never be stumped again as we feature a selection of electrical pump terms for contractors.


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An acronym meaning Direct on Line. DOL is a method of turning the pump directly on and off, e.g. with a simple contactor. This produces a large inrush current of up to six times the running current. 


Star-Delta is an older method of starting motors (no longer frequently used in the UK) which allows for connection of the pump motor with reduced voltage (across the motor windings). This reduces the inrush current. 

Soft Start

This is a frequently used method of starting larger three phase motors which limits large inrush currents by ramping up the initial voltage as the motor starts similarly to star delta, but much more gradually instead of in one step.



Inverter drives (also known as Variable Frequency Drive or the Variable Speed Drives) control pump motors by varying the frequency of the electrical power. 

Seal Fail Detection

An internal probe featured on some pumps for detection of mechanical seal failure. 

Single Phase Pump

A pump that runs on a single phase power supply. Its motor would usually be less than 3kW. 

Thermal Cut-Out Switch

A device which switches in response to temperature change. This can then open a relay or contactor to switch the pump off. Also see Thermister. 


An electrical resistor whose resistance is greatly increased (PTC) or reduced (NTC) by heating, used for measurement and control. 

Three Phase Pump

Usually found in industry on such as quarries, mines and oil rigs. Reduces voltage drop over longer cable lengths and is useful for much greater power transfer.


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