Pump Start Up Methods

by Bill Noakes March 2, 2021

DOL (Direct on Line)

DOL is turning the pump directly on and off, e.g. with a simple contactor.

This is only recommended on smaller pumps as this method produces the highest inrush current spikes (often up to 3 times the running current and potentially higher).

This is the simplest and cheapest method. These current spikes have to be taken into consideration when sizing power supplies e.g. generators, as well as contactors and breakers.

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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 also reduces the torque by a factor of three.

After a short period of time the delta configuration is then employed which makes the motor run normally.

This starting method has considerably reduced inrush current spikes and therefore the power supply and starting equipment does not have to be quite so oversized.

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 gradual instead of one step. Therefore the power supply and starting equipment does not have to be quite so oversized.

Soft start is of medium cost, is most widely used above 11kW and is the most-widely used starting method.

Inverter Drives

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

For instance in the UK standard line power is at 50Hz so a VFD may start a pump at 30Hz and increase it to 50Hz. This means that as frequency is directly proportional to running speed (and therefore consumed power in centrifugal pumps), the inrush current spike is almost removed, meaning that the power supply e.g. generator, and breakers etc. only have to be slightly larger than the running requirements of the pump.

In addition inverters can be used to produce variable pressure/flow with centrifugal pumps including submersible pumps. Inverters are the most costly method of pump starting and therefore used less often.

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