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Differences Between, Star-Delta, Soft Starter, DOL Start and a Variable Speed Drive Explained

Electric motors require a surge of electric current to start. This abrupt demand can cause voltage dips and a range of other unwanted effects. It’s essential to choose a control system that is both cost-effective and functional, so read on for advice about how to select the right starting method for your needs.

Direct online starting (DOL)

DOL starting involves turning the motor on in a single operation, via a direct connection to the power source. This method is usually used only for smaller loads because the starting current can potentially be up to ten times the motor’s typical running current. Generally, a contactor is used in order to switch power. An electronic or thermal overload relay is provided to protect the motor. This is the simplest and most cost-effective method, but serious thought needs to be given to the limitation for power supply capacity on starting.

Star-delta starting

With this method, the motor is connected first in a ‘star’ configuration that lets the motor gather speed, but without needing to draw excessive current. When a preset time has passed, or the motor is up to speed, it can be connected in the typical ‘delta’ configuration. While the star-delta starting method can slash the starting current requirement by 30%, it’s only suitable for applications in which the motor starts without a load (eg. when using a clutched gearbox).

Soft starting

A soft starter refers to an electronic device used to regulate voltage to the motor when starting. It works by slowly increasing the supply voltage so that a smooth start can be achieved, without requiring excessive current flow. This method is more expensive than the others discussed, but it’s widely used because of its simplicity and convenience.

Variable speed drives

Also referred to as a VFD or VVVF, a variable speed drive (VSD) is the term for an electronic device which provides control of the motor speed at all times, including when starting and stopping. It works by altering the frequency of power supplied to the motor. This method is very versatile, which is why it is often utilised in process applications which require a constant flow or pressure. Using a VSD can save significantly on power due to the fact the motor can run at lower speeds. While variable speed drives are more expensive than other methods, they are widely used due to their versatility.

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