What’s the difference between DOL start, star-delta, soft starter and a variable speed drive?

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An electric motor draws a large amount of electric current when starting; this sudden demand for current can lead to voltage dips and other undesirable effects. It’s important to select the best starting method to achieve a functional yet cost-effective control system. Read on for a quick crash course.

Direct online (DOL) starting

DOL starting is simply switching the motor on in one operation, with a direct connection from the power source. It’s generally only used for smaller loads, as the starting current can be up to ten times the normal running current of the motor. A contactor is generally used to switch power, and often a thermal or electronic overload relay is provided for motor protection. Direct online starting is the simplest and cheapest method of motor starting, but consideration needs to be made for power supply limitations on starting current.

Star-delta starting

Some motors can be started using the star-delta method: the motor is first connected in ‘star’ configuration, which allows the motor to gather speed without drawing excessive current. Once the motor is up to speed (or a preset time is elapsed) it is then connected in the normal ‘delta’ configuration. This method can reduce the starting current demand by 30%, but is only suited to applications where the motor is starting without load (e.g. where a clutched gearbox is used). Star-delta starting is relatively uncommon in Australia.

Soft starting

A soft starter is an electronic device which regulates the voltage flowing to the motor at startup. By slowly ramping up the supply voltage to the motor, a smooth start without excessive current flow can be achieved. Soft starters are more expensive than DOL or star-delta, but they are widely used due to their convenience and simplicity.

Variable speed drives

A variable speed drive (VSD), also known as a VFD or VVVF, is an electronic device which allows complete control of the motor speed including starting and stopping. It operates by changing the frequency of the power supplied to the motor. A VSD is extremely versatile and often used in process applications where a constant pressure or flow needs to be maintained. In addition, because the motor can be run at a slower speed and hence use less energy, use of a VSD can facilitate significant power savings. Variable speed drives are generally the most expensive method of motor starting, but their versatility means they are very widely used.