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What starting methods are available for electric motors?

An electric motor can be activated in one of several unique ways. Here we look at the modern choices for motor starting, with the pros and cons of each as well as some examples of where they’re used.

Direct Online Starting (DOL)

This is the most straightforward way to start an electric motor. A DOL starter applies three-phase power straight to the motor, which provides 100% torque but in an uncontrolled way. This can result in excessive mechanical and electrical strain on the motor which may lead to overheating and winding failure. Sudden starting is not suitable for all applications but is often used for water pumps, fans and compressors.

Star delta starting

The windings in an electric motor can be connected in either ‘star or delta’.  During start-up, the windings must be configured in ‘star’ and then switch to ‘delta’ once the motor up to speed, rather like a vehicle changing gear. This method limits the current to the motor during start-up to as much as 33% of DOL starting current, with the aim being to reduce shock load. Therefore, this method is used in larger motors to allow longer start-up time and reduce what would otherwise be a huge starting current. Star delta starting is useful for applications such as cranes and conveyor belts.

Variable-speed drives

For complete control over a motor’s starting and running torque and current, connecting it to a variable-speed drive (VSD) offers an all-in-one solution. Both acceleration and deceleration can be precision controlled so that the user achieves the greatest possible energy-efficiency and benefits from reduced maintenance costs. The cost of ordering and installing can normally be recouped via energy savings in around 12 months. VSDs are coupled with electric motors in all manner of industries, including food and beverage, textiles and printing.

Soft starters

A soft starter is a device that protects an electric motor from the sudden inrush of power during start-up by allowing a smooth climb up to full speed. Soft starters are fitted with timers to switch the current back to normal level once the motor is running. As with a variable-speed drive, a soft starter provides energy efficiency and reduced maintenance costs, with the difference being that the former provides controlled speed throughout operation, not only at the beginning. Soft starters are used in applications such as conveyor belts and lifts where a smooth start is essential for safety.

So there you have it. If there’s anything else you need to know about your motor-driven applications, give Paul Scott a call on 01621 868138 or email

Variable-Speed Drives, Electric Motors

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