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4 ways to start an electric motor

There are several unique ways to activate an electric motor. Here we examine the main options for motor starting, looking at the pros and cons of each as well as some common applications.

Variable-speed drives

For total control over a motor’s starting and running torque and current, connecting it to a variable-speed drive (VSD) offers a complete solution. Acceleration and braking can both be precision controlled so that the user achieves the best possible energy efficiency and can reduce maintenance expenditure. A drive’s purchase and installation cost can often be recouped through energy savings in less than 12 months.

Soft starters

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

Direct Online Starting (DOL)

This is the most straightforward way to start an electric motor. A DOL starter applies three-phase power directly to the motor, which provides 100% torque. However, it does so in an uncontrolled way, which can result in excessive mechanical and electrical strain on the motor and potentially 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

Electric motor windings 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 gets up to speed, much 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. This method is used in larger motors to allow longer start-up time and reduce what would otherwise be a massive starting current.

To discuss starting methods for your motor-driven applications, give Paul Scott a call on 01621 868138 or email

Electric Motors, Variable-Speed Drives

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