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What Is the Difference Between Direct on Line (DOL), Star Delta, Soft Start and Variable Speed Drive Starting Modes?

Published by Gavin Carvell
Gavin Carvell
on Wednesday 19 January 2022

Last updated on Wednesday 9 February 2022

The reason there are several different starting methods is because the required current for electrical induction motors spikes during initial start-up. For small motors this doesn't cause any issues, however, for large motors this spike is significant and so different starting methods are used to reduce this surge. The spike during start-up is because when stationary the electrical connection is a short circuit, i.e. live is connected to neutral via the motor winding, with no power being consumed. In the case of direct-on-line starting this current spike can be 7-10 times the running current, or higher if starting under load. The power supply to the site or application will have been designed for a specified power requirement. If electric motors are installed the running loads might be acceptable, but the requirement on start-up could exceed the design power supply, causing fuses to blow or cables to burn out.

Why do you need to select the correct option?

The use of voltage control in various ways reduces the motor torque on start-up, thus reducing the current spike to an acceptable level. However, consideration needs to be given to the application as not all starting methods are suitable in every situation. Some situations have variable torque characteristics, for example a fan running at half speed only requires 25% of the torque and 12.5% of the power. Other situations have constant torque, for example a loaded conveyor belt requires the same torque to move the load at low speed as at high speed, so the power requirement is directly proportional to the speed, i.e.50% speed = 50% power.

Direct online (DOL) starting

DOL is simply an on and off switch but usually in the form of a contactor; the power is turned on and the motor starts.

This method is usually only used for smaller motors so that the electricity can spike without tripping the supply. A thermal or electronic overload relay is often used as protection for the motor.

DOL is the simplest and cheapest starting mode but calculations need to made to ensure the supply can provide the needed starting current.

Star-delta starting

Some motors can be started using the star-delta method: the motor is first connected in ‘star’ configuration, i.e. the current flows through 2 motor windings in series, reducing the applied voltage. This allows the motor to gather speed without drawing excessive current. Once the motor is up to 80% of full speed (or a preset time is elapsed) it is then connected in the normal ‘delta’ configuration, i.e. a single winding at full voltage.

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 or for a fan motor). It is difficult to set the change-over from star to delta at the correct speed, and during change-over the motor acts as a generator, producing a 'transient' voltage, which can result in a second current spike.


Soft starting

A soft starter is an electronic or solid state device which controls the voltage flowing to the motor at start-up. By slowly ramping up the supply voltage to the motor, a smooth start without excessive current flow can be achieved. The torque characteristics must be specified when selecting the soft starter.

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 Variable Frequency Drive (VFD) or Variable Voltage Variable Frequency (VVVF), is an electronic device which allows complete control of the motor speed including starting, running speed and stopping. It operates by altering the frequency of the power supply 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.