This website uses cookies primarily for visitor analytics. Certain pages will ask you to fill in contact details to receive additional information. On these pages you have the option of having the site log your details for future visits. Indicating you want the site to remember your details will place a cookie on your device. To view our full cookie policy, please click here. You can also view it at any time by going to our Contact Us page.

Motor control: Soft starters and variable frequency drives

01 June 2020

Soft starters and variable frequency drives are used to limit start-up currents and high torque in electric motors, which helps extend the life of the motor and its drive components.

The choice of using a variable frequency drive or a soft starter largely will depend on the application, explains Martin Wyatt, Business Development Manager at Carlo Gavazzi UK.

A large amount of electric current is drawn by an AC motor starting up. In the few seconds between the application of electric supply to a motor and its actual acceleration up to its full speed, the starting current can be up to eight times the current for conventional motors and up to 15 times the motor rated current for high-efficiency motors.

This high start-up current – also termed inrush current – can induce severe torque oscillations for motors with a direct-on-line (DOL) starter configuration. This can lead to voltage sags and light flicker on the voltage network. Similarly, for motors with a star-delta starter, the high start-up current can lead to torque oscillations during the motor's transition from star to delta. In addition, the lack of a motor soft stop means water hammer, for example, can occur in centrifugal pumps which can increase wear and tear on the pump and shorten its lifespan.

Variable frequency drives (VFDs) and soft starters can both be used to reduce inrush currents and to limit torque on AC electric motors. Which is the most appropriate will depend on the application, the mechanical system requirements and both the capital- and life-cycle costs of the system. 

Read the full article in the June issue of DPA.

Contact Details and Archive...

Print this page | E-mail this page

Igus - Tech Up, Costs DownOmron Electronics