Motor control: Soft starters and variable frequency drives

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.