Which statement correctly differentiates soft starters from variable frequency drives?

• A. Soft starters limit voltage to reduce inrush; VFDs adjust frequency to control speed and torque.
• B. Soft starters alter frequency while VFDs limit current.
• C. Soft starters and VFDs perform identical functions.
• D. VFDs are cheaper and simpler than soft starters.

Answer: (A)

The key distinction is how each device affects the motor’s drive during starting and operation. A soft starter reduces the voltage applied to the motor during the start and ramps it up, lowering the inrush current and mechanical stress. It does not change the frequency, so once the ramp finishes the motor runs on full line voltage at the standard frequency. This makes soft starters simple and economical for a gentle start, but they don’t provide true speed control once the motor is running.

A variable frequency drive, by contrast, changes the frequency of the supply to the motor (and adjusts the voltage to maintain a proper V/Hz relationship) to control speed and torque. This allows precise speed variation and smooth acceleration/deceleration, along with potential energy savings, but comes with greater complexity and cost.

So, soft starters limit voltage to reduce inrush, while VFDs adjust frequency to control speed and torque.