A Rockwell Automation post by product manager Richard Anderson presents the economical and technology-based case for using soft starters in place of AC drives.

(Learn more about motor starters and contractors at Engineering360).

Savings can be realized when using a soft starter instead of an AC drive in the proper applications. Anderson says that AC drives continue to be applied at a rate of approximately 20:1 even in situations when soft starters may be sufficient to do the job. He says that AC drives will work in almost all applications but soft starters will not. He says that “substantial savings can be realized when using a soft starter instead of an AC drive in the proper applications.”

Those savings may include reduced cost, smaller physical size and improved energy efficiency. Anderson suggests that engineers consider torque requirements, speed control and power distribution concerns.

For soft starter applications, he presents these rules of thumb suggesting the ideal applications for soft starters:

•Lower starting-torque applications

•Lightly or moderately loaded applications

•Full-speed operation

•Limiting current is the prime reason for starting method

Soft starters are relatively simple devices--silicon-controlled rectifiers (SCRs) connect the AC input line to the motor. Comparatively, AC drives typically have more components, including a diode front end, DC bus and insulated-gate bipolar transistors (IGBTs). More components mean more cost. Anderson offers data showing relative cost of AC drives versus soft starters based on horsepower rating.

“AC drives can be 1.5 to 7 times the cost of a soft starter,” he says, “and AC drives can be 2.5 to 10 times the size of a soft starter.”

Anderson says that soft starters can achieve up to 99.9% efficiency, “depending on the size of the soft starter and the 3-phase voltage applied. After the starting process is complete, a soft starter with an integrated bypass pulls in an internal contactor. The SCRs are no longer firing and current flows through the contactor, enhancing efficiency.

“When a soft starter is up to speed and bypassed,” he says, “the motor current flows across the bypass contactor. No active solid-state components are on to generate heat.”