AC drives continue to be applied at a rate of approximately 20:1 compared with soft starters, even in situations when soft starters are sufficient to do the job, says a Rockwell Automation blog post that presents a range of applications where soft starters might make sense.

AC motor starters are used to turn-on and turn-off electric motors and motor-controlled equipment.Savings provided by using soft starters include reduced cost, smaller physical size and improved energy efficiency. To reap the benefits of these savings, engineers must carefully consider torque requirements, speed control and power distribution concerns.

The role of soft starters, designed for use with AC electrical motors, is to reduce stress on the motor and shaft, and on power cables and the electrical distribution network, during startup. In pumping systems, for example, a soft start can avoid pressure surges.

(To learn more about AC motor drives visit the Engineering360 product and service guide.)

AC motor drives are classified based on the type of AC motor being used—typically induction drives, synchronous drives, sensorless vector drives and servo drives.

Induction motors induce current into the rotor windings without any physical connection with the stator windings. They are adaptable to many different environments and can provide considerable power, and variable speed control.

Synchronous motors operate at constant speed up to full load, and find in applications requiring precise speed control.

Sensorless vector drives employ independent control of voltage and frequency supplied to the motor, for optimum speed control; low-speed torque output approaches that of DC motors. No feedback sensor, such as an encoder or resolver, is used.

Servo motors are typically permanent magnet synchronous motors that often have low torque-to-inertia ratios for high acceleration ratings.

For soft starter applications, consider 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