Drives & Motor Starters

A soft starter controls only the starting and stopping of an AC motor by ramping voltage, after which the motor runs at line speed, usually through a bypass contactor. A VFD changes frequency and voltage continuously, so it can run the motor at any speed. A VFD costs more but also saves energy on variable-flow loads.

Under the affinity laws, power varies roughly with the cube of speed for centrifugal loads, so a 20 percent speed reduction can cut power to about half, ignoring losses. Real savings depend on the system curve, static head and how many hours the load runs below full flow. The VFD savings calculator gives an estimate.

Match the VFD output current to the motor full-load current, with allowance for overload duty and ambient temperature, rather than selecting only by kW. Choose between variable torque and constant torque ratings. For high altitude or hot panels, derate. A qualified engineer should confirm the sizing for critical or heavy-starting loads.

A line reactor reduces harmonic current drawn from the supply and protects the drive from supply spikes, and is often recommended on weak supplies or with several drives. An output filter or dV/dt filter helps when the motor cable is long. The drive manual gives recommended cable length limits for your model.

DC drives are used where a DC motor is already installed, such as on rolling mills, winders and some cranes, and where the plant prefers not to replace the motor. New installations normally use an AC motor with a VFD, which avoids brush maintenance. The decision depends on the existing motor condition and downtime for conversion.