Instead of applying the full voltage available to start the motor when powered on, a soft starter ramps up the voltage according to the application. Douglas Yates, product manager, MCG Products, North America Motion Controls, Danfoss Inc., says that while a variable speed drive, as the name suggests, changes motor speed, a soft starter cannot. Inside, he says, a pattern of solid-state switches—called silicon-controlled rectifiers (SCRs) or thyristors—open in different intervals for voltage to ramp up to full speed. SCR-diode and SCR-SCR designs are available for three-phase motor applications. SCR-diode requires higher start current, causes more heat, and generates undesirable harmonics, Yates says; SCR-SCR provides full wave control.