Motors often require large amounts of energy when quickly accelerating to full speed. Soft starters and variable frequency drives can both be used to reduce inrush currents and limit torque--protecting equipment and extending the life of a motor by reducing motor heating caused by frequent starts and stops. But there are a number of differences between a soft starter and a variable frequency drive.
Soft Starters
A soft starter is a solid-state device that protects AC electric motors from damage caused by sudden influxes of power by limiting the large initial inrush of current associated with motor startup. They provide a gentle ramp up to full speed and are used only at startup (and stop, if equipped). Ramping up the initial voltage to the motor produces this gradual start. Soft starters are also known as reduced voltage soft starters (RVSS).
Soft starters are used in applications that require speed and torque control are required only during startup (and stop if equipped with soft stop) or where there is a need to reduce large startup inrush currents associated with a large motor is required. They are also used when the mechanical system (the load) requires a gentle start to relieve torque spikes and tension associated with normal startup (for example, conveyors, belt-driven systems, gears, and so on). And they are used for pumps to eliminate pressure surges caused in piping systems when fluid changes direction rapidly. 
Electrical soft starters temporarily reduce voltage or current input by reducing torque. Some soft starters may use solid-state devices to help control the flow of the current. They can control one to three phases, with three-phase control usually producing better results. Most soft starters use a series of thyristors or silicon controlled rectifiers (SCRs) to reduce the voltage. In the normal OFF state, the SCRs restrict current, but in the normal ON state, the SCRs allow current. The SCRs are engaged during ramp up, and bypass contactors are pulled in after maximum speed is achieved. This helps to significantly reduce motor heating. Soft starters are often the more economical choice for applications that require speed and torque control only during motor startup. Additionally, they are often the ideal solution for applications where space is a concern, as they usually take up less space than variable frequency drives.
Variable Frequency Drives
A variable frequency drive (VFD) is a motor control device that protects and controls the speed of an AC induction motor. A VFD can control the speed of the motor during the start and stop cycle, as well as throughout the run cycle. VFDs are also referred to as adjustable frequency drives (AFDs). VFDs are used in applications where omplete speed control is required, energy savings is a goal and custom control is needed.
VFDs convert input power to adjustable frequency and voltage source for controlling speed of AC induction motors. The frequency of the power applied to an AC motor determines the motor speed.
The VFD's input power comes from the facility power network (typically 480V, 60 Hz AC). It has a rectifier that converts network AC power to DC power. A filter and DC bus work together to smooth the rectified DC power and to provide clean, low ripple DC power to the inverter, which uses DC power from the DC bus and filter to invert an output that resembles sine wave AC power using a pulse width modulation (PWM) technique.
VFD's can save a lot of energy. And they are good at reducing peak energy demand or reducing power when not required. They offer fully adjustable speed (pumps, conveyors, and fans) and can control starting, stopping, and acceleration. they have dynamic torque control and they provide smooth motion for applications such as elevators and escalators. They can maintain speed of equipment, making them ideal for manufacturing equipment and industrial equipment such as mixers, grinders, and crushers. They have self-diagnostics and communications, advanced overload protection, PLC-like functionality and software programming, digital inputs/outputs (DI/DO), analog inputs/outputs (AI/AO) and relay outputs.
To learn more about soft starters and variable frequency drives, please download the attached document by Eaton, called "Choosing between a soft starter and a variable frequency drive," which was the source of information for this document.
| Choosing between a soft starter and a variable frequency drive.pdf |
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