Motor protection delivers reliable, efficient operation

How to make the right choice between a soft starter and a variable frequency drive


How to make the right choice between a soft starter and a variable frequency drive. Courtesy: EatonMotors 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 your valuable equipment and extending the life of your motor by reducing motor heating caused by frequent starts and stops. Choosing between a soft starter and a variable frequency drive often depends on the application, system requirements, and cost (both for initial start-up and over the lifecycle of the system). 

Extending motor life

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 start-up. They provide a gentle ramp-up to full speed and are used only at start-up (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 where:

  • Speed and torque control are required only during start-up (and stop if equipped with soft stop)
  • Reducing large start-up inrush currents associated with a large motor is required
  • The mechanical system requires a gentle start to relieve torque spikes and tension associated with normal start-up (for example, conveyors, belt-driven systems, gears, and so on)
  • Pumps are used to eliminate pressure surges caused in piping systems when fluid changes direction rapidly  

How does a soft starter work? 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. 

Figure 1. Soft Starter Schematic. Courtesy: Eaton

Figure 1. Soft Starter Schematic 

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 start-up. 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.

Driving energy efficiency

A variable frequency drive 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 used in applications where:

  • Complete speed control is required
  • Energy savings is a goal
  • 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, based on the following equation: 

Speed (rpm) = 120 x Frequency (Hz) x number of motor poles 

For example, a four-pole motor is operating at 60 Hz. These values can be inserted into the formula to calculate the speed:

120 x 60 x 4 = 1800 rpm 

Figure 2. The function of a VFD. Courtesy: Eaton

Figure 2. The function of a VFD

  • ac supply: Comes from the facility power network (typically 480V, 60 Hz ac)
  • Rectifier: Converts network ac power to dc power
  • Filter and dc bus: Work together to smooth the rectified dc power and to provide clean, low ripple dc power to the inverter
  • Inverter: 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.
  • Pulse width modulation: Switches the inverter semiconductors in varying widths and times that, when averaged, create a sine waveform 

Figure 3. Pus Width Modulated Waveform. Courtesy: Eaton

Figure 3. Pus Width Modulated Waveform 

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