VFD vs Soft Starter


Many electric drive users would like to understand what is the difference between a VFD and a soft starter? Why, in one case, you can use a soft starter, and in another case, only a VFD.

In this article we will try to explain in understandable language what is the key difference between a soft starter and a VFD.

What is a Soft Starter and When to Use It?
A soft starter, also known as Reduced Voltage Soft Starters (RVSS), is a device that protects electric motors from damage caused by sudden power influx by limiting and controlling the initial inrush current during the motor startup. It uses an array of 6 thyristors or Silicon Controlled Rectifiers (SCR) for reducing the current and torque of the motor. A thyristor can be descried as an electronic valve that when applied with a control signal, allows the flow of current in only one direction. Since there are 3 AC voltages that switch between polarity at 50 or 60 Hz, 2 thyristors are needed for each phase, each for a different polarity. Thus, an array of 6 thyristors are used in a soft starter. The soft starter cuts the voltage waveform and limits the inrush current as well as the starting torque. The SCR circuit limits the voltage, but the frequency stays constant at 50 or 60 Hz, which means the speed remains constant. However, this is not an issue in applications where the motor is used at full speed always. The following are the applications where a soft starter would be ideal: Speed and torque control are needed only during motor startup. When equipped with soft stop, the same can be done during stopping the motor. Cutting down huge inrush current for large motors. Mechanical systems like conveyors, gears, etc. that require a gentle start during the startup. Eliminating pressure surges in pumping systems due to rapid directional changes of fluids.

What is a VFD and When to Use It?
A Variable Frequency Drive (VFD) is a device that controls the motor speed and torque by varying the frequency and voltage input to the motor. In other words, a VFD can be used as a soft starter with the added benefit of controlling the motor speed depending upon the workload changes. The voltage and frequency control in a 3-step process: Rectifier converts supplied AC voltage to DC. The DC signal is then filtered to improve power quality. The inverted converts to DC power back into AD at the required voltage and frequency. A VFD can control both voltage and frequency, thus control the torque by adjusting the V/Hz ratio. Reducing the voltage alone of a running motor is detrimental for the performance and the service life of the motor due to overheating. However, when both voltage and frequency are controlled, the electric motor can be slowed down without any negative impacts. One of the biggest advantages of VFDs is energy savings. When motor have variable workloads, VFDs can achieve energy savings of at least 20% with speed reduction. In buildings, a VFD can ramp down the pumping system when water usage is low and can reduce ventilation rates when the occupancy is low. This translates to energy savings which is always beneficial for building owners. Apart from the above applications, some more are as follows: Elevators and escalators for providing smooth motion. Manufacturing equipment like mixers, grinders and crushers.