What Is The Difference Between VFDs And Soft Starters?

You can start and stop an induction motor safely with both of these devices, although they do have entirely different features. We’re going to make a brief comparison to identify the difference between VFDs and soft starters, but first, we’ll have a few quick descriptions. 

What Is A Motor Starter?

The name explains itself, describing a device used for the safe starting and stopping of induction motors. It allows the motor to start more safely by cutting down the volume of the starting current. This helps to prevent damage to the windings or mechanical parts that might otherwise occur during a jerky start-up. Motor starters provide protection against low voltage and overcurrent, making the windings and mechanical parts less prone to electrical damage and burnouts. 

How Does An Induction Motor Usually Start?

This type of motor has low winding resistance, which means that it requires a vast amount of electrical current to start up. This huge volume of current that’s consumed during the initial stages of the motor’s operation is called the inrush current. Turning on a motor is a sudden and jarring action that can damage its internal windings. The wear and tear inflicted by the abrupt starts and stops can significantly reduce a motor’s life span. In the worst cases, motors can burn out, which can cause serious damage not only to the motor but also to the machinery it drives.

Soft Starters

A soft start or starter is a particular variety of motor starter that offers even greater protection to an induction motor. It reduces the risk of mechanical stress and damage from the high inrush current to the motor. It does this by cutting the amount of voltage and torque applied to the motor during the stress-inducing start-up and shutdown processes. 

How Do Soft Starters Work? 

A soft starter is a more advanced electronic device than an ordinary motor starter. It consists of a logic circuit, which can gradually increase or decrease the voltage by controlling the firing angle of a gate signal. The voltage supply to the motor is controlled by a pair of semiconductor thyristors that manage current flowing in both directions. 

A three-phase soft starter consists of six thyristors, set in a back-to-back formation, each comprising three terminals: anode, cathode and gate. This provides simultaneous voltage reduction in all three phases, in either direction of current flow. The thyristor is triggered when the gate signal is applied to a silicon-controlled rectifier (SCR), which starts the current flowing to the motor. You can determine how much voltage the thyristor will allow by varying the angle of the gate signal, which is known as the firing angle. Cutting down the input voltage reduces the amount of current supplied to the motor, which can be gradually increased to bring the motor up to speed. 

What Is A Variable Frequency Drive?

A VFD is another type of electronically controlled motor starter that uses a semiconductor. It performs the same safe starting and stopping functions for electric motors as a soft starter. The difference is that a VFD has complete control of the motor speed throughout its operation. Not only does a VFD control the supply voltage, it’s also able to vary the input voltage frequencies to provide a greater range of motor speeds. 

How Does A VFD Work?

A VFD is comprised of three circuits: a rectifier, a DC filtering circuit and an inverter. The incoming power to the VFD is usually supplied in fixed alternating current (AC), which is converted by the rectifier circuit to variable direct current (DC). Passing this current through the DC filter smooths it out, to stabilise the voltage. The DC voltage is then converted back to AC using the inverter circuit. This conversion process in the VFD is continuous, so it can adjust the motor speed at any time during its operation. For this reason, VFDs are mainly used for applications requiring variable motor speeds, such as pumps, blowers, conveyors and machine tools. 

VFD Or Soft Starter? 

You’ll find several differences between a VFD and a soft starter: 

• Soft starters regulate AC voltage by means of thyristors, whereas VFDs rely on current conversion.

• Both devices are based on semiconductors, but the soft starter can only regulate starting and stopping. VFDs not only start and stop induction motors but can also vary their speed constantly. 

• VFDs operate continuously during the motor’s operation, whereas soft starters are bypassed by a contactor when the motor attains full speed.

• VFDs can vary both voltage and frequency, whereas soft starts can only alter the supply voltage. 

In addition, VFDs are physically larger, require filters and are comparatively expensive. 

Should You Choose A VFD Or A Soft Starter?

VFDs and soft starters are very similar, in that they both control an induction motor’s start-up process. The following factors outline the essential difference between VFDs and soft starters:

Purpose

• VFDs provide precise control over motor torque and speed throughout the motor’s operation.

• Soft starters only reduce the risk of mechanical or over-current damage during start-up.

Functionality

• VFDs are designed for processes requiring variable motor speeds.

• Soft starts work best with systems operating at fixed speeds

Control

• VFDs continuously adjust the input voltage and frequency, allowing more precise control throughout the operation.

• Soft starts only impact motor start-up, controlling inrush current.

Energy Efficiency

• VFDs are more energy efficient as they match motor speed precisely to demand.

• Soft starts only reduce start-up impact and use a normal amount of energy during operation.

Cost

• VFDs offer advanced capabilities and typically cost more.

• Soft starts are simpler and therefore cheaper.

In summary, soft starters provide smooth motor start-ups, while VFDs offer versatile, ongoing control for electric induction motors. This makes soft starters the best choice for simpler, fixed-speed applications, and VFDs ideal for dynamic systems.