VFD vs Soft Starter: Which is Best for Your System?

When it comes to controlling the motors in industrial applications, two common options are Variable Frequency Drives (VFDs) and Soft Starters. Both of these technologies are designed to control electric motors, but they do so in different ways. Choosing between a VFD and a Soft Starter can be challenging, as both have their own advantages and disadvantages depending on the specific needs of your system. In this article, we will explore the differences between VFDs and Soft Starters and help you determine which one is best suited for your industrial application.

Differences Between VFD and Soft Starter

The major differences between VFD and soft starter are:

1. Philosophy

A variable frequency drive (VFD) is a device that can control the speed of an electric motor by controlling the frequency and voltage of the electrical power supplied to it. When a VFD starts a motor, it initially applies a low frequency and voltage to the motor thus allowing high inrush currents to be avoided. On the other hand, the soft starter is a device used to limit the large initial inrush of current associated with motor start-up and provide a gentle ramp-up to full speed.

The soft starter can be referred to as a reduced voltage motor starter. Variable frequency drive can be referred to as variable speed drive, adjustable frequency drive, frequency inverter and drive.

2. Speed control

Variable frequency drives are used to control the speed of a motor by changing the frequency of the power. A VFD can control the speed at any point during the operation of the motor. The soft starter cannot control the speed of a motor. Soft starters are only used during startup to limit the inrush current. (and stop if equipped) The gradual start of the motor is produced by ramping up the initial voltage to the motor.

3. Operation

VFDs convert constant frequency and voltage input power to adjustable frequency and voltage sources for controlling the speed of AC induction motors. Soft starters use semiconductor devices to temporarily reduce the motor terminal voltage. This provides control of the motor current to reduce inrush and limit shaft torque. The control is based on controlling the motor terminal voltage in two or three phases. By limiting the voltage, a reduced torque is provided to start the motor.

Soft starters utilize silicon-controlled rectifiers (SCRs) to perform phase angle control while VFDs use insulated-gate bipolar transistors (IGBTs) to perform pulse-width-modulation (PWM) control.

4. Operating function

VFDs can also provide more flexible functionality than soft starters, including digital diagnostic information. With a VFD, you can receive feedback using many types of communications, from hardwired I/O points, if you just want minimum feedback, to Ethernet, if you want to have all the data in the drive available to your controller.

5. Cost

In many applications, it is required to continuously regulate the speed of the motor and a drive is then a very good solution. However, in many applications, a drive is used only for starting and stopping the motor, even though there is no need for continuous speed regulation. This will create an unnecessarily expensive solution if compared with a soft starter. In fixed-speed applications, soft starters are still the most economical and effective solution. However, while a VFD can be more expensive initially, it can provide significant energy savings which reduce operational costs over the life of the equipment for a lower total cost of ownership.

6. Market size

The market size of VFDs is extremely huge compared to soft starters. Most motors need speed control and VFD is a perfect solution. The global VFD market is projected to reach a market size of USD 27.57 billion by 2023. The global market for soft starter is expected to reach $2.0B by 2023.

7. Needed space

The drive has a much bigger physical size and requires more space. Soft starters are often a solution for applications where space is a concern as they usually take up less space than VFDs.

8. Harmonics

Since the drive changes the frequency and creates the sinus wave, a drive will cause harmonics in the network. Additional filters and shielded cables are used to reduce these problems but the harmonics will typically not be eliminated. The soft starter does switch over to running across the line using bypass contacts. So, it will be a linear load while running across the line. This means it will not inject harmful harmonic distortion into your plant’s ac power grid.

9. Weight

The drive is also much heavier than a soft starter. This makes it a less desirable solution on, for instance, ships where weight is important.

10. Heating and cooling system

Soft-starters are normally air-cooled due to their short operation time. For high powers or in case of reduced availability of cooling air, additional ventilators might be preferred. The integrated bypass function of a soft starter provides the opportunity to switch over to direct-online operation, after the initial start-up. By running partially direct-on-line, you win the advantages of reduced losses and the need for heat dissipation, thus saving energy due to decreased cooling requirements.

VFDs operate on a continuous and enable more sophisticated control. However, they use insulated gate bipolar transistors (IGBTs) and so must be designed with sufficient cooling capability.

11. Forward and reverse operation

On their own, soft starters cannot run motors in the reverse direction at full speed. However, forward and reverse operations can be achieved by using a forward and reverse contactor arrangement. Some soft starters also provide a part speed function that runs the motor at a slow speed in either the forward or reverse, without a reversing contactor. However, the reverse operation is limited to short periods at a fixed slow speed. On the other hand, VFD can operate in forward and reverse directions.

12. Maintenance

Soft starters do not require so much maintenance. Keeping the fan vents clear and clean is enough. Depending on the drive, annual maintenance or even less frequently, parts need to be inspected, cleaned, and or replaced. For example, on a drive operating 24 hours per day, in year 3, you should replace some parts of a VFD.

13. Energy saving

To determine energy saving when using VFD and soft starter, we need to calculate the average operation hour per day and energy consumption per day. Studies show that using VFDs in variable torque applications such as centrifugal pumps and fans can provide a very fast payback in energy saving. Some applications can even pay back the entire cost of a typical VFD within months, especially if the local utility offers a VFD rebate program. VFDs may offer greater potential to reduce your electricity costs over the life of the equipment.

15. Applications

A VFD can perform the same function as a soft starter and can be used in applications requiring complete speed control, energy savings and custom control. VFDs have also some microprocessor capabilities with inputs and outputs.

15. Inside delta connection

3 phased controlled soft starters can be connected inside the delta. Drives are sized for line connection (three leads from the motor), based on the full load current of the motor.