Difference between VFD and Soft Starter

 The main difference between VFD and Soft Starter is that a VFD can vary the speed of the motor, while a soft starter only controls the start and stop of that motor. When facing an application, the price and size favor a soft starter. VFD is a great option if you need speed control.

In this article, we'll look at the internal functioning of VFD and Soft starter, to determine what equipment you need. Look at some devices to better understand these differences.

Difference between VFD and Soft Starter
Variable Frequency Drive Control Panel

What is VFD and Soft Starter

Soft start and variable frequency drives are used in industrial motor control and pump control. Both devices are semiconductors. VFD and soft starters operate on the same working principle, but differ from each other.

Let us first know about the internal work of the two devices. A typical 3 phase soft starter uses six thyristors or silicon controlled rectifiers oriented in a parallel configuration to start electrical motors smoothly.

A thyristor is made with 3 parts; a logic gate, b cathode and c anode. When an internal pulse is get to the gate, it allows current to flow from the Anode to Cathode which then send current to our motor.

When the internal pulses are not applied to the gate, the SCRs are in the off position and hence they restrict the current to the motor.

These internal pulses slow down the applied voltage to the motor which slows down the current. The pulses are sent on a ramp time basis, so the current will be applied slowly to the motor.

The motor, attached to a soft starter, will start in a nice smooth stream and come out at a predetermined maximum speed.

The motor stay at a speed until when stop the motor where the used soft starter will ramp up the motor in the same way as the ramp up.

Variable Frequency Drives

Variable frequency drives are semiconductor devices used primarily for motor torque-speed control and protection. It can start and stop a motor smoothly. The speed torque characteristics of a motor can be controlled during an operation depending on the application using a VFD. The frequency of the motor can be controlled by varying the frequency of the motor from zero to its maximum rated speed. 

Soft starters

Soft starter also used semiconductor devices are  to start or stop a motor. As the name indicates, it provides a smooth start and smooth stop. Typically, soft starters are bypassed by a contactor-overload circuit as the motor gradually reaches its full speed and returns to action when the motor stops. Soft starters, when start and stop time temporarily lowering motor terminal voltage.

VFD Main Components

The VFD has three main components, 1, Rectifier 2, Filter 3, Inverter.

The rectifier acts like diodes, taking the incoming AC voltage and converting it to DC voltage.

Subsequently, the filter uses capacitors to clean the DC voltage making it a smooth incoming power.

Finally, the "inverter" uses a transistor to change the DC voltage and sends the motor a frequency in hertz.

frequency drives can change the motor RPM as we required. We can set the ramp repeatedly just like a soft starter.

So how do we know which tool is required for our application? The decision on the device you choose determines how much control your application needs.

If your application requires a large amount of current. But does not require speed control, a soft starter is the best option. If speed control required, you can select a VFD.

In addition, price can be a determining factor in a lot of real-world applications. Since a soft starter has fewer control characteristics, the price is less than a VFD.

And finally, if the size of our device is a determining factor, the soft start is usually smaller than most VFDs. Let us look at some real-world applications to help you see the difference between VFD and Soft starter.


How to work a VFD in application?

The application we will see is a cooling fan. In this scenario, the cooling fan will turned it on when the VFD air temperature hits a high temperature set-point.

Once the set-points reached, the system cooling fan will slow down to maintain the air temperature, until it falls below the low-temperature set point where it will shut off.

However, if the temperature continues to raise above high-temperature set point, the fan will need to rotate rapidly.

Which device would work best in this situation? This is right! A VFD would be better than a soft starter because the fan speed should vary.

If our scenario is simply turning the fan on or off depending on the temperature and not changing the speed, then a softer starter would be a better option.

In closing, a VFD and a soft starter can perform similar tasks when it comes up or down a motor.


Better Option in Motor Speed Control

The main difference between the two is a VFD that can vary the speed of a motor while a soft starter only controls the start and stop of that motor.

When faced with an application, the size and price favor a soft starter.

The VFD ("Variable Frequency Drive") is better option if you want speed control in your system. There good news that if an existing application has a soft starter and speed control later determined as a requirement, a VFD can easily replace a soft starter. The opposite is true and also, a soft starter can replace with a VFD.

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