Variable Frequency Drive vs Soft Starter: What's The Difference?

Which starter are you using for your motor? Or, are you in the market looking for the best motor starter? There are two most popular options that you are likely to come across. These are variable frequency drive (VFD) starters and soft starters.

What are they and what is the difference between them? Read on as we give you everything you should know about variable frequency drive vs soft starter.

What is a motor starter?

First things first, let’s define what a motor starter is. This is the part of the motor that is responsible for starting and stopping an induction motor as well as regulating its speed. The motor starter is a mechanical device that is placed between the electric motor and the source of power (i.e. electricity).

A motor starter has two basic parts: an armature and a field winding. The field winding produces a magnetic field when it is supplied with electricity, which in turn produces torque to start and run the motor. The armature, on the other hand, acts like a conductor that moves within this magnetic field to produce rotational movement. Once electricity stops flowing through the armature, it comes to a complete stop and stops rotating. This is how you control the speed with a motor starter.

What is a variable frequency drive?

A variable frequency drive (VFD) is a type of motor starter that regulates the speed of an induction motor by varying its frequency. It is used in industrial applications to control the speed, torque, or direction of an electric motor.

The VFD does this by varying the frequency at which the current passes through the armature. In other words, it changes the number of times per second that current flows through the armature.

This causes a change in rotational speed and torque for drives with AC motors.

A VFD will have an input voltage range and output voltage range. These are usually given as ranges because it depends on what sort of load you are using and what you intend to do with it.

For example, if you want to run a small fan then you can use a low-voltage output and low-speed input combination while if you want to operate something like a drill press then higher voltage and higher-speed combinations will be more appropriate.

How does a VFD work?

Variable frequency drives have three main components that determine their working mechanism and operation in general. These are the rectifier, the DC filter, and the inverter.

The rectifier takes the AC voltage from the power supply and converts it into DC voltage to be used by the inverter. It is used to convert the ac power supply into a DC power supply.

The DC filter provides a smooth and uninterrupted flow of DC to the inverter. It is used to smoothen out the voltage fluctuations to make a smoother flow of current for the smooth operation of the inverter.

The inverter changes the input voltage into output voltage with a variable frequency using high-frequency switching techniques. This is done by switching on and off thousands of times per second, based on a pre-defined frequency from the input side, which then determines how many times per second it will switch on and off to get an output of desired frequency.

From the working mechanism, you now understand how VFD can change the speed of a motor while it is still in operation. It simply changes the frequency.

What is a soft starter?

A soft starter is a device used to control the speed of an induction motor to provide a smooth start for the motor. The starter reduces the starting voltage applied to the circuit and this goes a long way in achieving a soft and smooth start.

It works by gradually increasing the current flow to the motor so that there is no sudden surge of current. This helps in reducing mechanical stresses in the motor and also reduces the load on it during starting. It also helps in reducing the motor’s starting current, which is very useful in applications that require the motor to start operation at a zero speed.

A soft starter is used in various applications such as pumps, fans, conveyors, textile machines, etc. It can also be used to control the speed of a DC motor.

How does a soft starter work?

A soft starter works on the principle of a variable resistor. The soft starter is a variable resistor that has a feedback circuit. The soft starter is supplied with power by an AC voltage source and it regulates the current flow to the motor depending on the voltage drop across its terminals.

The soft starter can be programmed to apply different starting voltages to achieve a smooth start for the motor. This reduces the stress on the motor and also reduces its starting current. It can also be programmed to apply different starting voltages at different speeds for varying loads.

The same happens when stopping the motor. The soft starter is programmed to apply different voltage levels to the motor when it is being stopped.

From the working mechanism, it is clear that the soft starter is applicable only when starting and stopping the motor. It cannot be used to control the speed when the induction motor is already in operation.

Variable Frequency Drive vs Soft Starter: What’s The Difference?

Now that you have the basic knowledge of these two motor starters and how they work, it is time to break down their differences.

Function

As we have seen from the introduction, a soft starter is a semiconductor-based device used for starting and stopping an induction motor smoothly. VFD is also based on semiconductors and starts/ stops the motor. However, it does one more job, varying the speed of a motor. So, if you want a device that will be changing the speed of the motor while in operation, VFD will be your perfect bet.

Operation

What happens when the motor is already running? Well, once the motor has achieved its full speed, the soft starter will be bypassed using a contactor. In other words, the starter will be of less importance while the motor is running.

The situation is very different for the VFD. The starter is designed to operate throughout the running of the motor. You will need it whenever you want to increase or reduce the frequency of the motor.

Current management

How the two starters manage their current is different. A soft starter only regulates the AC voltage thanks to its thyristors. It can’t deal with the DC voltage.

On the other hand, VFD converts AC to DC voltage, and later on, it will convert it back to AC from where it can manage or control the frequency when desired.

Voltage management

How do these motor starters deal with both current and voltage? Well, the soft starter, it can only deal with the supply voltage. It has nothing to do with the current.

VFD manages both the supply voltage and supply frequency.

Torque

In this context, torque refers to the rotational force of an induction motor. Soft starter generates low to medium range torque. It generates adequate torque required for starting an induction motor.

VFD starter on the other hand is designed to generate the high torque required for starting and running the motor. This explains why you can use this starter for increasing the rotational speed of the motor.

Harmonics

The term ‘harmonics’ in this context refers to the frequency of the AC voltage. Harmonics are generated in an induction motor due to the interaction of its rotating magnetic field with the stator windings. This interaction generates harmonics in the AC voltage which is a problem for soft starters and VFDs.

The soft starter generates low to minimum range harmonics. It also has an inherent limitation on its control so that harmonics are not too high or too low.

If your application is super sensitive when it comes to the issue of harmonics, you will be safer with the soft starter.

VFD starters on the other hand generate a high level of harmonics which can be controlled fully by the control system or other means such as external filters, etc. In fact, when buying VFD you should ask if it has features that will minimize the effects of the harmonics.

Filters and enhancers

Soft starters come ready for use. As we have seen, the working mechanism of soft starters won’t result in harmonics or other unwanted factors.

However, you will need filters when using VFD. From the working mechanism, we have seen that VFDs tend to generate harmonics, which may end up interfering with the circuit. The surest way of protecting your circuit is getting the right filters.

Size of the starter

There is also a difference in size between VFD and soft starters. VFDs are usually bigger than soft starters. This is because VFDs have to contain more components and a control system to manage and regulate the current. On the other hand, soft starters are smaller than VFDs. This is because soft starters are meant to be used in small applications where space is not a constraint.

Price

VFDs are more expensive than soft starters. This is because VFDs are made of heavy-duty components and have many features that make them more efficient.

Soft starters may cost a little less than VFDs, but they have been made with lighter components, which makes them cheaper and easier to handle.

The working mode and other technical specifications should tell you that you will spend more on VFDs than on soft starters.

So, if you are operating within a limited budget, a soft starter will be your go-to option-that is you don’t have to use VFD.

Energy-saving?

Between soft starter and VFD, which one will save energy?

It is true that VFDs have more power-consuming components than soft starters. But, soft starters also consume more energy than VFDs. This is because the energy used by a soft starter is proportional to the load, while the energy used by VFD is proportional to the current.

Soft starters are advantageous when you need a small and compact device with minimum power consumption and a long life span.

Heat generation

Which one generates more heat between soft starter vs VFD? Given their working mechanisms, VFDs tend to generate a significant amount of heat than soft starters. The heat generation of VFDs is also due to their nature of running for long beyond starting and stopping the motor.

The good news is variable frequency drive starters have heat management mechanisms that prevent incidences of overheating. Also, devices that use VFDs have their own systems that manage heat.

Applications: Soft starter vs VFD

Basically, both soft starters and VFD starters are used for starting and stopping induction motors. This implies that they tend to have common application areas, right?

Not necessarily.

From their operation, soft starters will be used only in an application where you won’t need to control the speed of the motor. In case your application will require varying the motor speed, then VFD will come in handy.

Where can I buy a soft starter and VFD?

Whether you have decided to use VFD or soft starter, where you buy matters. It will determine whether you get a perfect starter for your motor.

Buying motor starters online may be your top priority. You will find hundreds of manufacturers and suppliers selling both soft starters and VFDs.

Take your time to vet the suppliers. Ask questions. Make sure they are reputable and have a great client rating. Check out their motor starter reviews online to determine their quality.

Are you considering buying directly from manufacturers in China? You will be better off using a sourcing agent. An agent will help you find reliable suppliers at a reasonable price with excellent service.

This is exactly what ICRFQ does. We are a reputable sourcing agent for motor starters in China. Whether you want VFD or soft starters, we will deliver. Just contact us for details.