What’s the Difference? – Variable Frequency Drive (VFD) vs. Soft Starter

Variable Frequency Drives (VFD) and Soft Starters are electronic devices that are popularly used in a wide variety of industrial processes which require the utilization of AC motors. These devices share a common, specific purpose to which they are applied, particularly in the field of motor protection and lowering of energy consumption.

But what is the difference between a VFD and a Soft Starter?    

Device Terminology

Before we differentiate the difference between each of the devices, let us understand first the common industry terms used to refer to these devices.
A Soft Starter is commonly termed as Soft Starter, Motor Starter, or simply starter.
A VFD is popularly termed as Variable Speed Drive (VSD), Adjustable Frequency AC Drive, Adjustable Speed Drive, or simply an Inverter.

When to use a Soft Starter?

Soft Starters are generally used to protect the AC motor from the stress produced by a large amount of electrical inrush when the motor is started or stopped.

During start-up, high amount of inrush current is absorbed directly by the motor. Though motors are designed with the capacity of absorbing such inrush current, repeated Start and Stop cycles will definitely affect the operational lifespan of the motor. Not only that it affects the motor directly, but it also produces mechanical stress as an indirect effect of the sudden change from idle to operational state. This mechanical stress may produce rattling and high level of shock and vibration which may damage process connection, piping, instruments, and devices connected along the line. Soft Starters prevent these effects from happening by managing the electrical inrush and by providing a programmable start-up characteristic, specifically ramp-up and acceleration which is essentially Speed and Torque control.
Conversely, when a motor is stopped, the sudden and unmanaged change of speed from 100% to 0% produces similar stress to the motor and the system. Some soft starters are also equipped with different stopping modes, programmable deceleration and ramp-down sequence

When to use a VFD?

VFDs are popular choice for motor Speed and Torque control applications, however, it is also known that VFDs come with programmable Ramp-up, Acceleration, Ramp down, Deceleration, and varieties of Stop modes which are inherent features of Soft Starters. With this, it can be said that a VFD is also a soft starter.

Aside from featuring the capabilities of soft starters, the Speed and Torque Control function of a VFD provides a specific advantage, enabling the connected motor to be used in a variety of control strategies especially in automatic closed-loop control of motor utilizing Proportional + Integral +Derivative (PID) control algorithm.   

Construction / Operation

Soft starters are mainly composed of Thyristors or Silicon Controlled Rectifiers (SCRs). These SCRs are active during the ramp-up sequence and restrict current in OFF-state. During ON-state, it permits current then automatically switches flow to the contactors when rated or maximum speed is achieved.
VFDs are made of three (3) parts; Rectifier, Filter or DC Bus, and Inverter. The Rectifier is a standard network of Diodes used to convert AC power to DC power. The converted DC power is transferred to the Filter or DC bus to further condition the converted power. The DC bus voltage is then fed to the Inverter circuit where an output, similar to AC waveform, is provided through Pulse Width Modulation (PWM). This output resembles the sine wave of a real AC power which can be modified and characterized depending on the specific configured parameters of the VFD.

VFD vs Soft starter

To differentiate further VFDs and Soft starters, practical points may be taken into consideration. These points not only provide comparison points but may help guide users in deciding which device to use in their system.

Application

Soft starters are ideally used in applications where analog speed control is not required. As Soft starters are only capable of Speed and Torque control during start-up (and shutdown if supported), use this device in applications where fixed speed is sufficient to maintain control of the process or simply, where an ON/OFF motor control is applicable. Some compatible applications include conveyor and belt-driven devices or equipment as well as gear systems. 
VFDs are used where conventional ON/OFF motor control is not sufficient, particularly, where process variables are constantly changing, or process loads are fluctuating. As previously mentioned, VFDs permit closed-loop control using PID algorithm thus, Speed and Torque can be modulated and maintained anywhere from 0-100% of the motor’s rating. Some applications that require motor Speed or Torque modulation include industrial fan and pumps, Dampers, lifting, and material handling systems. 

Control Customization

VFDs simply outmatched Soft starters in terms of control customization. As VFDs are capable of implementing more varieties of control, it can meet simple or complex control requirements by customizing its built-in programmable parameters.

Embedded I/O

VFDs are typically equipped with more variety of I/O types enabling remote and local operation of motors. These are commonly composed of discrete I/O channels that permit carrying out basic motor control commands such as Motor Start / Stop; JOG / Forward / Reverse; Remote / Local operation. Additionally, advanced control can be implemented via the usually provided analog I/O channels that function as speed reference speed feedback. Aside from this, programmable I/O are usually also included which aid in the customization of control. Soft starters also have embedded I/O channels however, not as much variety compared to the embedded channels of a VFD.

Energy Savings

Energy savings is one of the primary reasons why Soft starters or VFDs are utilized. As previously mentioned, these devices limit electrical inrush, therefore, lowers current consumption. In actual comparison, VFDs offer more varieties of energy savings for lowering operational expense. Since VFDs can control Speed or Torque anywhere from 0-100% of the motor rating, VFDs allow motors to operate at a minimum or maximum speed as well as within an identified optimum speed of the process. These prevents the VFDs from cyclic start and stop and motor hunting.

Size

In terms of size or form factor, Soft Starters are generally smaller compared to VFDs when compared according to current rating, however, size matters only if Speed or Torque control is out of the selection consideration.

Cost

As Variable Frequency Drive offers more complex features; it is typically purchased at a higher cost. Additionally, since Soft starters are smaller, it requires lesser panel space, therefore, contributes to lower overall cost of installation. 

Accessories

Variable Frequency Drives typically require more accessories installed to its incoming and outgoing connections. Each of the internal circuitry (Rectifier, Filter or DC Bus and Inverter) may generate or prone to electrical noises therefore, VFD accessories may need to be installed such as AC Line filter, Harmonic filter, DC Choke, and Braking resistors. Soft starters may need lesser accessories as this device generate lesser electrical noise due to its simple operation.

CHARACTERISTIC VARIABLE FREQUENCY DRIVE (VFD) SOFT STARTER
Protective Function Start-up, Operational, Shut-down sequence Start-up and Shut-down sequence
Application Modulating Speed and Torque Motor Control On / OFF Motor Control
Control Customization Highly Customizable due to wide programmable parameters Customizable; fewer programmable parameters than a VFD
Embedded I/O Wide I/O covering basic  and advanced motor control Fewer I/O
Energy Savings During Start-up, Operational and Shutdown sequence During Start-up, Operational and Shutdown sequence
Size (compared with appropriate current rating) Larger Smaller
Cost Higher Lower
Accessories More may be needed (AC Line filter, DC Choke, Harmonic filter, Braking resistor) Fewer needed due to simple circuitry

VFDs and Soft starters are electronic devices that are used to improve control efficiency. Not only that these devices promote efficiency, but also contribute to increased uptime of the driven motor and the process it serves. At the same time, these devices help in maximizing lifespan by preventing unwanted electrical and mechanical stress to be absorbed by the motor. 
VFDs and Soft starter have commonality in features however, VFDs offer more functions compared to Soft starters, particularly continuous modulation of Speed and Torque. Though VFDs may be capable of implementing more complex task, this doesn’t mean that VFDs are always the ideal option. Selection highly depends on the suitability and adherence to the selection factors mentioned in this article.


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