Trying to decide between soft starters and variable frequency drives (VFDs) technology? The choice can be tough.
As a 37-year industry veteran working for Schneider Electric, a world leader in automation and control, I’ve helped hundreds of customers with this comparison. This blog focuses on the pros and cons of VFDs and soft starters.
When you should use a soft starter
Soft starters are a type of motor control device that provides slow ramp-up (soft start-up) of the motor to moderate mechanical shock and electrical peak demand. They are ideal for the following applications:
- Constant full-speed operation such as a continuous running conveyor belts
- Low or medium starting torque for gradual controlled starting, which is required to avoid shock and tension in certain mechanical system, such as inclined conveyors
- Lightly loaded conveyors having ten or fewer starts per hour
- When you are replacing direct online starters to prevent damage during starting and stopping on a conveyor
What are the benefits of soft starters?
- Reduce costs: Soft starters are typically less expensive than VFDs. Additionally, because soft starters provide a smaller footprint, it requires less panel space than a drive, as well as fewer internal and external components, which can lower maintenance costs.
- Protect the motor: Soft starters protect a motor by minimizing the in-rush starting current it receives, specifically by reducing the voltage applied to the motor. It uses silicon-controlled rectifiers (SCRs) to gate the voltage on and off during the starting cycle. It then switches to line power during the run cycle.
- Shrink energy bills: The in-rush of current provides energy demand benefits. Many utilities have peak demand charges, which result in excess billing for momentary demand loads, including motor starting. The use of a soft starter can help reduce peak demands, and as a result, lower energy bills from utilities.
- Extend equipment life: Soft starters can prevent mechanical shock, and many can provide a deceleration on the stop cycle, helping extend equipment life. For instance, on a belt-driven conveyor, belt life will be significantly extended with a soft start.
When you should use a VFD
A VFD controls AC motor speed and torque in an electromechanical drive system. It varies and regulates motor input frequency, as well as voltage supply and current supply to the motor. They are best suited for the following applications:
- Dynamic torque control and perfect speed control
- Conveyor systems where speed needs to be maintained regardless of load variations
- Conveyor or sortation applications with high starting torque
- Conveying systems requiring continuous feedback for critical position control
What are the benefits of VFDs?
- Eliminate mechanical shocks. VFDs provide constant torque across the operating range and give continuous feedback that is useful for critical position control, helping to eliminate mechanical shocks.
- Provide more control: In power distribution applications, a large motor starting at full voltage can place a high demand on the system, which can trigger voltage dips and increase wear on the motor. A VFD addresses these issues because it varies the frequency and voltage supplied to an AC motor, improving control of a motor’s acceleration, change of speed, and deceleration.
- Reduce power consumption: VFDs are designed to reduce power consumption when the motor is not needed. They can optimize business and process outcomes through improved life-cycle asset management and optimized energy consumption.
- Prolong equipment life: VFDs allow the motor to operate at variable speeds, which can improve processes, conserve energy, and extend the life of conveyor equipment life.
Energy efficiency comparisons of soft starters vs. VFDs
- Soft starters: Efficiency depends on sizing and the three-phase voltage applied. When operating at full speed and adequately loaded, soft starters are more efficient than VFDs. With an integrated bypass, current in the soft starter is carried across the contactor, so it runs cooler, as no active solid-state components are generating heat.
- VFDs: Active components such as insulated-gate bipolar transistors (IGBTs) stay on during run and stop functions. Active components are always controlling frequency and voltage, so these drives are inherently are hotter during operation.
Soft starters and VFDs pose different implications for harmonics.
- Soft starters. Silicon-controlled rectifiers (SCRs) turned on during starting and stopping provide partial voltage amplitudes and partial sine waves. In bypass condition, with the SCR switched off after start-up, almost no harmonics are generated.
- VFDs: Long cable or wire runs from a drive to a motor can create reflected wave issues, and wire type also must be considered. Line reactors, DC chokes, or a passive filter will prevent harmonics from feeding back to the power source and harming equipment.
Frequently asked questions: application considerations
When do you use soft starter vs. AC drive can be a challenging question? Make your final selection by examining the application with the following questions:
Question: Does the application require reduced voltage, current, and torque while starting? Answer: Either a soft starter or an AC drive can provide this feature.
Question: Does the application need full torque at zero speed?
Answer: An AC Drive can provide full torque at zero speed where a soft starter cannot.
Question: Does the application need speed control once the motor is at speed?
Answer: AC Drives are better suited to control speed of the conveyor once running.
Question: Does the application need precise starting and stopping times?
Answer: Although a soft starter can aid in precise starting and stopping, the control is limited when compared to an AC Drive.
Question: Is space a consideration?
Answer: Soft starters can provide a smaller footprint solution.
Question: Is cost a consideration?
Answer: Generally, AC Drives cost more to implement than soft starters.
In the table below, you can see a comparison of features for soft starters and VFDs.
|Reduced voltage, current, and torque while starting||Yes||Yes|
|Slow speed capabilities||Limited||Yes|
|Does the application need precise starting and stopping times?||Limited||Yes|
|Size of controller||Smaller||Larger|
|Full torque at 0 speed||No||Yes|
In this article, we focused on soft starters and variable frequency drives (VFDs) and the unique advantages of one technology over the other.
We find the common application for soft starters include full speed operation, applications with low or medium starting torque, and lightly loaded applications. Soft starters can help to reduce mechanical wear and damage to system. They are also ideal for controlling inrush current and power monitoring applications.
AC drives are commonly used for applications requiring speed control during the run mode, single-phase applications on certain drives, and applications with high starting torque. They provide continuous feedback for critical position control and constant torque at zero speed. An added benefit includes reduced mechanical wear and damage to the conveyor system.
 Based on internal data, not a guarantee of future performance.