At EMA, we get this question a lot: “Do I need a softstarter or a VFD for this?” and the purpose of this article is to help answer that. Softstarters and VFDs both control induction motors but there are several distinct differences that are important to understand when determining which is best for you; some of the major distinctions are the application, available power grid, and starting torque required.
The application of the motor is a major consideration when determining whether a VFD or a softstart is necessary. A softstarter is a much less complex device than a VFD and therefore is typically on much less complex applications as well. If your application needs any kind of speed control whatsoever then a softstarter will not do the job. Softstarters use SCRs or Thyristors to reduce the initial voltage which limits initial current during startup, but once the motor is up to speed, the softstarter has done its job. Most softstarters close bypass contactors once the motor is up to speed, switching the motor to line power. Because the electronics are only in the circuit a very short amount of time, softstarters will typically last for a very long time and require minimal maintenance. Most softstarters are not designed for continuous duty so they are limited to a certain amount of starts per hour. Because softstarters utilize the phase rotation of the utility feeding it, reversing contactors are needed if you want to run the motor in forward and reverse directions.
VFDs for speed
VFDs on the other hand are designed to run the motor at any speed for any duration of time, however, auxiliary cooling may be necessary on the motor if it is running at very low speeds. If your application requires the motor to run at ANY variance of speed, then a VFD is necessary. VFDs work by creating a pulse-width modulated output waveform created by transistors, which varies the voltage as well as the frequency to the motor. Because VFDs create their own output by drawing power from the DC bus, they can regulate the switching pattern at any time to increase the speed, decrease the speed, or even reverse the direction of the motor without any additional, external components.
Available Power Grid
There are times when an application is perfect for a softstarter but the available power grid simply cannot handle the inrush current used by softstarters on the initial start. Although softstarters significantly reduce the inrush current on starting for motors, the only limit it to 200-250% inrush current. There are areas, typically in rural areas, where the power grid cannot handle that, the initial inrush pulls the line down. In situations like these, a VFD may be the only option because a VFD limits the inrush current to 100-115%. There was an interesting situation recently in Nebraska where EMA used a VFD for this very reason.
Starting Torque Required
In the world of variable frequency drives and softstarters, there are 4 major types of loads:
- Constant Torque Load: Torque Loading is NOT a function of speed
- Variable Torque Load: Torque Loading IS a function of speed
- Constant Horsepower Load: Loading is typically above base speed (rare)
- Intermittent Load: Loading is intermittent, not connected to speed
Constant Torque vs Variable Torque
For the purposes of this article, we’ll focus mainly on constant torque verses variable torque and why that is so important to understand when determining whether a softstarter or a VFD is the most applicable for you. Variable torque loads (mainly pumps and fans), require very little torque to get started and the torque needed increases exponentially as you increase the speed on the load. These basic affinity laws are why VFDs on variable torque applications save large amounts of energy.
Softstarters are typically designed for variable torque loads because by nature, they reduce the starting torque during start as well as the current and voltage.
Constant torque applications are more common in industrial settings on loads such as conveyors, extruders, mixers, reciprocating compressors, etc. These applications require full load at any speed, including low speeds, making them ideal candidates for VFDs even if speed control is not necessary.
VFD by nature allows full load torque at any speed so starting torque is not a problem as it is with softstarters. Applying a softstarter to a constant torque application can lead to frustration and stalls because the motor may simply not be able to generate enough torque to get the initial load going.
Still, confused? Contact us by giving us a call: 770-448-4644, email us or start a chat by clicking the chat to your right. We pride ourselves in being unbiased, third-party solutions providers in all things VFDs and softstarters. Let us prove to you that “No One, ANYWHERE Is Better At Drives Than We Are!