What is the difference between a VFD and a Soft-Start?

One of the most common misconceptions that we hear from those that may be newer to the realm of industrial electronics is that a Soft Start and VFD (Variable Frequency Drive) are essentially the same device. The confusion is not really that surprising as there are dozens of different industry terms for motor controls that practically have the same function. But Soft starts and VFD’s could not be more different electrically and serve two very different roles.

What is a Soft Start?

In Motor Controls, a starter is a device which is used to turn the power to a motor on and off. In its simplest form a direct starter simply applies full voltage when on and conversely cuts off power to coast the motor to a stop. This direct startup at full power – or what we call inrush current – is generally unappealing in most applications as it applies a significant amount of mechanical shock to the motor.

A Soft Starter solves this problem ramping the motor up slowly, reducing the impact of the high amounts of current it would normally be subject to during start up. It does this through the use of devices called SCRs – or Silicon Controlled Rectifiers – that can restrict or allow current based on their “on or off” states. This control of current allows the limiting of motor torque during start-up and reduces the amount of stress the motor undergoes. This is helpful for many applications that must endure sudden unexpected spikes of mechanical stress such as a pump application where fluid pressure can change dramatically or conveyor applications that can receive unexpected loads either during operation or at start up. The limiting factor of a Soft Start is that it cannot be used to control speed once full voltage is applied. They are mostly only used to start and stop a motor safely.

What is a VFD?

A VFD or Variable Frequency Drive is a type of motor control that takes a power frequency coming in from an input source and converts it to a frequency that is variable. AC induction motors are designed so that at a known frequency, in the US the common being 60 hertz, the motor’s shaft will rotate at a certain RPM, (rotations per minute) depending on the configuration of its internal windings. The ability of a VFD to vary this frequency is how it can be used to control the speed of a motor. Further refinements such as resolver or encoder options allow the VFD to also be able to control shaft position as well. While with a VFD you have more control of a motor, typically you will not have as much surge protection as you would get with a Soft Start of comparable specifications.

If you have any trouble with any of your motor controls, whether it be a Soft Start or VFD, give us a call! Also be sure to visit us online at gesrepair.com or call us at 1-877-249-1701 to learn more about our services. We’re proud to offer Surplus, Complete Repair and Maintenance on all types of Industrial Electronics, Servo Motors, AC and DC Motors, Hydraulics and Pneumatics. Please subscribe to our YouTube page and Like Us on Facebook! Thank you!

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