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VFDs for Soft-Starting: Not Glamorous, but Necessary

Picture courtesy of Pumps and Systems Article:

Variable Frequency Drives (VFDs) can be used in a variety of applications for a variety of purposes. As a service engineer, I’ve seen some interesting applications using VFDs: tunnel boring machines, carnival rides, mechanical bulls, and electric cranes are just a few that come to mind. On a list of interesting tasks assigned to VFDs, using one for soft-starting is at the bottom, yet it is very common and often necessary. Because a VFD varies the voltage as well as the frequency to a motor, it can be used to “softly” ramp up A/C induction motors as opposed to across-the-line starts that draw upwards of 600 % inrush current for a short period of time.

An A/C induction motor has two main elements: the stator (the stationary windings) and the rotor (the rotational windings). When voltage is introduced to the motor, the stator is instantly saturated with voltage, creating a large difference between the magnetic fields of the stator and the rotor. It takes a lot of torque to get the rotor physically moving and overcome that initial difference and that torque requires a significant amount of current. This inrush can be terrible on the motor and other mechanical system elements like valves, seals, bearings, belts, and gear boxes (not to mention extreme stress on the guy pushing the start button). Aside from that, rural areas without a strong power grid are often unable to handle the additional load when motors, particularly large motors are started across-the-line.

VFDs virtually eliminate inrush current when applied to a motor, preventing power grid dips and reducing mechanical damage and stress. EMA was recently involved in a compressor project in rural Nebraska where a 10,000 HP, 13.8KV motor was unable to be started across the line due to the lack of grid power. EMA used a 5,000 HP Toshiba medium voltage drive with a 13.8 KV input simply to get the motor started unloaded. The VFD was able to eliminate the inrush and get the motor up to speed. Once up to speed, we utilized a synchronous bypass configuration designed by EMA engineers to put the motor across the line so the compressor could be fully loaded.

EMA Engineer Franklin Flores at the job site in Nebraska

In all total, the VFD is in circuit for under a minute, yet is a completely necessary element to this application.

Now I know you may be asking “But Trey, aren’t there soft-starters that can do the exact same thing?” Excellent question! While softstarters do exist and are often excellent options when low starting torque is needed, they are limited, and are only able to reduce inrush current to around 200-250% of motor FLA; for additional information, see blog here. This is obviously much better than 600% like an across-the-line application, but it often won’t do the job like a VFD will.

So, although it’s not the most exotic application for VFDs, soft starting is a very necessary one. By the way, we sell Medium Voltage VFDs! If you have an application where a Medium Voltage VFD is needed, we may have one (new or refurbished) that will work for you. Please call us to discuss further or for help deciding which option is best for you. No One, ANYWHERE Is Better At Drives Than We Are!


-Trey Mayfield, EMA Inc.


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