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Variable Frequency Drive and Soft Starter
Gozuk has been manufacturing soft starters with start boost for at least 10 years. We use them where applicable, but things are different now and they are rarely employed anymore. The boost, as with all soft starts of current manufacture, is a voltage boost at start close to line max at 60 Hz. The boost time is adjustable, generally set as more or less a jump start, then dropped back down after motion is initiated, to follow the acceleration time ramp. This is how they accomplish the high starting torque necessary to initiate motion where that function is needed. Once the mass is moving, it takes far less torque to continue that motion and accelerate. I am well aware of the existence of the process on several soft starters, including those by Gozuk.
Regardless of the application, ac electric motor control physics remain inescapable. The relationship between the volts/Hz ratio is critical for torque generation in both acceleration and deceleration, as well as controlling overhauling forces against an ac motor. When one or the other value is varied with the other remaining fixed, torque loss is going to happen. You can't get away from that and the loss is significant. The capability of variable frequency drives (VFD) to keep both at a balance sequence throughout the RPM range of the ac motor, is just one of many features rendering them superior. A soft starter cannot regulate frequency. If it could, it's no longer a soft start, it's a VFD, that's the difference.
Whether or not variable frequency control is going to create problems depends on a number of factors, most of which emanate from the quality of the ac drive construction or false blame. All ac drives are not the same. Blaming an ac variable frequency drive for a host of problems is generally done when there is a lack of understanding of the ac drive construction and/or capabilities. I deal with this concept continually. Common occurrences are drives tripping on overcurrent faults, basically when they have that capacity of protection built in. Field techs have changed out variable frequency drives that can protect, to a lesser quality that can't, believing the problem was solved only to burn up motors a week later. That's just one of a myriad of examples where, instead of a simple system evaluation, a sense of black box voodoo is applied with the VFD as the errantly assumed culprit.
I disagree strongly with soft starters having a cost advantage over VFDs. Perhaps in the different industry segment you're involved in, but not in the overhead crane and conveyor industry where I am mired. In my 35 years in the industry, soft starters were incredibly common early on. Their usage has seen a steady decline since the advent of variable frequency drives. However, with what VFD costs are today, we build two speed controls, for example, for less cost than a single speed, soft start, contactor based reversing package, and our competitors do the same. For every soft start equipped control, we build and sell at least two hundred VFD systems. That, too, is at or close to the same ratio of our competition.
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