No industry standard employs the term “soft starting”. In popular usage, it means reduced voltage starting of a motor. That’s possible in a half-dozen different ways.
The most recently developed method of reduced-voltage starting uses an electronic device called a “solid-state starter,” or SSS as we’ll abbreviate it here. Many people have assumed, incorrectly, that “soft starting” and “solid-state starting” are the same. They are not. Just because an animal has four legs doesn’t mean it’s a horse. Likewise, just because a motor controller provides reduced voltage for starting doesn’t mean that solid-state electronics is involved.
The solid-state starter is capable of varying only the voltage applied to the motor. It cannot vary the frequency, and therefore cannot affect motor speed. It is normally bypassed, exerting no further control over motor operation, once that motor reaches full speed.
In contrast, an adjustable speed drive or ASD uses some of the same electronic hardware to vary both voltage and frequency applied to the motor. That results in some important differences between the ASD and the SSS. We’ve already mentioned two: the impossibility of speed control with the SSS, and that control’s lack of any influence over motor performance at full speed.
Here’s another important difference: The ASD can be used to start the motor from rest, just as the SSS does, but in a different way. The variation of both voltage and frequency allows the motor to develop high torque — almost up to the stalling or breakdown value — at any speed, including zero rpm at the instant of starting. At worst, the motor can put out at least 100% of its rated torque throughout the entire acceleration. Contrast that with acceleration on a SSS, during which the available motor torque is reduced at least by the ratio of applied voltage squared, which may mean half or less of the value available with ASD starting.
If a start is “severe,” in the sense that the torque required by the load is high during acceleration, using the SSS — which reduces the motor’s own output torque — just makes matters worse. On the other hand, by allowing the motor to develop much higher torque, an ASD start can greatly reduce the thermal stress in the accelerating motor. At the same time, the burden on the power system is reduced, because that torque is supplied at only 100% to 200% of rated motor current instead of the 600% to 800% normally demanded at full voltage and frequency, or the 300% to 500% for the SSS at normal frequency.
In light of all those advantages, why not use an ASD to handle any difficult motor starting situation? The simple answer is that the ASD costs several times as much as the SSS, which in turn can be four times as expensive as a conventional motor starter. The ASD is “overkill” unless speed variation is needed. Also, the ASD can introduce harmonic or voltage transient problems for both power system and motor. Any such phenomena originating with the SSS are a concern only during the brief starting period.
Summing up: Both SSS and ASD have their practical and economic places in motor control. Don’t confuse one with the other. And don’t use either one unless you really need the particular characteristics each can provide.
Richard L. Nailen, P. E