What is motor “soft starting”? There’s no industry standard. Many people think of a “soft starter” as an electronic solid-state device for reducing motor voltage (and current draw) during acceleration. But there’s no difference in principle between that and any other “reduced-voltage starting” method — autotransformer, series reactor, etc. Each reduces the current (and therefore the voltage drop) in the power supply system during motor acceleration. Which kind of reduced-voltage or “soft” starter to use depends upon equipment cost, the amount of voltage/current reduction needed, and the motor design (for example, can the motor start its load safely at the proposed voltage reduction?).
Whatever the type of starter used, two benefits result. One is the mitigation of a severe voltage dip on the system when the motor starts. System limitations or utility rules may require this. A second benefit — though unimportant in some drives — can be the reduced mechanical shock to equipment resulting from lower accelerating torque.
Starter suppliers (especially of solid-state equipment) sometimes claim that a third benefit is reduced energy cost. Reduced starting current supposedly means the motor “consumes less power.” This is untrue. To accelerate a load consisting only of pure inertia requires expenditure of a fixed amount of energy within the motor. At reduced voltage/current, the acceleration takes longer — but the energy expended is the same. When load torque is also considered, as well as inertia, the amount of energy expended is actually increased when motor torque is lowered. (The ultimate example is a voltage so low that the motor cannot accelerate past, say, 3/4 speed. Energy input becomes infinite in theory, because the acceleration can never be completed. If the motor isn’t tripped off the line, it will eventually burn out.)
What’s usually meant by this supposed benefit is a reduced demand charge on the power bill, because it’s claimed that the “soft start” will lower the motor’s peak demand. Utility engineers have been debunking that fallacy for years, but it persists.
In the first place, kilowatt demand is normally metered only for the larger customers — facilities using large blocks of power. Many industrial and commercial customers have no such metering.
In the second place, when a motor starts, its current peaks at 6 to 8 times rated. But motor power factor during acceleration is so low that power drawn from the line seldom rises more than 10-20% above the full-load value. And that occurs for only the brief period during which the motor is starting. Such a small “blip” in kilowatt demand will not noticeably affect the reading of the usually 15-minute integrating demand meter.
Use soft starters, then, when it’s appropriate — but don’t expect them to influence the monthly utility bill.
Richard L. Nailen, P. E