What, actually, is “soft starting” of an electric motor? There are several types of soft starting methods, and no industry-standard definition exists.
In years past, common types were electro-mechanical in nature, i.e., they used multiple electro-mechanical contactors to apply line power to the motor in ways that reduced the current draw at the time the motor is started. In Brithinee Electric’s marketplace, probably the part-winding-starter was most common. This starter applies full line-voltage to half of the coils in a motor, which causes the motor to begin rotation. After one to two seconds, another contactor is engaged that applies full voltage to the remainder of the motor’s coils, and the motor accelerates to full speed. Other types of electro-mechanical soft starter are autotransformer and the big favorite outside the United States, the wye-delta starter. Some of the electro-mechanical soft starters require that the motor have a particular internal connection.
Because of declining costs for electronic components, probably the favorite in the USA today is the solid state starter. With the use of thyristors, which are solid state switches, the voltage is effectively “ramped” from a starting voltage, say 50% of line voltage, to full voltage, over a length of time, usually about 15 to 30 seconds. Note that the power is delivered to the motor at full line frequency, which is 60 cycles per second in California. Also note, no special connection is required of the motor; it is started with the same connection that’s used to run it.
A motor soft starter reduces the voltage drop which occurs every time the motor starts. In addition, it reduces the torque that the motor delivers to the load at start-up. Electrically, line flicker is reduced, and your electrical utility definitely does not want your motor flickering your neighbor’s line. Mechanically, wear and tear on power transmission equipment is reduced.
Some installations are located at the end of long power transmission lines and require extremely low starting current, which can only be achieved using a variable frequency drive (this controller ramps up both voltage and frequency). In most cases, the cost of this solution is a multiple of the cost of a soft-starter, but sometimes it is the best solution.
For insight into whether a soft starter reduces energy consumption, visit our website and take advantage of the lucid articles prepared by Mr. Richard L. Nailen, P.E., who originally wrote the articles for the benefit of customer service representatives of a major electrical utility.
Call our control’s department at 909-825-7971 – extension 35. Our service and support will help keep things running smoothly and efficiently. By Donald P. Brithinee