Doing some modeling as of late, I thought it might help to share some experience with motor soft starters since I was exposed to little to none of this in school. In distribution systems, particularly rural distribution systems, reduced voltage starters are a common device used to decrease “flicker” on the electrical system. There are 5 popular ways for the motors to start like this.
1. Auto Transformer
This is the most common type of reduced voltage starting. For light duty motors you’ll typically see taps at 30%, 37.5%, and 45%. Medium duty units require more power to get them turning, so you’ll typically see taps at 65% and 80% for units up to 50hp (37kW), and 50%, 65%, and 80% for units above 50hp. For motors in the heavy duty range, typically the taps are specifically designed for the application and the motor, since the motors get so expensive and their demand grows quite large.
Typically the motor is turned on with reduced voltage, under no or very light loading, and allowed to reach full operational speed. Then the motor is turned off (while being allowed to free-wheel) and then reconnected with full voltage. With a motor, V does not equal IR obviously, so the line current is reduced by the square of the voltage reduction.
2. Reactor Starting
Reactor starting uses a reactor in series with each phase of the motor, again until the motor is at or near full speed. At this point the reactor is removed from the circuit; typically they are just shorted and not physically moved or transferred. With reactor starting, line current is reduced proportionally to line voltage reduction.
3. Resistance Starting
Resistance starting is similar to reactor starting, however unlike reactor starting the losses are real (i.e. massive amounts of heat) and are typically more economical than reactor starting. Further the power factor of the motor is not adversely effected like it is with reactor starting.
While working at one facility, I came across a very novel concept of reactor starting involving moving blades and a conductive liquid. At the beginning of motor start, the blades were entirely out of the liquid, and the motor was stopped. As the blades entered the liquid, a circuit was completed, and as they moved through the liquid the resistance was lessened as there was more surface area for the conduction to occur. In this way the motor was “ramped up” to operational rotational velocity, and once there the circuit was shorted across and the motor was brought fully online.
4. Part-Winding Starters
Part-Winding starters use windings which are typically connected in parallel. This reduces starting current, and depending upon their design and construction can be either “full acceleration” or “increment” starters. Full acceleration uses one set of windings to bring the motor to operational speed. Increment starters use a series of windings to progress the motor up to speed.
5. Wye-Delta Starting
This method connects the motor in wye during starting, and then switches to a delta connection after acceleration has been completed. This reduces the voltage to about 57% of nominal. Torque is however only reduced 33%. When the motor is reconnected in delta, you can see large transients however. I’ve never come across this method and seems to be more popular abroad. (non-United States)
