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Gaining Inertia Gradually
Electric motors are heavy. They spin on a shaft. It takes extra energy to start their motion. For example, it generally takes much more energy to move a motor from a dead stand-still up to 500 rpm than it does from 2,000 to 2,500 rpm, even though at the higher rpm, the motor is doing more work. Soft starts help start the motor with less energy by getting the motor up to speed gradually or incrementally.
Electricity Requirements
If you turn all the lights on in a home shop, run a compressor, then start your table saw, you might see your lights dim. It may even trip a circuit breaker. The example demonstrates what happens with large-motor hard starts. Electricity drawn right when the motor is switched on is many times higher -- maybe six or eight times -- than the electricity drawn just to operate it. If the motor is large enough relative to the power supply, a hard start will rob current from everything else connected to that power grid.
Mechanical Stress
When powerful electric motors are "hard started," it jars every direct mechanical connection in the motor. It puts additional strain on the bearings in the motor. It also puts more strain on whatever mechanisms are connected to the motor. For example, if the motor runs a saw mill, a hard start may mechanically jar and strain parts of the mill assembly.
Ways of Soft Starting
There are a variety of soft starter types. Some limit voltage so there is one or more steps in the starting process. This is analogous to starting at, say, one quarter throttle, letting the motor get up to speed, then stepping up again. Some soft starters are mechanisms to start, then gradually increase the voltage as the motor accelerates to full speed. If a motor isn't designed to soft start or retrofitted with a soft starting device, but it is connected to a transmission, the concept can be employed by reducing the load on the motor via the transmission. Lower gears will draw less voltage than higher gears.