Motor Starting with Soft Starters

Induction motors make up 38% of the total load on the US Grid. Industrial 3-phase induction motors make up 17% of the total U.S. electrical energy used and within the industrial sector about 62% of the energy used goes to motor driven equipment. This is according to the U.S. Department of Energy. Motor starting accounts for a major source of power network instability and maintenance headaches. When an AC motor is energized a high inrush current occurs. Typically, during the initial half cycle, the inrush current is often higher than 20 times the normal full load current. After a fraction of a second the motor begins to rotate and the starting current drops to 5 to 8 times the normal current. This type of starting can be analogous to 'dropping the clutch' on a manual transmission car or otherwise called a 'hard start'. The U.S. power grid runs at a fixed rate near 60Hz. Depending on how strong your system that motor will get an inrush of current and torque in an attempt to sync with the 60Hz source. In a hard start this trouble happens essentially instantaneous. System voltage drops in response to the current inrush until the electrons can catch up to the demand and/or until the motor reaches the normal running speed and nominal current. This stress is felt at every point, including the motor itself, the conductors and electric machines (i.e. transformers and generators) supporting the motor. This requires consideration when setting over current protection devices and designing in the ampacity of the system from conductors to electric machines. Ampacity has a linear cost. Instantaneous mechanical force or instant torque has a cost, as well. A hard start results in mechanical and thermal stresses within the motor, as well as, mechanical stresses to the drivetrain and mechanical load. Particularly, if the drivetrain has any looseness or play, repeated hard starts can result in excessive stress and wear - causing a big maintenance headache.

All of this can be reduced by limiting the current inrush or soft starting the motor. Returning to the manual transmission analogy, when the driver of a manual transmission gradually engages the clutch less inertia is required by the power plant. The more gradual the engagement, the less throttle is needed and the jarring effect is greatly reduced. Power plants, gear boxes, drivetrains and tires - all see greater life and higher reliability by easing into a start as appose to 'dropping the clutch'. Don’t drop the clutch unless it’s a rental car.

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Waveform Capture of a Typical Soft Start.

Not only is the inrush reduced by half (when compared to a Hard Start in the example below) but the maximum current happens near the end of the start process.  At this point the motor is well on its way to full speed and the drive train is fully engaged.  This slow ramp up also allows for the electric network to keep up and maintain the voltage.

Example Current Waveform of a Hard Start 

Peak Current is almost instantaneous and does not allow for the Network to maintain voltage or drive train to engage.