There are multiple ways to control an AC motor. For example, a direct starter, also known as a full-voltage non-reversing (FVNR) or across-the-line starter, switches a constant speed motor on and off. Because the full line voltage is supplied when the motor is being started, a high inrush current is applied to the motor, and both the motor and the controlled equipment experience significant mechanical shock.
A full voltage reversing starter (FVR) switches a constant speed motor on and off in both the forward and reverse directions. Again, since it this is full-voltage starting method, in-rush current and mechanical shock are high.
The typical in-rush current for a motor being started with a full voltage method is 600% or more of its full load amp rating. This currently drops off quickly as the motor comes up to full speed. However, the power system must be sized to allow for the potential current surge. A Soft Starter may be a solution to help reduce this stress on the electrical power system.
A Soft Starter is a reduced-voltage solid-state starter (RVSS) that smoothly ramps the motor up to full speed, reducing the in-rush current and mechanical shock. A Soft Starter can reduce the in-rush current to 200-300% of the motor full load amp rating. This starting time or ramp or ramp is typically adjustable which allow for a more controlled start. However, this ramp time is dependent on the motor load, and may vary. Soft Starters can also be configured with a “Soft Stop”, to control the ramp down time to gradually stop a motor. This will reduce the time it takes an unloaded motor to come to full stop, during the testing process. Additionally, some Soft Starters include integrated safety features that can be utilized for emergency stop procedures.
Soft starters, however, cannot control the speed of the motor after it has reached full voltage. If speed control is required, an VFD will be required.
The Soft Starter allows the Motor Test Lab to start the motor being test at a reduced voltage. It replaces the need for multiple contactors to manually step the motor up from a lower starting voltage up to full voltage. The reduction of inrush current will also help to reduce brownouts or dimming of office or shop lighting when starting larger motors during the testing process.
