Employing an Adjustable Frequency Drive with a Soft Starter Bypass Improves System Reliability

Today’s comprehensive drive packages typically combine several solutions to provide a reliable and robust system. Using a drive with a soft starter built into the bypass offers a spectrum of advantages. In general, soft starters are designed for customers requiring reliable and efficient soft-starting of three-phase motors.
Advantages of using a soft start include:
  • Minimizing shock to mechanical components
  • Extending the life of the system
  • Increasing reliability
  • Reducing downtime
  • Lowering costs
Lower in-rush current causes less stress on electrical components and junctions, further maximizing system life. A soft starting device reduces slippage, squealing and stretching, and extends belt life two to six times that of traditional across-the-line starter. Reducing the shock to products on conveyors and material handling equipment reduces the costs associated with damage to handled products. In pump applications, soft starters are able to soft stop the motor, thereby alleviating what is typically known as the water hammer effect.
Employing Contactors, Soft Starters, Adjustable Frequency Drives and Bypass Configurations
Contactors, soft starters, and drives can be used to control a motor.
Contactors: Simply connect the motor directly across the AC line. A motor connected to the AC line will accelerate very quickly to full speed and draw a large amount of current during acceleration.
Soft Starters: Used to slowly ramp up a motor to full speed, and/or slowly ramp down the motor to a stop. To reduce both current draw and the mechanical strain on the system, soft starters are used in place of contactors. By providing smooth acceleration and deceleration of the load, soft starters are able to reduce wear on belts, gears, chains, clutches, shafts and bearings. Many large pumps and fans require at least a 30-second ramp time to prevent mechanical damage to the system. Typically, soft starters are more common on larger horsepower systems. They are often critical for applications where an abrupt stop of the load may cause system or product damage.
Adjustable Frequency Drives: Have the ramping ability of a soft starter, allow speed to be varied, and offer more flexibility and a variety of additional features.
Enclosed adjustable frequency drives often include a bypass when used in applications that can not afford downtime. A bypass configuration consists of three contactors: one contactor disconnects the drive from AC power, another disconnects the motor from the drive, and the last contactor connects the motor directly to the AC line. See Figure 1 for a typical three-contactor bypass configuration.

Figure 1: Typical Three-Contactor Bypass Configuration
A bypass enables the system to run continuously, even in the event of a drive failure. A soft starter can be added in line with the bypass contactor that connects the motor directly across the line. See Figure 2 for a typical three-contactor bypass with an integral soft starter.

Figure 2: Typical Three-Contactor Bypass with Integral Soft Starter
If an application is using an adjustable frequency drive because a contactor is not acceptable, then a contactor-only bypass should not be acceptable either. Adding a soft starter to the bypass allows the motor to be ramped up to full speed, thus reducing the mechanical and electrical stress on the system.
Optimum motor control packages start with a drive to provide the most control, flexibility, and protection. Adding a bypass with an integral soft starter provides a backup system to run the motor should the drive fail. Large drive systems should have soft starters in the bypass, otherwise the application risks negatively affecting the power system, damaged bearings, or bent shafts once the bypass is engaged.

About The Author

Timothy R. Skell is an Application Engineer at Eaton

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