Soft starter 101: How do they work?

Soft starters are designed to increase the voltage in steps and keep the motor from receiving the full voltage right away, which saves the motor from damage over time.

By David Manney, L&S Electric May 16, 2017

Motor damage takes place as a result of mechanical wear and tear and huge inrush currents. Typically, variable frequency drives (VFDs) help prevent wear and tear to mechanical components. VFDs slow starting and stopping of the drive train. A soft starter is a viable option if it is impossible to use a VFD with a motor. A soft starter minimizes the initial impact that a motor receives upon starting. The soft starter eases that impact once the operation stops.

How do soft starters work?

Motor-driven applications rely on VFDs or soft starters. These devices prevent damage, or a significant amount of stress, to the product transported by the machine. Among the types of equipment that benefits from a soft starter are pumps, fans, and conveyors. Plus, other applications such as moving walkways and escalators use soft starters to save energy. This equipment stops and starts on an automatic basis when necessary.

What soft starters do is to increase the voltage in steps, depending on the application. Thus, motors do not receive the full voltage right away. The delayed power spares them from damages over time.

Unlike a variable speed drive (VSD), a soft starter does not change the motor speed. There are thyristors such as silicon controlled rectifiers (SCRs), or a pattern of solid state switches in a soft starter. These components allow the voltage to increase at a slower rate and cause the motor to run at full speed at differing intervals.

One, two, and three phases

Three-phase motor applications utilize SCR-SCR and SCR-diode designs. The SCR-SCR offers a full wave control. The SCR-diode tends to generate undesirable harmonics and requires a higher start current.In a single-phase unit, it fails to reduce the start current, although it controls the start torque. This issue is why this unit is not suitable for applications that handle high inertial loads or frequent cycling. As for a two-phase unit, a motor needs a circuit breaker or thermal relay for protection. Moreover, this unit does not isolate all the phases of the motor.

Unlike the single- and two-phase unit, three-phase units are capable of providing full and maximum control. Thus, these units provide the best control of both the torque and the current.

Working with open or closed loops

A soft starter either runs an open or closed loop. An open loop design does not provide any current feedback loop. The open loop controls the starting phase with a pre-selected voltage profile without protecting the motor. On the other hand, a closed loop design provides current functions and motor protection. This combination allows users to select the preferred start current level.

The start voltage profile on open loop systems follows a pre-determined ramp regardless of the motor speed or the current drawn. The most suitable setting offers the breakaway torque to the electric motor upon starting. Although these starters make the motion gentler, they are not capable of producing any specific torque. In the case of a closed loop starter, it monitors the output and adjusts the input voltage automatically to reach the target voltage.

Other closed-loop units only adjust the voltage to maintain the constant acceleration. There are voltage ramp systems that monitor the input current on one phase. The next step is a comparison to a set point. The final step includes ramping once there is a higher amount of current generated.

When it comes to machines that require varied onset torque such as load conveyors, a current-ramping type of soft starter is advisable. This device takes the current from the start value to a predetermined limit over a specific length of time.

For the most part, applications that use 75 HP or more benefits from soft starters. Some of these include grinding mill machines and paper manufacturing machinery. This equipment needs to reach the full load within a defined period to prevent damages to certain equipment located in the drive train. A limited level of acceleration is necessary to avoid too much gear teeth stress on heated cylinders on paper dryers, as an example.

Why install soft starters

There are several advantages linked with the installation of soft starters working with motors. As a primary benefit, these devices promote the smooth starting of a motor without glitches or jerks. This method of starting motor results in controlling the starting current in steps by altering the initial voltage.

Other reasons to install a soft starter in a motor include improved efficiency and controlled acceleration. These benefits translate to increased productivity due to fewer breakdowns in motors and unplanned downtimes.

David Manney is a marketing administrator at L&S Electric. This article originally appeared on L&S Electric Watts New Blog. L&S Electric is a CFE Media content partner.

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