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A soft starter is used to reduce inrush current, thus reducing the load on electrical circuits that supply power to motors. Inrush current is the initial surge of current into the windings needed to overcome inertia when the motor is being started. If yuo're worried about the current requirements of your motor during startup, a soft starter is an easy solution.

Soft Starter

Why use a Soft Starter?

A soft starter temporarily reduces the load and torque during startup, reducing stress on the motor, as well as the power cables and electrical distribution network.

Unlike an across-the-line soft starter, the voltage will not immediately spike, and torque can be controlled.

While VFDs give more precision when it comes to ramping up voltage, they will not automatically go above rated voltage, and may not provide the power to start up a motor.

A soft starter also provides protection to machines with a high inertia. The soft starter will slowly ramp up the speed, still going above rated voltage to start up the motor, but not immediately, which may result in overheating or stress on parts of the motor.

Soft Starter Types

WEG SSW07 Digital Soft Starter

Digital Soft Start

The most commonly used type of starter. They provide longer performance with virtually no maintenance, since they don't have contacts that can burn out or armatures that can break. Solid state starters (also called digital soft starters) have integrated motor protection, which other starts do not have. They can include digital programming as well as other control features.

Digital soft starter details »
Reduced Voltage Soft Starter

Reduced Voltage Start

Reduced voltage start systems often use autotransformers, which can be used to adjust the voltage during starting. Once the motor gets up to full speed, the autotransformer will then be switched out, allowing the motor to run at full voltage.

The autotransformer has taps, or preset voltage allowances, that limit the voltage to either 50%, 65%, or 80% of full-voltage.

Reduced voltage start details »
Part Winding Soft Starter Motor

Part Winding Start

Loads that require for low starting torque, such as low inertia fans and blowers, can be started using part winding starters. The motor will initially draw full voltage but only 65% to 70% of full current.

A part winding system does not provide internal motor protection, so it must be added separately.

Part winding start details »

Other Types of Starters

Some other starters include variable frequency drives (VFD) and across-the-line starters, which differ from each other in how they start up a motor.

Variable Frequency Drives

Both VFDs and soft starters will limit the starting current of motor and are designed for applications that require the inrush current to be limited during acceleration. This may be due to limitations in available power or the negative effects of inrush current and the resultant voltage sag on machinery surrounding the motor that is starting up.

Typical motors have start up current requirements anywhere from 600 to 800% above rated current, but a soft starter can reduce it to as low as 150% above the rated full load current. Depending on the starter and the load on the motor, inrush current may go up to 400 or 500% above rated full load current.

In comparison, a VFD is used to slowly ramp up the speed of the motor by controlling frequency, voltage and current. By doing so it will control the load applied on the motor as well as the current drawn by the motor. You can program the VFD to limit the current so it never even reaches the full load amperage. Unlike soft starters, VFDs can provide full torque even at low motor speeds.

Across-the-line Starters

Across-the-line starters, also known as direct on-line (DOL) starters, will give the motor full voltage, achieving full inrush current and locked rotor torque. This is the most common way to start a motor.

An across-the-line soft starter typically has inrush currents up to 700% higher than the running current, but can be as high as 1000% above. They also have starting torque that is 300% higher than running torque.

The high torque puts stress on the motor and can reduce its service life, while the high current puts stress on the power supply and can lead to voltage dips.

How do I select a soft starter?

View our Motor Soft Starter Selection Help page for more information on how to select a soft starter. Our guide will go over the specifications you will need to pick the rigth starter for your application.

Call us at 877.474.8209 to speak with an application engineer about your project.