A soft starter is an excellent addition to an AC electric motor, but it's essential to understand your needs and choose the right one. There are five different types of soft starters, each offering its own unique benefits and applications. In this blog, we'll explore these soft starters and the various manufacturers to help you make the right choice, as well as when to utilize these in your AC electric motor.

What is a Soft Starter?

Soft starters are devices added to AC motors to provide a different startup method for reducing the electrical and mechanical stress that would otherwise come from the typical full voltage power-up. Essentially, the soft starter, or reduced voltage starter, is placed between the motor and the incoming utility line, applying increasing voltages to the motor slowly and gradually for a smoother acceleration rather than the usual burst of power. This smooth acceleration helps prevent damage to the motor as well as the machine itself. Soft starters also reduce wear on the motor's circuits and help reduce torque. The soft starter amperage will depend on the type used, although some models offer the ability to adjust both the starting voltage and the length of time until the motor speed reaches full power.

Soft starters help to reduce energy usage, lower the risk of a power surge, offer adjustable acceleration times, increase the possible starts per hour, reduce overheating risks, improve operating efficiency, and extend the lifespan of the motor.

The Five Main Types of Soft Starters

Each type of motor requires its own type of soft starter. There are five main types of soft starter classifications, each with its own benefits and unique uses. Whether it's exploring the different types of soft starters for large motors or looking at those for smaller motors, we will help you find the right reduced voltage starter for your needs.

1. Primary Resistor

Primary resistors are one of the original soft starters, introduced in the early 1900s. As the name suggests, resistors are involved, one for each of the three current phases. When in place, the resistors resist the current flow during motor start-up, providing a drop in voltage. Once the motor accelerates to the pre-determined point, a timer closes the sets of contacts to remove the resistors from the circuit, allowing full power to the motor.

Known for their smooth starts, primary resistors offer two-point acceleration, ideal for motors that need limited torque to prevent damage to machinery, such as squirrel cage motors.

2. Autotransformer

Autotransformers are one of the most effective soft-starting methods. When the starting current is drawn from the line and must be held to a minimum but maximum starting torque per line amp is necessary, autotransformers are preferred over primary resistors. Rather than relying on resistors, this motor starter utilizes taps on the transformer windings with built-in flexibility to control the power input to the motor. By activating any one of the three taps on the windings, different amounts of current are sent to the motor. Autotransformers are a great power supply that provides current to the motor while keeping voltage low.

When you need to start star or delta-connected, 3-phase induction motors, autotransformers are an excellent choice.

3. Part Winding

With the part winding method of soft starter, the motor windings must be divided into two or more separate sets intended for parallel operation. During start-up, power is only applied to one set of windings, but as the motor reaches speed, power is applied to the other set to resume normal running. Windings that are energized this way produce a reduced starting current and torque. Part winding also minimizes voltage drop and excessive heat during start-up.

The part winding method is ideal for motors that supply moving centrifugal force like those in pumps, fans, and blowers.

4. Wye-Delta

A Wye-Delta soft starter can only be used in motors that have connection points to each of the three coil windings, specially wound with six leads for both Delta and Wye connections.

The Wye configuration - named such because of its "Y" shape - results in a connection of line voltage that is applied to an electrically larger winding, helping to reduce the line current. After a pre-determined amount of time, the starter electrically switches these windings over to the Delta configuration - named for its resemblance to the Greek "delta" - during which the windings are connected normally, with every winding receiving full voltage.

There are two different types of Wye-Delta starters:

• Open Transition: The starter is placed at the transition point in which the motor must disconnect and reconnect when switching from Wye to Delta. This may cause a slight hitch in operation and allow some inrush current.
• Closed Transition: Using an extra contact and set of resistors, the motor is kept online during the Wye to Delta transition. This eliminates any inrush current concerns, but the cost for these soft starters is slightly more than open transition.

Wye-Delta starters are one of the most common reduced voltage starters used in large horsepower and three-phase induction motors.

5. Solid-State

Solid-state starters are the newest and most effective method, replacing mechanical components with electrical ones using a silicon control rectifier, or SCR. During motor acceleration, the SCR controls the motor's voltage, current, and torque. It also can rapidly switch heavy currents, meaning the soft starter can provide smooth, step-less acceleration, which is considered to be the smoothest soft start method. A motor across the line (full voltage) will draw 5-7 times the rated amount of current for a brief timeline at start-up. This surge of current draws the power consumption up and is what you get charged for in your utility bill. The Solid State Starter (Soft Starter) controls this inrush current and reduces the electrical losses, therefore saving you energy costs as well as mechanical wear and tear.

As an AC motor starter, solid-state starters are used most often in industrial applications as well as HVAC systems, processing equipment, elevators, mining, and more.

When to Use Soft Starters

Understanding when to use a motor soft starter is just as important as knowing which is right for your unique application. When you require control of ramp up and ramp down on your motor control, or when you do not need speed control, a soft starter is a more economical choice over a variable frequency drive. Although you cannot limit the speed of a soft starter, you can still offer all the same protections of a VFD. They are also ideal when space is limited as a soft starter will typically take up less space than a variable frequency drive.

Choosing a Soft Starter Manufacturer for Your Needs

Looking for specific product information? We have all of the resources and information you need regarding product specifications so you can perform accurate research for your motor needs and performance recommendations. We offer soft starters from brands such as Eaton, Weg, ABB, and Toshiba, available in various amperages and voltages to fit your needs. Contact our Service and Support team for more information.

Eaton Soft Starters

There are various frame sizes of Eaton soft starters, including the S811+N37N3S, S801+T24N3S, S801+ line, S811+ line, and more. The reduced voltage soft starters from Eaton are compact, multi-functional, easy to install, and easy to program, designed specifically to control acceleration and deceleration of three-phase motors.

Weg Soft Starters

With Weg soft starters you'll find the GPH2, SSW, and TPH series soft starters. No matter which you choose or what you need, you'll be pleased with the quality and increased efficiency for your electrical motor.

What is the Life Expectancy of a Soft Starter?

In general, you can expect your soft starter to have a life expectancy of 10 years or more. However, this electrical service life is dependent upon various conditions, including temperature, humidity, stresses, operating utilization, control voltages, and more.

What are Alternatives to Soft Starters?

Soft starters may not be your first choice when looking for motor control options, particularly if you require a greater in-rush current for a more abrupt motor start. Another reason may be those looking for more control over the motor speed and torque than what is offered by a soft starter. If you are looking for an alternative to soft starters, there are a few other options you may choose from:

• Variable Frequency Drive (VFD): Similarly to soft starters, a VFD applies a lower starting voltage to reduce the current and torque. However, a VFD can also control frequency, offering improved control over the starting torque. These are ideal in applications that require varied workloads, such as HVAC and pumping systems. Although they consume less power when running at reduced speeds, they do require more complex circuitry for installation.
• Direct On-Line (DOL): Comprised of a main, three-phase supply, the DOL starter method is wired into the circuit through a circuit breaker or fused disconnect with a manual handle. Power is sent through a contractor on its way to the motor, which is often paired with an overload relay. DOL starting systems do start more abruptly than soft starters, though, causing a high in-rush current. These are mainly utilized in motors with a low HP rating or when maximum currents won’t cause damage, such as with small compressors, conveyors, pumps, and fans.

Find the Right Soft Starter for Your Needs

Whatever type of soft starter you need for your dc or ac motor, you'll find the high quality products you need at Wistex. We carry everything you need from soft starters to other electrical product needs, from AC drives to circuit breakers, fuses, power supplies, voltage monitors, and much more. For questions, help with a purchase, or more information, contact the Wistex team today.