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 unique benefits and applications. In this blog, we'll explore these soft starters and the various manufacturers to help you choose, and when to utilize these on your AC electric motor.

What is a Soft Starter?

Definition and Purpose

Soft starters are electrical devices used with AC motors to control and reduce the stress of motor startup. A soft starter gradually increases the voltage supply instead of an immediate surge of full voltage, allowing for a smoother, more controlled acceleration. This process helps minimize electrical stress, reduce mechanical wear, and prevent 100% instantaneous torque that damages the motor or connected equipment over time.

By integrating a soft starter for efficient motor control, businesses can enhance motor performance while extending operational lifespan. 

Explore our range of soft starters designed for efficient motor control.

Benefits of Using Soft Starters

Using a soft starter offers several key advantages: 

• Energy Efficiency: By limiting the initial power surge, soft starters help lower overall energy consumption, leading to cost savings. 

• Extended Motor Lifespan: Reducing sudden torque and overheating minimizes wear and tear, helping motors last longer.

• Improved Operational Efficiency: With controlled acceleration, motors run more smoothly, reducing strain on electrical systems and mechanical components.

• Reduced Power Surges: Soft starters prevent high inrush currents, which can cause voltage drops and disrupt other electrical equipment. 

• Adjustable Acceleration and Deceleration: Many soft starters allow users to customize startup and shutdown speeds to match operational needs. 

Learn more about the benefits and applications of soft starters.

The Five Main Types of Soft Starters

Several types of soft starters are available, each designed to accommodate different motor requirements and operational needs. These devices help reduce electrical and mechanical stress during startup, improving motor efficiency and lifespan. Below, we’ll explore the five main types of soft starters and their applications.

1. Primary Resistor Soft Starters

Primary resistor soft starters are among the earliest soft starters, dating back to the early 1900s. As the name suggests, they use three resistors, one for each phase of the electrical current, to limit the inrush current during startup. Gradually increasing voltage allows for smoother motor acceleration. Once the motor reaches a set speed, a timer removes the resistors from the circuit, enabling full-power operation.

These starters provide two-point acceleration and are best suited for applications requiring limited torque, such as squirrel cage motors used in pumps and fans.

2. Autotransformer Soft Starters

Autotransformer soft starters are among the most efficient motor startup options. Instead of resistors, they rely on transformer windings with adjustable taps to regulate voltage and current. By selecting different taps, operators can control the amount of power sent to the motor, ensuring a smooth and controlled startup.

These starters are particularly useful for star or delta-connected three-phase induction motors, where maintaining low voltage while achieving maximum starting torque per line amp is critical.

3. Part Winding Soft Starters

With a part-winding 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 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, such as pumps, fans, and blowers.

4. Wye-Delta Soft Starters

A Wye-Delta soft starter can only be used in motors with 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 line voltage connection 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, where 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: During the Wye to Delta transition, the motor is kept online using an extra contact and resistors. This eliminates any concerns about inrush, but the cost for these soft starters is slightly higher than for an open transition.

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

5. Solid-State Soft Starters

Solid-state starters are the most common overall, as well as the newest and most effective method, replacing mechanical components with electrical ones using a silicon control rectifier, or SCR. The SCR controls the motor's voltage, current, and torque during motor acceleration. It also can rapidly switch heavy currents, meaning the soft starter can provide smooth, step-less acceleration, which is considered 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, saving energy costs and mechanical wear and tear.

Solid-state starters are most often used as AC motor starters in industrial applications, such as processing equipment, crushers, conveyors, elevators, mining, and more. They are also used in HVAC systems, on fans, pumps, and where constant volume is needed.

Choosing the Right Soft Starter for Your Application

Factors to Consider

Understanding when to use a motor soft starter is just as important as knowing which is right for your unique application. If your primary goal is to control motor acceleration and deceleration without needing variable speed control, a soft starter is a cost-effective alternative to a variable frequency drive (VFD). While soft starters don’t adjust motor speed, they provide essential protection against electrical stress and mechanical wear. Additionally, their compact design makes them ideal for applications where space is limited. 

But not all soft starters are made equally. Now that solid states are taking over the market, there’s something else you need to know and pay attention to: Should you select a 2-phase or 3-phase solid state SCR-controlled version, and what does this mean? 

A 2-phase type only uses SCR control on two of the three phases, so the third phase is 100% across the line. A 3-phase type uses SCR control on all three phases, so the amps come up balanced on all three phases, and the motor is electrically balanced on all three phases at the start and during the ramp. 

When selecting a product for the best MTBF, we always like to see 3-phase types, but when cost or even space is a factor, customers use 2-phase on smaller horsepower motors.

Choosing a Soft Starter Manufacturer for Your Needs

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

Eaton Soft Starters

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

Weg Soft Starters

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

What is the Life Expectancy of a Soft Starter?

Generally, you can expect your soft starter to have a Mean Time Before Failure (MTBF) of 10 years or more. However, this electrical service life depends on various conditions, including environmental conditions like temperature, humidity, vibration, and operating conditions, such as power & control voltage variations, utilization (# of starts per Hr/Day), sizing, and more.

What are Alternatives to Soft Starters?

Soft starters may not be your first choice 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 a soft starter offers. 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 current and torque. However, it can also control frequency, offering improved control over the starting torque. VFDs are ideal for applications requiring varied workloads, such as HVAC and pumping systems. Although they consume less power when running at reduced speeds, they 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. However, DOL starting systems start more abruptly than soft starters, causing a high inrush 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.

Contact Wistex LLC for Soft Starter Solutions

Get in Touch

Whatever type of soft starter you need for your DC or AC motor, Wistex has the high-quality products you need. We carry everything you need, from soft starters to other electrical products, from AC drives to circuit breakers, fuses, power supplies, voltage monitors, and much more. Contact the Wistex team today for questions, help with a purchase, or more information.