What are the benefits of using soft starters to reduce inrush current and mechanical stress?

If you use variable frequency drives (VFDs) to control the speed and torque of your electric motors, you may have encountered some challenges when starting them up. High inrush current and mechanical stress can damage your motors, reduce their efficiency, and increase your energy costs. That's why you may want to consider using soft starters to reduce these problems and improve your VFD performance. In this article, we'll explain what soft starters are, how they work, and what benefits they offer for your VFD applications.

1 What are soft starters?

Soft starters are devices that gradually ramp up the voltage and frequency of the power supply to your motors, instead of applying the full voltage and frequency at once. This reduces the inrush current, which is the peak current that flows when you start a motor. Inrush current can be up to 10 times higher than the normal operating current, and it can cause overheating, voltage drops, and circuit breaker tripping. By using soft starters, you can lower the inrush current to about 2-4 times the normal current, depending on the load and the soft starter settings.

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Tom W.

Head of Engineered Solutions I Sales Engineer
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Solid State Soft Starters are beneficial to reduce the amount of inrush Amps from ATL or Across the Line starting method of starting. The HP of the motor drives a lot of the decision process of applying VFD or SS Starters. Since SS Starters DO NOT vary the frequency, only the voltage. On a SS Starter the drive will increase the voltage over time until the until the motor reaches break away torque. reduced voltage starters have little starting torque, so SS starters may require near locked rotor amps as you would see in a ATL starter. If the load is easy to turn such as a centrifugal pump or a fan, then a SS starter could be a great choice. PD Pumps, long Conveyors, and High Inertia loads could be better suited to run on a VFD.

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2 How do soft starters work?

Soft starters use thyristors, which are semiconductor switches, to control the amount of voltage and frequency that reaches the motor terminals. The thyristors are connected in series with the power supply, and they can be turned on and off at different angles of the AC waveform. By adjusting the firing angle of the thyristors, the soft starter can vary the voltage and frequency applied to the motor, creating a smooth and gradual acceleration or deceleration. Soft starters can also provide other functions, such as current limiting, overload protection, and soft stop.

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Tom W.

Head of Engineered Solutions I Sales Engineer
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SS starters start the voltage at 0 then ramp up voltage over time to bring things up to speed. The frequency is 60hz at 1 volt and remains constant at 60 hz as motors ramps up. Excessive ramp times over 15-20 seconds could overheat SS starter and the motor cause it to trip on over current and temperature. Ideal ramp times are in the 5-14 second time frame. Until you reach full voltage, the motor is actually running in an undervoltage state. After the SS starter reaches full voltage, a by pass contactor closes and removes the Solid-State portion of the drive out of the circuit. Major advantage of SS starters. Above Size 5 ATL, they are generally less expensive. After they are at full voltage, ambient temps are not the concern that VFD's are.

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3 What are the benefits of using soft starters for VFDs?

Using soft starters for your VFD applications can offer several benefits, such as reducing mechanical stress on motor components and energy consumption, as well as improving process control. By avoiding sudden changes in speed and torque, soft starters can extend the lifespan of motors and lower inrush current and power factor. This, in turn, can reduce maintenance costs and improve energy efficiency. Additionally, soft starters can help you adjust start and stop times and speeds of motors to optimize process performance and quality, while avoiding water hammer in pumps, material spillage in conveyors, or product damage in crushers.

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4 How to choose the right soft starter for your VFD application?

When selecting the right soft starter for your motor, there are several factors to consider. Firstly, you need to match the soft starter rating to the motor power and voltage, as well as check the compatibility with the motor type, such as induction, synchronous, or wound rotor. Additionally, you should know the load type, duty cycle, and requirements. Lastly, you should compare the functions and options that different soft starters offer. These may include current limiting, soft stop, bypass contactor, communication interface, or fault diagnosis.

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5 How to install and configure your soft starter for your VFD application?

Once you have chosen the right soft starter for your motor and load, you need to adhere to some best practices when installing and configuring it. This includes reading and following the user manual and wiring diagram of your soft starter, as well as having the proper tools and protective equipment. Additionally, you must connect the soft starter to the power supply and motor terminals according to the wiring diagram, and connect any auxiliary devices or sensors that you may need. Lastly, it is essential to adjust the soft starter settings according to the motor and load specifications, such as start voltage, start time, current limit, stop time, and overload protection.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the benefits of using soft starters to reduce inrush current and mechanical stress?

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1 What are soft starters?

2 How do soft starters work?

3 What are the benefits of using soft starters for VFDs?

4 How to choose the right soft starter for your VFD application?

5 How to install and configure your soft starter for your VFD application?

6 Here’s what else to consider

Variable Frequency Drives

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