Understanding Soft Start Inrush Current and How to Reduce It

You’ve probably heard that “softer” is better, and you might even know that soft starts can protect your motor from sudden surges of current, but have you ever stopped to consider what exactly is meant by “inrush current” and why it should be reduced? Understanding soft start inrush current and knowing when and how to reduce it isn’t easy…in fact, deep diving into electrical engineering topics is often intimidating and overwhelming.

However, it doesn’t have to be. In this blog post, we will dive into the electrical engineering aspects of soft start inrush current and how to reduce it, making them easier to understand and giving you the power to tackle your power-related problems. We’ll be exploring topics like what inrush current is, what causes it, and how to reduce it so that you can reap the maximum benefit from a soft start solution. Ready to get started? Let’s do this!

Quick Insight into Key Points

Soft start inrush current is the initial surge of current when the circuit breaker is switched on. This current is usually lower than the steady-state current, providing gradual pressure on the circuit’s power supply.

Overview of Inrush Current

Inrush current is an electrical surge caused by the charging of capacitors when an AC motor, transformer, solenoid valve, or other inductive load is first turned on. It occurs because the device’s capacitors must be charged almost instantaneously to full value. This can create a momentary high-current demand that can be many times greater than the normal operating current. If not managed properly, this surge of inrush current can cause any number of problems, especially in sensitive electronics.

The amount of inrush current depends on several factors such as the size and type of inductor, the magnitude and shape of the voltage supply waveform, and system resistance. Smaller sized inductors tend to have higher levels of inrush current for a given input voltage level in comparison to larger inductors with the same voltage supply waveform. Additionally, for a given power supply voltage level, the peak inrush current tends to increase with an increase in the frequency of AC power.

Some argue that it might be beneficial to limit this initial surge to protect other components connected to the same circuit like HVDC drives and electronic circuitry from potential damage caused by excessive current demands. Others contend that allowing this surge protects against over-heating by allowing sufficient electricity into the device at start up while maintaining operational stability under varying load conditions.

Either way, understanding inrush current and how to reduce it is important when attempting to preserve the life of sensitive electronics and create a safe working environment. The following section will discuss what exactly causes this surge known as ‘Inrush Current’.

What is Inrush Current?

Inrush current, also known as input surge current, is an initial rush of current that flows into an electrical system when the power is first turned on. This large spike in current occurs because of components charging up within the circuit, such as capacitors and inductors. Unlike regular running current, inrush current is usually much larger and can range from 8 to 10 times the amount of running current for most circuits. As a result, it can cause damage over time to components as well as create noise or vibration within the system.

The arguments surrounding inrush current center around whether this initial surge of current should be controlled or ignored. Some argue that ignoring inrush current can cause costly damage to systems and must be carefully managed. On the other hand, there are those who argue that controlling inrush current—for example by using special techniques like soft start—are overkill for most circuits and add unnecessary cost. Ultimately, the decision depends on individual requirements and the level of protection needed for each application.

In any case, understanding what causes high inrush current is essential for designing systems that are able to handle the sudden influx of energy and minimize component damage. The next section will discuss some common situations causing high levels of inrush current and how to address them.

Causes of High Inrush Current

High inrush current is caused when an electrical motor is powered on or first connected to the supply. In particular, the capacitance of the windings needs to be charged and this can lead to a surge of inrush current, often several times higher than normal operating levels. This is because there are several impedance sources inside the motor that cause a power spike.

On one side of the argument, some suggest that higher current ratings will result in more inductor capacity which reduces high inrush currents and protect other components of the motor from damages due to inrush. However, on the other side of the argument, higher current ratings will also increase power consumption which impacts on efficiency. Moreover, a higher rating would require a bigger casing and extra protection devices as part of the system.

Therefore, it is essential to assess all parameters before choosing the most suitable solution. Future section will discuss various methods used to reduce high inrush current associated with electrical motors.

  • A study found that soft start inrush current can reduce initial starting currents of up to 70% and increase the life expectancy of connected equipment.
  • When using soft start technology, power supply manufacturers have found that their products’ mean time between failure (MTBF) can be improved from around 20,000 hours to 40,000 or 50,000 hours.
  • According to a survey conducted by the International Electrotechnical Commission (IEC), most industrial facilities set their maximum limit of inrush current at 250A.

Key Summary Points

High inrush current is caused when an electrical motor is powered on or first connected to the supply and can lead to a surge of inrush current, often several times higher than normal operating levels due to internal impedance sources. There are two sides to the debate over choosing the appropriate solution: one advocating for higher current ratings due to increased inductor capacity, but other bemoaning at the cost of efficiency, extra casing and protection devices. Ultimately, it is essential to assess all parameters before deciding on the most suitable solution and various methods to reduce high inrush current associated with electrical motors will be discussed further.

Electrical Motors

Electrical motors have become a critical component in many industries, from machinery manufacturing to robotics. However, it is important to understand that starting an electrical motor requires much higher levels of current compared to its normal operating current. This is known as the inrush current, and it can cause serious damage if not managed properly.

Electric motors employ two main strategies for controlling the inrush current – soft starts and reduced voltage starter motors. Soft start systems rely on a circuit that gradually increases the voltage supply to the motor, which slowly increases its rotational speed over time. This helps reduce sudden increases in electrical load and minimizes the risk of overload. Conversely, reduced voltage starter motors manage the inrush current by reducing the voltage supplied to the motor at start-up, allowing them to accelerate more smoothly than with traditional starters.

Understanding when, why and how to implement soft start systems and reduced voltage starter motors is key to improving safety, longevity and performance on any electric motor powered system. It’s important not just to consider inrush currents but also serviceability – having educated personnel who understand this technology is pivotal in modern mechanical engineering applications.

Now that we have discussed electrical motors, let’s examine inductive loads in the next section.

Inductive Loads

Inductive loads function in an entirely separate way from resistive ones. Whenever electric current flows through a circuit of this type, it creates a magnetic field that self-inducts and works against the current itself. This means that the higher the current entering the circuit, the higher the opposition to that current. Motors are by far the most common inductive loads; their circuitry contains motors, relays and other components with windings that tend to generate huge amounts of starting current (or inrush) which can often exceed 10 times the normal operating level.

The high current generated by an inductive load can wreak havoc on any connected switches, power sources or transformers if not monitored closely. This is why any electrical system containing an inductive load must also include an efficient starting control device capable of controlling inrush current and protecting both system components and other connected pieces of equipment.

That said, it’s important to remember that even though excessive inrush current can be problematic for some systems, it can also sometimes be beneficial as it allows for prolonged stability and efficient motor operation. It is therefore essential for any power engineer to have full knowledge of a given motor’s characteristics (including its size, resistance and voltage) before deciding which starting control device would be best suited for that application.

Soft start inrush current protection is just one possible solution for reducing or controlling excessive levels of inrush current produced by inductive loads. The next section will discuss several approaches to soft start inrush current protection as a way of mitigating potential problems associated with this often overlooked but important aspect of power engineering.

Soft Start Inrush Current Protection

Soft start inrush current protection is a technique used to minimize the damaging impact of high inrush current on an electrical system. Inrush current, often referred to as surge current, occurs when a system first powers on and is typically much higher than the normal operating currents. This spike of current can cause damage to the system by putting excess strain on contacts or circuit breakers or causing other components to overheat and fail prematurely.

To reduce this risk, soft start inrush current protection utilizes resistive elements or capacitors when turning the power supply on so that the peak inrush current is reduced. By limiting the amount of inrush current, component life expectancy is increased and excessive wear and tear avoided. Even though soft start inrush current protection generally comes with some slight efficiency losses compared to just turning a power supply on without any protection, manufacturers usually find that its advantages far outweigh its drawbacks.

Despite its benefits, some people argue that soft start inrush current protection does more harm than good in certain situations since too little or too much insuring current can both reduce system efficacy. They suggest that it’s smarter to leave the system unprotected and replace any burned-out components rather relying strictly on soft start for protection. The debate between utilizing soft start inrush current protection versus leaving a system unprotected can create confusion for inexperienced engineers and technicians, which could lead to poor choices being made and costly mistakes being made as a result.

Ultimately, whether soft start inrush current protection should be implemented depends on the individual application and preferences of the engineer working on it. Going forward into our next section, we’ll look at ways that equipment and systems can be designed with soft start capability built-in so they are less prone to premature wear and tear due to surges of high voltage or currents.

Equipment and Systems

Understanding inrush current is essential to the design, operation, and maintenance of electrical equipment and systems. As a critical protection measure, the ability to anticipate and control inrush current is paramount for scenario-based preventive measures.

In power electronic applications, controlling inrush current allows proper system operation which prevents device failure due to overloading during startup. Whether it’s motor drives, capacitors, rectifiers, transformers or other power components, these devices must meet expected inrush performance requirements.

Other systems have their own inrush current behavior as well. For example, fiber optic systems require a relatively low emergency start-up inrush so that their high voltage units don’t exceed safe limits. Additionally, linear regulator systems may exhibit triple-point behavior when transitioning from deceleration to acceleration and vice versa.

Standalone solutions are also available for mitigating inrush currents such as contactors that can limit the start-up current draw. However, these solutions must be properly applied depending on the application since they come with their own limitations and drawbacks that must be taken into consideration.

One challenge associated with inrush current is its continuous evolution as technology progresses leading to increasingly complex system designs. This means running cost/benefit analyses to determine if mitigation techniques are necessary or can be reliably implemented on any given system design for manageable outcomes.

All of this underscores the importance of having a clear understanding of inrush current when dealing with the design, operation and maintenance of electrical equipment and systems. With this knowledge comes confidence when navigating many of the involved complexities and implementation decisions that accompany today’s sophisticated power electronics applications regarding inrush current reduction techniques. The next section will discuss some of these techniques.

Inrush Current Reduction Techniques

Inrush current reduction techniques attempt to limit the harmful impacts from an unexpected high inrush current on both the power supply and connected equipment. As a result, there are several approaches to attempt to mitigate this issue.

Using Inrush Limiters: One method of reducing inrush current is by attaching a circuit called “inrush limiters” across two separate points on a high DC voltage circuit. Inrush limiters use thermistors, also known as negative temperature coefficient (NTC) resistors, which begin with weakly conducted state when exposed to cold conditions. These thermistors are designed so that they can decrease their physical impedance over time as the electrical current begins to rise with the power up sequence. When it reaches its rated maximum value, the NTC can limit said currents and eventually maintain it at its rated maximum.

Separating High Voltage Bus Bars: Obviously, if components are built next to each other closely or connected side-to-side on one bus bar, power supplies will be subject to sizable inrush currents due to mutual inductance – especially during any switching events. Using multiple isolated bus bars within a power supply design’s architecture is a good method for preventing system components from interacting inductively. As such, minimizing the effects of each component’s inrush current allows for more reliable operation of the entire system and limits any possible excessive peak inrush current levels.

Time Delay Circuits: Time delay circuits are also known as soft start delay circuits which can be very effective in reducing peak inrush currents as they allow for controlled gradual power to be delivered from one point across to another over predetermined intervals of time instead of all at once – thus reducing said peak current levels which would not occur with delayed onset of gathered amount of energy or charge being released.

Debate both sides: While some people may argue that using these three aforementioned approaches could lead to potentially better performance against inrush current problems overall, it should be noted that there are still limitations and downsides associated with their usage which must always be taken into consideration before implementing them into any given design architecture. For instance, adding circuit delay devices can lead to longer response times for operations amplifiers between signal input and output stage amplifiers – leading to reduced efficiency and higher operating costs due to poorer usage of resources used over time.

In conclusion, while various forms of inrush current reduction techniques have been proposed based on limiting physical inductance between connected systems or gradual introduction timing-based circuits; precise judgement must always be exercised before any particular technique is chosen for implementation into an electrical system architecture as certain advantages amongst these might come coupled with certain disadvantages which cannot necessarily be ignored depending on what best suits operational needs in question. With that being stated however, it’s certainly safe to conclude that soft start processes can provide valuable protection against dangerous effects of sudden spikes in rush currents experienced by multiple electronic components within a single system framework if leveraged correctly.

Therefore, understanding the benefits provided by soft start processes is indeed quite important when attempting to secure one’s own system from potential damage caused by sudden high inrush currents – making exploration into this field even more pertinent for effective management and design strategies moving forward within numerous applications today. And now that we have more fully contextualized our understanding around how we can address and reduce dangerous situations related to high inrush currents via various techniques available today; let us transition towards further exploring how these processes might advantageously benefit our systems going forward through proper implementation. Therefore, our next section will proceed by taking a more thorough look into the “Benefits of Soft Start Inrush Current Protection”.

Benefits of Soft Start Inrush Current Protection

Soft start inrush current protection offers a range of benefits, including increased safety and energy efficiency, to any equipment using motors. The most obvious benefit is that it helps protect both the motor and other power supply components from damage due to high inrush currents. High inrush currents can cause large voltage spikes that can potentially burn out or damage critical electronic components, reducing the life of any machinery or equipment.

Soft start inrush current protection also improves energy efficiency by providing for a gradual ramp up of the motor’s current draw. This makes a motor much more energy efficient as it does not need to immediately draw huge amounts of current. In this way, soft start inrush current protection can save considerable amounts of energy and reduce costs significantly.

In addition to safety and cost savings, using soft start inrush current protection increases the reliability of the system and reduces wear on motors over time. By controlling the amount of higher-than-normal amount of startup current, these circuits can help ensure that systems are both safe and reliable over their lifetime.

Despite these potential benefits, there are some potential drawbacks associated with using soft start inrush current protection circuits such as higher cost and complexity to setup. If not configured correctly, these circuits can have an adverse effect on performance and reliability; they need to be carefully monitored when turning on larger appliances with high power requirements.

Overall, the use of soft start inrush current protection has several overall benefits but should be weighed against its potential drawbacks before implementation. As such, careful consideration is needed for any system installation or upgrade. With this understanding in place, we are now ready to explore considerations for inrush current protection components and designs in the next section.

Considerations for Inrush Current Protection Components

When it comes to understanding soft start inrush current and how to reduce it, considerations for inrush current protection components are essential. With the vast array of failure and safety mechanisms available, the selection process can be overwhelming if deciding on suitable protection devices unaided. Before selecting any components for controlling inrush currents, the following factors and associated specifications should be taken into account:

First and foremost, an appropriate power supply component must be selected according to power rating, tolerance level, switching frequency, saturation voltage, transient response and other key parameters. Lower-capacity devices such as semiconductor fuses, thermistors and mechanically operated switches may take a prolonged period of time to react to high currents produced by inrush events and thus do not represent suitable options.

Second, the total supply voltage should be taken into consideration when selecting a component to protect against inrush current. When dealing with variable AC voltages–such as variable frequency drives (VFD)–therefore a component that is capable of handling dynamic stress must be chosen. It has to remain capable of satisfactory operation even after a transient surge has passed. Various types of varistors or VDRs provide reactive protection against voltage surges but have limited ability to dissipate heat. Careful attention must also be paid to their ratings when choosing the component most suitable for the application.

In many cases, electro-mechanical devices such as circuit breakers or contactors may represent useful forms of inrush current protection. Although they may require additional hardware control systems for operation, they prove more reliable than some electronic counterparts under certain conditions as they are highly adaptive to transient loads/surges without necessarily employing load interruption techniques.

Overall, the choice of inrush current protection component depends on two main criteria: Rating certification by accredited organizations (ETL/UL/CE) that confirms its compatibility with relevant environmental standards; and electrical ratings that guarantee satisfactory performance against expected worst conditions (such as surge voltage peaks). Weighing both aspects will inform you whether the device represents an economically feasible solution for your application before further investing money into installation and maintenance costs associated with it.

Therefore it is critical to understand soft start inrush current and how best to reduce it prior to making any selection regarding inrush current protection components. Now that we have discussed considerations for these components, let’s move on to the Conclusion section which will bring us together all we have learned up until this point about understanding soft start inrush current and reducing it effectively.


Soft start inrush current is a critical component of maintaining electrical circuit stability, and has the potential to damage or short out components if not managed properly. In order to reduce the likelihood of equipment failure due to high current draw, engineers must consider designing their power systems with appropriate soft start control technology and/or using inrush limiters. A complete and comprehensive analysis of the power system is essential to understand precisely where and how much power is needed when the circuit is energized, as this can help identify the right type of soft start technology for specific applications.

While it is possible to reduce soft start inrush current, there are some drawbacks in certain cases. For instance, delays caused by a slower ramp-up time may be unacceptable in some industrial processes that require an immediate response. Additionally, because soft starters add an extra level of complexity, they necessitate a greater upfront cost investment. However, assessing the overall project cost savings from avoiding expensive repairs due to unexpected issues such as melt down makes investing in soft starter technology a smart choice for engineers looking for long term system reliability.

Overall, understanding soft start inrush current and employing suitable approaches to regulate it is critical for maintaining electrical safety and achieving optimal system performance. Planning ahead and taking proactive steps such as utilizing soft starters or install inrush limiter will help increase circuit resilience and save money over the long run.


How does a soft start limit inrush current?

A soft start works by limiting the inrush current of an electric motor or other inductive load. This is done by gradually introducing the voltage over a certain amount of time, thus limiting the rate at which it can draw current and reducing the peak current that would otherwise occur during start-up. By encouraging a slower buildup of current, it allows for a smoother transition from off to running; this reduces stress on electrical components and helps prevent problems such as overloads and voltage dips. Additionally, a soft start also prevents large mechanical shocks due to sudden torque spikes that can cause damage to any interconnected equipment.

What type of components are most commonly used to provide a soft start?

When it comes to providing a soft start, the most commonly used components are resistors, capacitors, and PTC thermistors. Resistors slow the flow of current from the device being powered, helping to minimize inrush currents. Capacitors store energy and gradually release it, providing a steady power ramp up and reducing startup surges. Finally, PTC thermistors are semiconductor devices that increase their resistance with increasing temperature. When used in combination with resistors and capacitors, they reduce inrush current by helping regulate when power is slowly phased in.

What causes a high inrush current?

A high inrush current is caused by the discrepancy between the starting power of a motor or transformer and its permanent operating power. When a motor is first turned on, it will draw significantly more power than when it reaches its steady-state operation level due to the initial surge of energy needed to spin up its components. This increase in power demand causes an inrush current, which can be hazardous to the circuit if it is not adequately managed. The primary contributing factors to this high initial inrush are:

1) Magnetic core saturation: At the moment the supply voltage is applied to the motor, the stator flux creates strong magnetic fields that need time to stabilize. During this period, the magnetic core becomes saturated and will draw more current for a short period of time until it stabilizes.

2) High capacitive load: Certain loads such as transformers, compressors or motors with large capacitors connected across their windings can cause a high amplitude inrush surge depending on how they are wired. Since capacitors store energy when connected across a winding, they will discharge that stored energy while powering up the circuit causing an abrupt increase in current flow through the system.

3) Voltage applied before start command: Under certain conditions, if a supply voltage is applied prior to sending a start command, an instantaneous current spike could be observed due to various components present between start and supply voltage being momentarily overloaded. This can act as an additional source of large inrush currents.

Reducing this sudden inrush of excessive current requires designing circuits and components properly along with proper protection measures such as fuses, breakers or contactors.