While inrush current has a peak current value, the term “Inrush Current” is commonly used khổng lồ describe the current that is required to energize an AC powered device or hàng hóa when first applying voltage & power khổng lồ it. This is especially true for inductive sầu loads such as transformers, Inductors and electric motors. It also applies to lớn AC/DC power supplies that use a simple rectifier/capacitor input stage. These initial currents can surge & be quite a bit higher than the normal operating current or what is called “steady state” current. An example of an electric motor inrush current is shown in Figure 1. It shows the peak current for the first half cycle as being cthảm bại to lớn 30 amps and then decaying over subsequent half cycles as the motor spools up.

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A different example of inrush current is an AC/DC input đầu vào stage that uses a rectifier, capacitor circuit where the capacitor needs to be charged up lớn its nominal voltage as shown in Figure 2. In both cases, it is apparent that the inrush current is considerably larger than the steady state current.


Peak current on the other applies to all AC currents, either inrush or steady state. An AC current waveform has an RMS value representing the effective or DC equivalent current but it also has a peak value, both positive and negative peaks where the current reaches it maximum & minimum value during each cycle. The absolute ratio between the RMS value & the peak value is called crest factor (CF). For a sinusoidal current as encountered with a resistive sầu load, the crest factor will be the square root of 2 or ~1.4142 to lớn 1. This crest factor or ratio is shown in Figure 3.


Other wave forms have sầu different crest factors as shown in Table 1 below for some typical other AC wave forms.

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Why Does it Matter?

When using an AC power source khổng lồ determine the required inrush current for a unit under chạy thử, it is important to lớn note that the AC source should be capable of delivering significantly more current for a short period of time than is required lớn run the unit under kiểm tra in a steady state condition. In the case of Motors và Inductors, the inrush current can be 10 to 30 times the nominal current. For toroidal inductors, this value may be up khổng lồ 50 times nominal.

Source current limitation may be both in terms of RMS current rating & peak current rating. For motors and inductor loads, the crest factor of the inrush current is only 1.414 so if the source can tư vấn the RMS current, the peak value will be supported as well. For rectified AC đầu vào equipment, the current crest factor is generally much higher than 1.414, up lớn 2 or 3 lớn 1 so not only the RMS rating but also the peak current rating must be considered. Most available AC power sources will tư vấn current crest factors from 2.5 to 4 at max RMS current output.

Current Limiting Effects

If the source is unable khổng lồ deliver the required inrush current, it may still be used for testing normal operation but the required inrush current cannot be determined as the power source will go inkhổng lồ current limit – either RMS or Peak or both – và limit the voltage in doing so. This means the unit under kiểm tra will typically still start up or turn on but not as fast as it would when operated from the utility.

AC Source Voltage Distortion

High peak currents & distorted current wave sầu forms also impact the AC power source distortion as they work against the output impedance of the power source. The lower the power source’s output impedance, the less this effect will be. Figure 4 shows the effect of highly distorted current on output voltage distortion. As the current reaches its peak near the top of the voltage wavekhung, the voltage is pulled down causing some flat topping lớn occur.


To alleviate this effect, a programmable output impedance feature may be offered on some AC source models that allows the output impedance the be reduced. See our expert blog post called “What is Prog Z on an AC power source and why does it matter” at https://kulturbench.com/2017/03/programmable-z-ac-source-work-matter/

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