While inrush current has a peak current value, the term “Inrush Current” is commonly used khổng lồ describe the current that is required lớn energize an AC powered device or product when first applying voltage & power to lớn it. This is especially true for inductive loads such as transformers, Inductors và electric motors. It also applies to lớn AC/DC power supplies that use a simple rectifier/capacitor input đầu vào 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 cthua thảm to lớn 30 amps và 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 lớn be charged up to 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 khổng lồ 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 & negative peaks where the current reaches it maximum and minimum value during each cycle. The absolute ratio between the RMS value và the peak value is called crest factor (CF). For a sinusoidal current as encountered with a resistive load, the crest factor will be the square root of 2 or ~1.4142 khổng lồ 1. This crest factor or ratio is shown in Figure 3.
Other wave sầu forms have different crest factors as shown in Table 1 below for some typical other AC wave sầu forms.
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Why Does it Matter?
When using an AC power source to lớn 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 khổng lồ run the unit under demo 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 lớn 50 times nominal.
Source current limitation may be both in terms of RMS current rating & peak current rating. For motors & 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 input đầu vào equipment, the current crest factor is generally much higher than 1.414, up to 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 lớn 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 into 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.
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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 khổng lồ 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 và why does it matter” at https://tekkenbasara.mobi/2017/03/programmable-z-ac-source-work-matter/
Chuyên mục: KHÁI NIỆM LÀ GÌ