GLENAIR
load will vary based on gauge, conductor material, and insulation, it is best to evaluate the temperature rise data for each unique assembly. However, SAE AS50881 does provides a conservative estimate which may be used as a universal reference point. The second primary current derating factor is the number of wires in a harness bundle . A single wire can freely dissipate heat through convection since all outer surfaces are exposed to ambient air. However, as additional wires are added to a bundle, wire surfaces are in contact with each other and therefore lose the ability to dissipate heat. This is especially true for wires at the center of a large bundle. Since the wires are no longer able to dissipate heat efficiently, current must be limited to reduce the aggregate heat produced. The offset of heat produced versus heat dissipated will ensure temperature rise does not exceed the maximum allowable. While “bundle derating” is ideally evaluated for the unique assembly, again AS50881 provides a conservative estimate which may be used as a universal reference point. The third primary current derating factor is altitude . At sea-level (standard pressure), air is dense and able to convect substantial heat away from the wire. However, at higher altitudes (low pressure), air density is reduced. Reduced air density means there are fewer air molecules available (per unit volume) to convect heat away from the wire. Since the air is no longer able to convect heat as efficiently, the current must be limited to reduce the heat produced. The offset of heat produced versus heat dissipated will ensure temperature rise does not exceed the maximum allowable. While “altitude derating” is ideally evaluated for the unique assembly, again AS50881 provides a conservative estimate to be used as a universal reference point. The fourth current derating factor is source frequency . In an AC system, the current alternates direction along the conductor. When current travels one direction, a magnetic field is generated which supports the current flow through inductance. As current reverses, the magnetic field collapses, and the inductance opposes the current reversal. This is known as counter-electromotive force (CEMF), or “back EMF.” The CEMF is strongest at the core of the conductor, leaving the outer surface less effected and more
available to conduct the source current. The “skin effect” is where AC current tends to
only conduct on the outer surface of the conductor, leaving the core useless. At higher frequencies, the period of reversal is reduced, and CEMF has less time to dissipate. As a result, higher frequencies lead to more core left unutilized, pushing more current to the outer surface. We can calculate the “skin thickness” (δ) to estimate how much of the conductor is in fact utilized. If the skin thickness is greater than the conductor radius, then we would expect the conductor to be fully utilized. However, if the skin thickness is less than the conductor radius, we would expect core loss. As shown in the table below, standard power frequency (60 Hz) will utilize 100% of all conductors up to 4/0. However, 400 Hz MEA power will only utilize 100% of conductors up to 2AWG, suffering loss on larger conductors. For AEA applications with high frequency and shallow skin depth, the alternating current will utilize only a small portion of large conductors. With the current only traveling across a small percentage of the conductor, conductor efficiency is lost, and conductor performance may be estimated using an elevated current at lower frequency. Glenair engineering routinely analyzes any application where the conductor may experience skin effect loss to more exactly estimate current rating and temperature rise. Conductor Percent Utilization due to Skin Effect at High Frequency Conductor Source Frequency (Skin Thickness, inch)
60 Hz (0.372)
400 Hz (0.131)
1000 Hz (0.083)
1500 Hz (0.068)
2000 Hz (0.059)
2500 Hz (0.053)
AWG Radius
8 4
0.064 100% 100% 100% 100% 99% 97% 0.102 100% 100% 97% 89% 82% 76%
2 0.129 100% 100% 87% 78% 70% 65% 1/0 0.162 100% 96% 76% 66% 59% 54% 2/0 0.182 100% 92% 70% 61% 54% 49% 4/0 0.230 100% 82% 59% 50% 45% 41% Calculated estimates for copper (ρ = 1.72×10 -8 Ω-m, α = 4.29×10 -3 °C -1 ) at 25°C
QwikConnect • July 2021
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