You're right to be concerned about skin and proximity effects at 600 Hz, especially with those current levels and cable configurations. It's a complex problem, but let's break it down and address your questions.

Understanding the Effects Skin Effect

At higher frequencies, current tends to flow closer to the surface of a conductor. This reduces the effective cross-sectional area, increasing resistance and therefore heat generation.

Proximity Effect

The magnetic fields generated by adjacent conductors induce eddy currents in each other, further increasing the current density on one side of the conductor and further reducing the effective cross-sectional area. The effect is greater when conductors are close together, which is likely the case with multi-core cables.

Challenges and General Approach1. Standard Tables are Limited

You're correct. Most standard cable ampacity tables are based on 50/60 Hz operation. Derating factors for higher frequencies are not commonly found in these tables. You will likely not find any easy to apply data for multicore cables.2.

Complex Calculation

A precise calculation requires finite element analysis (FEA) or specialized software that can model electromagnetic fields and heat transfer. This is computationally intensive.3.

Simplifying Assumptions

Since a precise calculation is difficult, we need to make reasonable assumptions and use available information to estimate the effects.

Steps to Estimate Derating

Here's a structured approach to estimate the derating

A. Calculate the resistance and Inductance of the cable
Use a cable calculator: Some cable manufacturers provide calculators to determine resistance and inductance. These calculators normally do not take into account proximity and skin effects. Take the information from the datasheet

Datasheets often contain resistance and inductance values at 50/60 Hz.

Derive from measurements

Ideally, you should measure the resistance and inductance of the cable at the frequency of interest (600Hz). This can be done using an impedance analyzer.

B. Estimate the Skin Effect Derating


Skin Depth Calculation

Calculate the skin depth (δ) at 600 Hz using the following formula:δ = 1 / sqrt(π f μ σ)Where: f = frequency (600 Hz) μ = permeability of the conductor (approximately 4π x 10-7 H/m for copper) σ = conductivity of the conductor (approximately 5.8 x 107 S/m for copper)This will give you the depth to which the current effectively penetrates.
2.

Effective Area

Compare the skin depth to the radius of your conductor. If δ is much larger than the radius, the skin effect is negligible. (Unlikely in your case.) If δ is significantly smaller than the radius, the effective area is reduced. The cross-sectional area can be calculated as the circumference of the cable multiplied by the skin depth.3.

Increased Resistance

Calculate the AC resistance (Rac) using the reduced effective area. Rac will be higher than the DC resistance (Rdc). Calculate the ratio Rac/Rdc. This ratio provides an initial estimate of the increase in resistance due to the skin effect.

C. Estimate the Proximity Effect Derating

This is harder to calculate analytically without FEA. Here's a simplified approach and factors to consider:1.

Cable Configuration

The closer the cables and conductors are to each other, the greater the proximity effect. Consider the spacing between the cores within the multi-core cable and the spacing between the four multi-core cables.2.

Qualitative Assessment

The Proximity effects in multicore cable are generally significant and can be even stronger than the skin effect itself. They increase the current density on one side of each core.3.

Adjusting Derating

Increase the derating obtained with the skin effect based on your qualitative assessment of the proximity effects.

D. Total Derating


Combined Effect

Multiply all the derating factors together to get a final derating factor for the cable ampacity.

E. Apply Derating Factors


Ambient Temperature

As you mentioned, you also need to apply derating factors for the ambient temperature. Use the cable manufacturer's data for this.2.

Installation Method

Consider the installation method (e.g., in free air, in conduit, buried). This affects heat dissipation.

F. Ampacity Calculation


Determine Base Ampacity

Find the base ampacity of the cable size (95mm² or 120mm²) at 50/60 Hz and a standard ambient temperature (usually 30°C) from the cable manufacturer's datasheet.2.

Apply Derating

Multiply the base ampacity by all applicable derating factors (frequency, ambient temperature, installation method). This will give you the allowable current for your application.

Addressing Your Specific Questions Stripped Length Impact

The stripped section could act as a slight "heat sink." However, the dominant factor will be the increased resistance and heat generation due to skin and proximity effects. The heat dissipated in the stripped area will be conducted from the sections of the cable that are not stripped. I don't expect it to have a big impact on the current capability.

Short Length

A shorter length reduces the overall heat generation in the cable, but it doesn't eliminate the skin and proximity effects. The effects are still present at every point along the cable. Therefore, the fact that the length is short will not help to derate the cable.

Recommendations1. Consult Cable Manufacturers

The most reliable approach is to contact cable manufacturers directly. Provide them with your specific application details (frequency, current, voltage, cable configuration, ambient temperature, installation method). They may have data or be able to perform simulations to estimate the ampacity.2.

Consider Litz Wire

For high-frequency applications, Litz wire can significantly reduce skin and proximity effects. Litz wire consists of many thin, individually insulated strands that are twisted together. This increases the surface area of the conductor and minimizes eddy current losses.3.

Cable Spacing

Maximize the spacing between the multi-core cables if possible. This will help to reduce the proximity effect.4.

Forced Air Cooling

If possible, consider using forced air cooling to improve heat dissipation.5.

Over-Sizing

As a safety measure, consider over-sizing the cables.6.

Monitoring

After installation, carefully monitor the cable temperature to ensure it is within safe operating limits. Use thermal cameras to detect hotspots.7.

FEA Simulation

If you have access to FEA software and expertise, this is the most accurate way to determine the current carrying capacity of the cable.

Example Calculation (Simplified)

Let's say you're using a 95mm² cable with a base ampacity of 300A at 30°C.1. Skin Depth

Assume δ is calculated to be 2mm.2.

Effective Area

Assume the cable radius is 6 mm. Since δ < radius, the skin effect is present. Effective area is reduced.3.

Increased Resistance

The AC resistance is higher than the DC resistance.4.

Proximity Effect

Qualitatively assess that it adds another 20% derating factor.5.

Temperature Derating

Assume a 0.8 derating factor for 55°C ambient temperature.

Total Derating

0.75 (Skin Effect) 0.8 (Proximity Effect) 0.8 (Temperature) = 0.48

Allowable Current

300A * 0.48 = 144AIn this simplified example, the allowable current is significantly reduced due to the combined effects. Therefore, with a current of 181A, this is not adequate.

Important Disclaimer: This is a simplified example and should not be used for actual design. Always consult with qualified electrical engineers and cable manufacturers for accurate ampacity calculations and cable selection.This is a challenging problem, and a detailed analysis is essential to ensure the safe and reliable operation of your motor drive system. By following these steps and consulting with experts, you can make an informed decision about cable selection and derating. Good luck! Flag for review