Shielded control cables for Variable Frequency Drives (VFDs) are essential for minimizing electromagnetic interference. Accurate cable sizing and selection ensure optimal VFD performance and longevity.
This article covers the IEC standards for shielded control cables, detailed calculation methods, and practical application examples. Learn how to select and calculate cables for VFD systems efficiently.
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- Calculate shielded control cable size for a 15 kW VFD at 400 V, 50 Hz.
- Determine cable length and voltage drop for a 7.5 kW VFD with 100 m cable run.
- Estimate shielded cable cross-section for a 22 kW VFD with 3-phase supply.
- Calculate maximum allowable current and cable size for a 30 kW VFD system.
Common Values and Parameters for Shielded Control Cable for VFD – IEC
| Parameter | Typical Values | Units | IEC Reference |
|---|---|---|---|
| Nominal Voltage Rating | 300/500, 450/750 | Volts (V) | IEC 60502-1 |
| Conductor Material | Copper (Cu) | – | IEC 60228 |
| Conductor Cross-Sectional Area | 0.5, 0.75, 1.0, 1.5, 2.5, 4, 6, 10, 16, 25 | mm² | IEC 60228 |
| Insulation Material | PVC, XLPE, EPR | – | IEC 60502-1 |
| Shield Type | Aluminum foil, Copper braid | – | IEC 60502-1 |
| Maximum Operating Temperature | 70, 90, 105 | °C | IEC 60502-1 |
| Maximum Voltage Drop | 3% to 5% | Percentage (%) | IEC 60364-5-52 |
| Frequency Range | 0 to 500 Hz (typical for VFD) | Hz | IEC 60034-17 |
Key Formulas for Shielded Control Cable Calculation in VFD Applications
1. Voltage Drop Calculation
The voltage drop across the cable must be limited to ensure proper VFD operation and motor performance.
- Vd: Voltage drop (Volts)
- I: Load current (Amperes)
- R: Resistance per unit length of cable (Ohms/meter)
- X: Reactance per unit length of cable (Ohms/meter)
- cosφ: Power factor (dimensionless)
- sinφ: Reactive component (dimensionless)
- L: Cable length (meters)
Resistance and reactance values depend on cable construction and frequency. For VFDs, harmonic currents can increase reactance.
2. Current Carrying Capacity (Ampacity)
Determining the maximum current the cable can safely carry without overheating is critical.
- Imax: Maximum current (Amperes)
- P: Motor power (kW)
- V: Line-to-line voltage (Volts)
- η: Motor efficiency (typically 0.85 to 0.95)
- cosφ: Power factor (typically 0.85 to 0.95)
3. Cable Cross-Sectional Area (A)
Based on allowable voltage drop and current, the cable cross-section is selected.
- A: Cross-sectional area (mm²)
- ρ: Resistivity of conductor material (Ω·mm²/m), copper ≈ 0.0175
- I: Load current (Amperes)
- L: One-way cable length (meters)
- Vd: Allowable voltage drop (Volts)
Note: For three-phase systems, the factor 2 accounts for the return path.
4. Shield Coverage and Effectiveness
Shielding reduces electromagnetic interference (EMI) and noise, critical for VFD control cables.
- Induced Noise: Noise voltage measured on the cable
- Source Noise: Noise voltage emitted by the VFD or motor
IEC 60502-1 and IEC 61000-5-2 provide guidelines for shielding requirements and testing.
Real-World Application Examples
Example 1: Calculating Shielded Control Cable Size for a 15 kW VFD at 400 V
A 15 kW, 400 V, 50 Hz VFD drives a motor located 80 meters away. The motor efficiency is 90%, and power factor is 0.9. The allowable voltage drop is 3%. Calculate the required cable cross-sectional area.
- Step 1: Calculate maximum current (Imax)
- Step 2: Calculate allowable voltage drop (Vd)
- Step 3: Calculate cable cross-sectional area (A)
Step 4: Select the next standard cable size, which is 10 mm² copper conductor with appropriate shielding.
This cable size ensures voltage drop is within limits and supports the current safely.
Example 2: Voltage Drop and Shielding Considerations for a 22 kW VFD with 120 m Cable Length
A 22 kW VFD operates at 415 V, 50 Hz, with a motor efficiency of 92% and power factor 0.88. The cable run is 120 meters. Calculate voltage drop and recommend shielding type.
- Step 1: Calculate load current (I)
- Step 2: Assume cable resistance R = 3.5 mΩ/m and reactance X = 0.08 mΩ/m (typical for 10 mm² copper cable)
- Step 3: Calculate voltage drop (Vd) assuming power factor angle φ = cos-1(0.88) ≈ 28.4°
Calculate cos and sin:
- cos28.4° ≈ 0.88
- sin28.4° ≈ 0.48
Calculate inside the parentheses:
Calculate voltage drop:
Step 4: Calculate percentage voltage drop:
This exceeds the typical 3% limit, so a larger cable size or shorter cable length is recommended.
- Step 5: Shielding recommendation: Use copper braid shield with 85% coverage for EMI reduction.
- Step 6: Verify cable meets IEC 60502-1 and IEC 61000-5-2 standards for shielded control cables.
Additional Technical Considerations for Shielded Control Cables in VFD Systems
- Harmonic Currents: VFDs generate non-sinusoidal currents increasing cable heating and reactance.
- Skin Effect: At higher frequencies, current tends to flow near conductor surface, increasing effective resistance.
- Shield Grounding: Proper grounding of cable shield at one or both ends reduces EMI and ground loops.
- Installation Environment: Temperature, chemical exposure, and mechanical stress affect cable selection per IEC 60502-1.
- EMC Compliance: Shielded cables must comply with IEC 61000 series for electromagnetic compatibility.
- Derating Factors: Adjust cable ampacity for ambient temperature, grouping, and installation method per IEC 60364-5-52.
Summary of IEC Standards Relevant to Shielded Control Cable for VFD
| IEC Standard | Scope | Relevance |
|---|---|---|
| IEC 60502-1 | Power cables with extruded insulation and their accessories | Defines cable construction, voltage ratings, and insulation types |
| IEC 60228 | Conductors of insulated cables | Specifies conductor materials and cross-sectional areas |
| IEC 60364-5-52 | Electrical installations of buildings – Selection and erection of electrical equipment | Guidelines for voltage drop and cable sizing |
| IEC 61000-5-2 | Electromagnetic compatibility (EMC) – Installation and mitigation guidelines | Requirements for shielding and EMI reduction |
| IEC 60034-17 | Rotating electrical machines – Part 17: VFD-fed induction motors | Guidance on cable and motor compatibility with VFDs |
Practical Tips for Selecting Shielded Control Cables for VFDs
- Always select cables with copper conductors for superior conductivity and flexibility.
- Choose cable insulation rated for at least 90°C to handle VFD heat dissipation.
- Use cables with 85% or higher shield coverage to minimize EMI and signal distortion.
- Consider cable construction with low capacitance and inductance to reduce signal loss.
- Verify cable compliance with IEC standards and local electrical codes.
- Factor in installation conditions such as ambient temperature, conduit fill, and grouping.
- Consult manufacturer datasheets for detailed cable parameters and derating factors.
For further reading and official standards, visit the International Electrotechnical Commission (IEC) website.