We recently encountered an issue with an 18.5 kW motor, rated at 37A, connected via a 16 mm² cable. On paper, this cable should handle up to 80A, suggesting it was more than adequate for our motor's needs. However, the motor unexpectedly began to fail repeatedly. Upon investigation, it was discovered that the motor was only receiving 340V, far below the expected 380V, due to voltage drop over an extensive 1000-meter cable run.
Understanding Voltage Drop:
- Fundamentals of Voltage Drop:
- Voltage drop happens when cable length is excessively long with an undersized conductor, increasing the cable's resistance. The standard operational voltage for this motor is 380V, with an acceptable voltage fluctuation of ±5% (ranging from 361V to 418V).
- A drop exceeding 19V (below 361V) can lead to motor damage or destruction.
Fundamentals of Voltage Drop:
- Voltage drop happens when cable length is excessively long with an undersized conductor, increasing the cable's resistance. The standard operational voltage for this motor is 380V, with an acceptable voltage fluctuation of ±5% (ranging from 361V to 418V).
- A drop exceeding 19V (below 361V) can lead to motor damage or destruction.
- Consequences of Low Voltage on Motors:
- Lower voltage leads to higher current draw, heating up the motor's windings. When voltage drops by more than 10%, this can cause excessive heat buildup, impacting the motor's lifespan or even causing catastrophic failure.
- Voltage Drop Calculation:
- Voltage Drop = Current × Resistance
- Current: The motor draws 37A.
- Resistance: For copper with a resistivity of 0.0175 Ω/m and a 16 mm² cross-section, resistance = 1000 m × 0.0175 Ω/m ÷ 16 = 1.1 Ω.
- Voltage Drop = 37A × 1.1 Ω = 40.7V, meaning the motor was supplied with only 339.3V, well outside safe operating parameters and explaining the motor burnouts.
Cable Sizing with Voltage Drop in Mind:
- Sizing Copper Cable:
- Cross-sectional Area = (Current × Length) ÷ (54.4 × Permitted Voltage Drop)
- For our 18.5 kW motor over 1000 meters, the calculation yields: (37A × 1000m) ÷ (54.4 × 19V) ≈ 35.8 mm², indicating we should use a cable size no less than 35 mm².
- Sizing Aluminum Cable (if applicable):
- Cross-sectional Area = (Current × Length) ÷ (34 × Permitted Voltage Drop)
- This calculation gives (37A × 1000m) ÷ (34 × 19V) ≈ 57.3 mm², suggesting no less than a 50 mm² aluminum cable.
Conclusion and Best Practices:
When designing electrical systems for long runs, consider not just the current requirements but also the potential for voltage drop. This case illustrates the critical need for accurate cable sizing or alternative solutions like voltage compensators to ensure equipment operates within safe voltage thresholds. Remember, proper planning can prevent costly failures and extend the life of your electrical installations.