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Voltage Imbalance - Derating Factor in Polyphase Motors

Efficiency of electrical polyphase motors decreases with increased voltage imbalance

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Electrical motors are designed to operate with a high efficiency within some variation in the electric power supplied.

  • voltage variation should not exceed +/- 10%
  • frequency variation should not exceed +/- 5%
  • combined arithmetic sum of voltage and frequency variation should not exceed 10%
  • voltage imbalance between phases should not exceed 1%

Voltage Imbalance Derating Factor

Voltage Imbalance
Derating factor
0 1.00
0.5 1.00
1.5 0.97
2.0 0.95
2.5 0.93
3.0 0.89
3.5 0.85
4.0 0.82
4.5 0.78
5.0 0.76
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  • en: electrical motor voltage imbalance polyphase efficiency
  • es: tensión del motor eléctrico polifásico eficiencia desequilibrio
  • de: Elektromotor Spannungsungleichgewicht mehrphasigen Effizienz
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  • Engineering ToolBox, (2004). Voltage Imbalance - Derating Factor in Polyphase Motors. [online] Available at: [Accessed Day Mo. Year].

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