The Hidden Dangers of Running Motors Under Incorrect Voltage

Negative Consequences of Operating Electric Motors Under Deviated Voltage Conditions

Like all electrical devices, electric motors are designed to operate under a specified rated voltage. Any deviation from this rated voltage can negatively impact the motor’s performance and longevity. High-end electrical equipment typically includes protection mechanisms that disconnect the power supply when abnormal voltage is detected. Precision instruments often use voltage stabilizers to ensure a constant voltage supply. However, industrial electric motors rarely utilize voltage regulators and rely more on power-off protection in the event of voltage irregularities.

For single-phase motors, voltage deviation usually falls into two categories: over-voltage and under-voltage. In contrast, three-phase motors not only face high or low voltage issues but also voltage imbalance among the phases. All three situations can lead to increased current or unbalanced current, resulting in harmful consequences.

According to motor technical specifications, the allowable voltage deviation should not exceed ±10% of the rated voltage. The torque output of a motor is proportional to the square of its terminal voltage. If the voltage is too high, the iron core of the motor becomes magnetically saturated, causing a significant increase in stator current. This leads to excessive heating of the windings and may even result in serious damage such as winding burnout.

When the voltage is too low, motors—especially those operating under load—may struggle to start. To maintain operation under load, the motor must draw more current, which again results in overheating and potential winding damage. Prolonged low-voltage operation poses even greater risks to motor life and performance.

Voltage imbalance in three-phase motors is often a power supply issue. It causes current imbalance within the motor. The negative sequence component of the unbalanced voltage generates a magnetic field in the air gap that opposes the rotor’s direction of rotation. Even a small negative sequence voltage can cause a disproportionately large increase in current through the windings. The current flowing through the rotor bars can reach frequencies nearly twice the rated frequency, leading to significant skin effect losses. These losses in the rotor windings are much greater than those in the stator windings, resulting in increased temperature rise in the stator compared to balanced conditions.

Voltage imbalance also reduces the motor’s locked-rotor torque, minimum torque, and maximum torque. In severe cases, the motor may fail to operate properly. When running under full load with unbalanced voltage, the rotor slip increases due to additional rotor losses, slightly reducing the motor’s speed. As voltage imbalance intensifies, motor noise and vibration may worsen. Excessive vibration can damage the motor or the entire drive system.

To identify voltage imbalance issues, one can monitor the power supply voltage or analyze current fluctuations. Most equipment includes voltage monitoring instruments that allow for data comparison and analysis. In systems without such monitoring, periodic voltage checks or current measurements should be conducted. For motors that support bi-directional rotation, switching two phases of the power supply without affecting the driven load can help observe current changes and indirectly assess voltage balance. If voltage imbalance is ruled out, potential issues such as inter-turn or inter-phase short circuits should then be considered.