What happens to a three-phase motor when it experiences an unbalanced load?

What happens to a three-phase motor when it experiences an unbalanced load?

• A. It maintains speed
• B. It becomes single-phased and overheats
• C. It increases current but cools
• D. It short-circuits

Answer: (B)

When three-phase motors run under balanced conditions, each winding carries the same current, spaced 120 degrees apart, so the magnetic fields sum to a smooth rotating field and torque stays steady. If the load becomes unbalanced, the currents in the windings are no longer equal. This introduces a negative-sequence component that causes torque pulsations and extra copper loss, which shows up as overheating in the windings. If the imbalance is severe, the motor can draw current predominantly from one phase or behave as if it’s running on a single phase, which greatly increases heating and can lead to damage. That’s why overheating from an effectively single-phase condition under heavy imbalance is the key consequence. The other options don’t reflect the typical outcome: the motor won’t reliably maintain speed under abnormal current distribution, it isn’t inherently a short circuit, and it doesn’t cool simply because the load is unbalanced.

When three-phase motors run under balanced conditions, each winding carries the same current, spaced 120 degrees apart, so the magnetic fields sum to a smooth rotating field and torque stays steady. If the load becomes unbalanced, the currents in the windings are no longer equal. This introduces a negative-sequence component that causes torque pulsations and extra copper loss, which shows up as overheating in the windings. If the imbalance is severe, the motor can draw current predominantly from one phase or behave as if it’s running on a single phase, which greatly increases heating and can lead to damage. That’s why overheating from an effectively single-phase condition under heavy imbalance is the key consequence. The other options don’t reflect the typical outcome: the motor won’t reliably maintain speed under abnormal current distribution, it isn’t inherently a short circuit, and it doesn’t cool simply because the load is unbalanced.