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When a motor is designed for a certain line voltage, the magnetic field it creates is proportional to the applied voltage. If you feed a 230V three-phase motor with 208V, the flux is weaker, so it can't develop as much torque. Torque roughly scales with the square of the voltage, so the available torque drops to about (208/230)^2 ≈ 0.82 of the rated torque—roughly 18% less. If the load needs the full torque, the motor won’t maintain speed and will slip more. That increased slip raises rotor losses and heat, leading to overheating. Under light loads it may run okay, but under heavier loads or during startup the reduced torque is the cause of potential overheating.