According to the IEC, voltage unbalance is defined as the ratio of negative sequence voltage to the positive sequence voltage. Briefly explained, the three phase voltages can be mathematically expressed as a sum of positive, negative and zero sequence components. Positive sequence voltage creates flux in the direction that the motor is intended to rotate, and negative sequence voltages rotate in the opposite direction. This creates flux in the opposite direction, however, since the positive sequence voltages are always much larger than the negative sequence voltages the direction of motor rotation is not affected.
IEC 60034-1 imposes a 1% negative phase sequence voltage limit on the supply feeding machines. However, EN 50160 states that imbalances of up to 3% can be expected, and indicates that an acceptable supply system standard is that “under normal operating conditions, during each period of one week, 95% of the 10-minute mean rms values of the negative phase sequence component of the supply voltage shall be within the range 0 to 2% of the positive phase sequence component”.