It’s impossible to balance line-to-line voltages perfectly in three-phase circuits, so they typically differ by a few volts or more. However, if voltage unbalance exceeds 1%, it can markedly decrease the performance and energy efficiency of three-phase motors while increasing the likelihood of premature failure.

The three-phase squirrel cage induction motor is a very robust machine that can operate over a wide range of voltage conditions; it is the workhorse of the industry. However, for optimum performance and life expectancy, the voltage supply should be a balanced three-phase, sine-wave voltage, and its magnitude should be as close to the nominal voltage stated on the motor nameplate as practical. 

Any deviations from these conditions may cause a depreciation in the motor performance and expected life, and increase energy costs. Benchmarking the service entry voltage and the motor performance may be a useful tool to assist in knowing when reasonable limits have been exceeded and that corrective action needs to be taken. 

The following detrimental conditions may alone or in combination exist at various times or continuously: 

1. Voltage variation (+/- nominal)
2. Voltage Unbalance
3. Transient voltage (surges)
4. Voltage waveform distortion (harmonics)

Unbalanced voltages are unequal line-to-line voltage values on 3-phase circuits that can exist anywhere in a power distribution system. Unbalanced voltages can cause serious problems,particularly for motors and other inductive devices. Perfectly voltage-balanced circuits are not possible in the real world. Typically, the circuit line-to-line voltages may differ by a few volts or more. It's when voltages differ by more than 1% that problems tend to occur.