Phase Unbalance
Unbalance of a three-phase system is less extreme than a complete loss of phase, but may have similar consequences. On new installations of three-phase power systems, careful attention is given to balancing the loads on each phase. However, as single-phase loads are added to these originally balanced systems, an unbalance may occur. Thermal overloads, magnetic breakers, and other such devices will not detect this gradual unbalance and therefore will not provide adequate protection.
Voltage unbalance of a three-phase system is expressed as a percentage value, and is often defined as the maximum deviation from the average of the three-phase voltages or currents, divided by the average of the three-phase voltages or currents. This voltage unbalance is calculated as shown below:
Voltage Unbalance = 100 x Maximum Deviation from Average Voltage
Average Voltage
With phase-to-phase voltages of 230, 232, and 225, the average would be 229 volts.
230 + 232 + 225
3 = 229
The maximum deviation from the average is 4 volts.
229 - 225 = 4
Therefore, the unbalance is 1.75 percent.
100 x 4
229 = 1.746
Phase voltage unbalance causes three-phase motors to run at temperatures greater than their published ratings. This excessive heating is due mainly to negative-sequence currents attempting to cause the motor to turn in a direction opposite to its normal rotation. These higher temperatures soon result in degradation of the motor insulation and shortened motor life. The percent increase in temperature of the highest current winding is approximately two times the square of the voltage unbalance. For example, a 3 percent voltage unbalance will cause a temperature rise of about 18 percent.
3 squared x 2 = 18
The greater the unbalance, the higher the motor winding temperature and the sooner the insulation will fail. NEMA standards recommend a maximum voltage unbalance of 1 percent without derating the motor. The motor can be derated down to 75 percent for a maximum of a 5 percent voltage unbalance. If the voltage unbalance exceeds 5 percent it is not recommended that the motor be operated. A rule of thumb states that for every 10°C a motor is operated over the rated temperature rise, insulation life (and therefore motor life) is reduced by half.
