List hazards of phase failure and phase reversal.

If two of the lines supplying power to a three-phase motor are reversed, it will cause the motor to reverse the direction of rotation. This can be a serious problem with some types of equipment. Unintended reversal of direction can cause gear teeth to shear, chains to break, and the impeller of submersible pumps to unscrew off the end of the motor shaft. It can't only cause dam age to equipment but also injury to operators or personnel in the vicinity of the machine.

Phase failure occurs when power is lost to one of the lines supplying power to a three-phase motor. The motor will continue to operate but will draw an excessive amount of current. In this condition, the overload relay should cause the motor starter to disconnect the motor from the power line if the overload heaters have been sized correctly. Single phasing will cause the two phases that remain energized in a three-phase motor to increase current by an average of 173%.

Effects of Voltage Variation on Motors

Motors are affected when operated at other than their rated nameplate voltage. NEMA rated motors are designed to operate at plus or minus 10% of their rated voltage. Fgr. 1 shows the approximate change in full load current and starting current for typical electric motors when operated over their rated voltage (110%) and under their rated voltage (90%). Motors are in tended to operate on systems with balanced voltage (the voltage is the same between all phases). Unbalanced voltage is one of the leading causes of motor failure.

Unbalanced voltage is generally caused when single phase loads are supplied by three-phase systems.