Common causes of motors single phasing on the secondary side are:
• Damaged motor-starter contacts – one pole open. Worn starter contacts are a common cause of single phasing. Wear and tear of the starter contacts can cause contacts to burn open or develop very high contact resistance, resulting in single phasing. This most likely is to occur on automatically started equipment such as air conditioners, compressors and fans.
• Burned-open overload relay (heater) from line-to-ground fault on a 3-or4-wire grounded system. This is more likely to occur on smaller-size motor starters that are protected by noncurrent-limiting devices.
• Damaged switch or circuit breaker on main, feeder or branch circuit.
• Open fuse or open pole in breaker on main, feeder or branch circuit.
• Open cable or bus on secondary of transformer terminals.
• Open cable caused by overheated lug on secondary side (caused by vibration) or any pullbox.
• Open winding in motor.
• Open in one phase of transformer winding.
Source: McGraw-Edison Co., St. Louis
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