Q: What happens to a 3-phase motor when one of the phases is lost?
The most extreme case of phase unbalance is the total loss of one of the three phases and is known as phase loss or single phasing. Phase loss can be caused by a broken power line, a lightning strike, an open supply transformer winding, a mechanical failure in switching equipment, or when a single fuse blows. A 3-phase motor started in a phase loss condition may stall under load. The current flowing in the windings of a stalled motor will increase to the motor locked rotor current which is around 600% greater than the motor’s normal full load current. Motor winding insulation subjected to locked rotor current may fail in as little as 15 to 90 seconds. A 3-phase motor already running when single phasing occurs will continue to run. The current in the remaining connected windings will increase to 150% of the normal full load current. The extra heat created by the increased current causes the winding insulation to break down and fail. To protect motors from failure due to phase loss conditions, voltage monitor relays are available to stop the motor or prevent it from starting when single phasing occurs. Detecting a phase loss can be tricky. During phase loss, a 3-phase motor can regenerate more than 92% of the supply voltage in the open phase. Kele offers several voltage monitors (TimeMark 258 and 269, and Littlefuse 201A, 460, 250A, 355-200, 355-400, & 355-600) that detect phase loss and other harmful voltage conditions regardless of regenerated voltage. Let Kele help you protect your motors instead of replacing them.