Tech Talk: 3-Phase Motor with One Lost Phase

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.

4 thoughts on “Tech Talk: 3-Phase Motor with One Lost Phase

  1. Common causes of phase loss can occur at engagement contacts as well as loosened terminal post. I suggest tightening all terminals used in the path of power to your motor. These can loosen from vibration and temperature variations. Consider replaceming relays, contacts and fuses as these can operate intermently.

  2. I really appreciate you for publishing this blog here about the tech talk 3 phase motor with one lost phase. This is really appreciated that you have shared the tips for hiring an emergency electrician, I love all the information shared. Great article!I have also found this resource useful and its related to what you are mentioning.

  3. If fuse 2 and 3 were bad. What would be the voltage be around fuse 3? And the voltage with the leads on lines 1 & 2? And the fuses were blown when the motor was running. So all of the contacts are still closed by the 24 volt control circuit. Fuse 1 and line one is still good.

    1. Hello Jason,

      If a 3 phase motor loses 1 or 2 phases, it’s called “single phasing” and the voltage on the remaining phase should not change (277V if 480V line to Neutral, for example) The motor will draw more current and possibly continue running at a lower RPM and the internal heat will increase and eventually damage the motor.

      If you have more questions, please feel free to call our technical sales department.

      Kele Inc

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