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| Subject - three phase imbalance | |
| tgjb | Does anyone know what effect the loss of one phase of a three phase power system have on the voltage supply of the remaining phases? I've heard it can cause the other phases to develop high voltages. Anyone have any experience with this? |
| JimmyDee | I don't think technically you can only loose one phase of the system. You can loose one leg and that would bring you down to a single phase situation. It should not affect the voltage of the other phase. Jim |
| Electricman | I had an service call at an motel where the service was three phase with one leg opened, the only problems they were having was AC units single phasing and some rooms had no power at all. |
| ricky | I do HVAC/R work.When you lose a phase on 3 phase it is called single phasing.The amperage in the other two phases increases and also in the windings of compressors and other motors.This will burn out the windings of the motor.The voltage actually drops because of the extra load.It is common to install a phase protector which senses a phase drop and turns the equipment off to avoid a burnout.Phase sensors get expensive and elaborate,but they either sense the missing voltage,or an increase in current in the remaining phases.They then cut the 24volt control voltage until 3phase power is restored.I have installed many of these.Blowing a fuse will cause single phasing also.Hope this info helps. |
| jackleung | Loss of one phase in three phase system, i.e. two phases left with 120 degree phase shift. If the neutral point is not installed and earthed for some equipment (e.g. no neutral in 3 phase motor), the remaining two phases will develop a line voltage which is sqrt(3) times phase voltage. Since the voltage is boost up, the insulation or protective devices may get into trouble. |
| JimmyDee | quote: This may be true if you are using a Y connected system for lighting but if the system is used for powering a 3 phase motor, a loss of one leg will cause the loss of 2 phases and will result in a single phasing of the motor. Motor will burn up if not protected. Only voltage changes will be from IR loss resulting from amp increase caused by single phasing condition. Jim |
| boccinono | Loosing a leg of a 3 phase system will not cause your voltage to rise on the other 2 legs. If you loose a leg on a 3 phase motor while running, it will continue to run, albeit, at a higher amperage, until its shut down either manually, or by thermal overloads. Once a 3 phase motor is shut down , it will not start after single phasing. |