Loosing a phase of a 3-phase system
Loosing a phase of a 3-phase system
(OP)
If I have a 4.16kV system that is stepped down through a 4.16kV Delta to 480V Y transformer what happens when I blow a fuse on the 4.16kV system?
Obviously I know that all motors on the 480V system (MCC) will single phase and I understand the effects of single phasing. What will be some of the other adverse effects that would occur besides single phasing? Obviously smaller 3-phase 120/208 transformers would be effected as well.
I read somewhere once that when a phase is lost the single phasing motors somehow act to create this missing phase when a fuse blows. I cannot seem to remember where I read this.






RE: Loosing a phase of a 3-phase system
This is if there is no load. Connected motors may change the voltage somewhat because of back emf.
RE: Loosing a phase of a 3-phase system
I am assuming that the 180deg phase difference is due to the fact that on the primary since phase A is open then this just becomes a series circuit from phase C throgh the junction at A and back to B. AB and CA are therefore in phase with each other ane are the equivelent of a CB phasor. This CB phasor is 180deg offset from the BC phasor which is at 4.16kV.
On the secondary wye the phasor relationships would be reflected from the primary. Phases A-N and C-N would be in phase with each other at a magnitude of 138V and at an angle of 0deg. Phase B-N would be 277V and would be at and angle of -180deg reflected from the primary.
Am I looking at this correctly?
RE: Loosing a phase of a 3-phase system
RE: Loosing a phase of a 3-phase system
( or lose a phase....same difference to the motor ) the motor may continue to run, providing its not loaded up. If its loaded up, it will trip eventually.
RE: Loosing a phase of a 3-phase system
jghirst
I am not following exactly what you mean by rotating the secondary by 90deg? Why would there not be a 30deg phase shift between L-L and L-N voltages?
So from looking at this in the case of a motor if you go take voltage measurments at the motor you will not read 0V L-G on any of the leads but will read 138V and 277V respectivaly. However these are only seperated by 180deg so this is still a single phase case.
If the open circuit were to happen on the 480V side we would read 0V on the open circuit motor lead L-G however we would still have a single phase situation. So the only difference betwwen an open circuit on the primary of a transformer and the secondary are the voltage readings you will see at the loads. The same will apply for load transformers.
RE: Loosing a phase of a 3-phase system
RE: Loosing a phase of a 3-phase system
RE: Loosing a phase of a 3-phase system
Would the secondary L-N voltages be 138V, 138V and 277V as I mentioned above?
RE: Loosing a phase of a 3-phase system
You are correct about the secondary Ø-N voltages being 138V, 138V, and 277V.
RE: Loosing a phase of a 3-phase system
Usually one motor will trip out first and through more load on the other motors, which will quickly follow suit and also trip out. After all motors have tripped off-line, the system will revert to the single phase and 50% voltages described above.
Another possibility is that the load that the motors, acting as induction generators, are trying to support is so large in relation to the motor capacities that voltage drop drops out the motor controls almost immediately.
Bill
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"Why not the best?"
Jimmy Carter
RE: Loosing a phase of a 3-phase system
RE: Loosing a phase of a 3-phase system
RE: Loosing a phase of a 3-phase system
RE: Loosing a phase of a 3-phase system
Does zero zequence current not transfer through a Delta Wye transformer?
RE: Loosing a phase of a 3-phase system
RE: Loosing a phase of a 3-phase system
RE: Loosing a phase of a 3-phase system