Open primary winding, 3 phase xfmr, feeding a motor

[rbyrne15]

I am trying to understand the cause of the increase in line currents from the secondary of a three phase transformer, when the primary winding of the same transformer has faulted and has opened. (Assuming a balanced motor load).

My Bussmann "Selecting Protective Devices" book shows only the final figures. While this is OK, I would like to know why (or how) these figures come to be.

A link would be great (my Google searches yielded no mathematical explanations) if people don't have time to explain.

Thanks,

Richard.

 

[waross]

You may have more luck searching induction generators.
With three-phase power you have three winding sets producing shaft power.
With one phase missing the motor is working partly as an induction generator feeding power back into the line and partly as a motor.
In a delta connected motor you have one winding providing all the power to turn the shaft and also supplying the power to supply the other two windings as they act as an induction generator and feed power back into the line. The windings that are acting as an induction generator are trying to slow the shaft down just as the normal load does and the energized winding must supply the energy to support this mechanical load in addition to the normal load.
I chose a delta connection for an example because it is easier to visualize than a wye winding with a floating wye point to complicate things.
The numbers may be a little more confusing with a wye connected motor but the effect is similar.
The motor back-feeds power and gets real hot, real fast. If you're lucky it will trip off line before it damages itself.
yours

 

[jghrist]

If the transformer is delta-wye, an open primary phase will result in 1/2 voltage on two secondary phases, with one phase at the same phase angle as the good phase and one 180 deg out of phase. Two of the primary windings are series connected between the two good phases, with the voltage on the open phase being midway between the two good phase voltages.

The secondary currents depend on the load. If the load is resistive, then the current in two of the phases will be 1/2 because the voltage is 1/2. If the load is motors, you will hopefully get zero secondary current because you will have phase loss protection and the motors will trip off line. Otherwise, you may get higher currents for a while until the motors burn up. ;-)

 

[waross]

Hello jghrist
You are looking at it from the point of view of the transformers and you're correct.
I'm looking at it from the point of view of the motors, which are trying valiantly to act as induction generators and supply the missing phase back to the transformers.
Either way
"If the load is motors, you will hopefully get zero secondary current because you will have phase loss protection and the motors will trip off line. Otherwise, you may get higher currents for a while until the motors burn up."
This says it all.
respectfully