wye connected transformer secondary

[gjoo]

On a wye connected transformer secondary if you by accident connect one phase backwards, will this reverse phase rotation and how?

 

[john monks]

Yes. In a three phase motor the rotor is dragged around in the direction of the winding that is being feed by next winding that has the next phase of 120 degrees later. If you draw a picture you will find that there are only two different phase relationships coming from a three phase source and if you reverse any two you have the opposit phase relationships.

 

[woodchips]

No. Think about it as the vector diagram. With correct connection the you have three vectors spaced at 120 degrees. Reversing one phase gives the three positive peaks at 0, 60 and 120 degrees.

The motor is being dragged around by the voltage between two phases. This is normally root3 of the phase voltage. Reversing one phase gives entirely different phase to phase voltages, two of sin60 of phase and one of root3 of phase. The motor can only respond to the phase voltages applied, not what it ought to be. The motor will run very roughly, and if any load will not run at all.

Reversing the motor direction is done by swapping any two connections from transformer to motor, assuming the transformer is a correct wye connection. So if it is Aa Bb Cc then reversing is by Ab Ba Cc or similar.

You can get double wye transformers where by deliberately connecting the phases in normal and reversed order you can get a 6 phase supply which makes high power rectification and smoothing very much simpler.

 

[john monks]

What you are saying makes some sense but not from the motors point of view. I clearly did not explain clearly enough. I should have said that reversing any two wires you have a totally opposite phase relationship. Keep in mind that looking at the graph we are looking at everything referenced to ground. And in a three wire system you don't have the ability to simply reverse one of the polarities by 180 degrees. We must look at the circuit as the motor sees the circuit. And in that vain the motor only sees three phases shifted by 120 degrees. And there are only two combinations of phases. One which causes the motor to turn one way and one the other way. So from the motors point of view we only have two different phase relationships.

 

[woodchips]

It is possibly a question of semantics here. By backwards I understood one phase to be connected so the three star winding connections had one incorrect. This is different to swapped, where two of the three phase wires are interchanged one for the other.

It is not rare for motors or transformers to have 6 winding connections allowing star or delta so being able to connect one phase backwards is very easily achieved. At that point of course you have a three wire system, completely wrong but still three wires. Doesn't matter which phases you swap then the motor still won't run correctly.

Perhaps the original poster could clarify what their thoughts were before posting the question.

 

[john monks]

You are correct. The original post could use further clarification. I just assumed from my personal experience that the three phases are hard wired into the utility transformer and that we don't have access to them. So I assumed that the original post meant that if two wires were reversed what would happen. Because if you connected the motor using a wye windings and one of the windings inside the transformer was reversed then the motor would not work correctly. Semantics does count.