# Negative sequence currents in an alternator

Homework Helper
Gold Member
We recently performed one experiment regarding measurement of negative sequence impedance of an alternator(unloaded) in our lab session. The alternator was star connected and we shorted Y and B phases together, leaving R phase open(an L-L fault). How does this arrangement generate negative sequence currents? How is the direction of rotation of magnetic field reversed? Please help. Many thanks in advance!

anorlunda
Staff Emeritus
https://en.m.wikipedia.org/wiki/Symmetrical_components

In electrical engineering, the method of symmetrical components is used to simplify analysis of unbalanced three-phase power systems under both normal and abnormal conditions. The basic idea is that an asymmetrical set of Nphasors can be expressed as a linear combination of N symmetrical sets of phasors by means of a complexlinear transformation. In the most common case of three-phase system, the resulting "symmetrical" components are referred to as direct (or positive), inverse (or negative) and zero (or homopolar). The analysis of power system is much simpler in the domain of symmetrical components, because the resulting equations are mutually linearly independent if the circuit itself is balanced.

Homework Helper
Gold Member
We measured the open circuit voltage VRY and short circuit current IYB. The ratio VRY/IYB is the negative sequence impedance. How?

anorlunda
Staff Emeritus
Well, the very definition of impedance is the ratio of voltage to current. The only question is which impedance it is it?

I suggest that you study symmetrical components enough to solve the homework problem of making an equivalent circuit for a L-L fault. Once you have that, it should be obvious which impedance you are measuring.

We recently performed one experiment regarding measurement of negative sequence impedance of an alternator(unloaded) in our lab session. The alternator was star connected and we shorted Y and B phases together, leaving R phase open(an L-L fault). How does this arrangement generate negative sequence currents? How is the direction of rotation of magnetic field reversed? Please help. Many thanks in advance!
You should be able to plug and chug, but I'm not able to use a program to give me a quick and dirty answer, and I don't have time to do the calculation by hand.

See if this can shed any light onto your problem...
https://cdn.selinc.com//assets/Lite...etrical-Pt1_AR_20130422.pdf?v=20150812-081134