Three Phase Alternators - Capacitive Loads?

Thread Starter


Joined Apr 18, 2007
I'm new to this forum and have a few electrical questions...Here is one...

Why does the terminal output voltage on a threephase alternator increase as a capacitive load is added?



Joined Apr 2, 2007
let me start the explanation form 3 facts: (since explanation of these facts wud require some graphs and vector dia and is also a little lengthy)
however in case u want an explanation of those i wud try to explain it to you
or you can refer some book for it.
these facts are for purely resistive loads
1- armature mmf remains constant with time
2-it is 90 space degrees behind the main field mmf (rotor field), thus it only distorts the main field
3- it rotates sync around the armature.(stator)
2nd one is of little more consequence.
for purely inductive load the the armatur currents are delayed in time by 90 deg hence armature mmf gets shited by another 90 deg hence main field mmf and stator mmf are now 180 deg out of phase.
for a purely capacitive load effect is opposite of purely inductive load and armature mmf advances by 90 deg in phase. hence it is now same phase (90-90=0 ) with the main field mmf thus it strengthens it and the effect is magnetising thus causing an increase in the term voltage.
for any other intermediate pf the effect is combination 2 of the above cases
depending on leading or lagging pf.