Although some generators are still produced with brushes and
sliprings, most now have a brushless excitation system. One of the main
techniques for synchronous generators is capacitor excitation and this is
described in the following sections.
The rotor is usually of salient-pole construction as
described previously, but in this case the rotor winding is shorted through a
diode. On starting, the residual flux in the rotor body induces a small voltage
in the stator excitation winding and a current flows through the capacitor.
This current produces two waves of magnetic flux around the
air gap of the generator. One wave travels in the same direction as the rotor,
to create the armature reaction.
The second wave travels in a direction opposite to the
rotor, and induces a voltage in the rotor windings at twice the output
frequency. The current circulated in the rotor windings by this induced voltage
is rectified by the diode to produce a dc current.
This dc current increases the magnetic flux in the machine,
which in turn drives more current through the stator excitation winding, which
in turn produces more rotor current. This self-excitation process continues
until the flux reaches a point at which the magnetic circuit is saturated, and
a stable voltage results.
Dear Sir
ReplyDeleteIn this type of generators, if the residual magnetism is lost, then how to recover the magnetism and in where we have to apply dc voltage to recover residual magnetism either in rotor winding or in stator auxillary winding. please clarify.
If you apply a DC voltage (12v) across the capacitor wires to provide an intial field you can re-excite the field! Sometimes required after long term storage!
Deletewhat are usually the value of the capacitor of the exciter winding?
ReplyDeleteI have a generator with 3 40uF caps and produces 120v at NOLOAD and 107v at 4000 watts of load. I would like to increase the voltage to 130v at NOLOAD near 120v at 4000 watts, so I am interested in increasing the capacitors value to do so. Would 45 uFds help? or 50 uFds? or am I barking up the wrong tree
DeleteWhy do you want to increase the voltage?
Deletemore load drops the voltage to less than 100v. I put in 3 50uFD caps, and voltage came up to 124v noload, 118v at 4000w and 112v at 5500w, I am good
ReplyDeleteYou must try to install a separate voltage regulator to help the Capacitors to maintain a constant voltage.
ReplyDeletehow do you install a seperate voltage regulator
DeleteI have one, 5KVA continuous, single phase 120/240. Bought used, running but not.making power, from an owner who said he acquired it in that state with purchase of a hunting cabin. So, twice removed from the last owner who got power from it. Testing gave 2V and 3V AC on each 120v circuit. Self excited brushless generator, capacitor tests good, all windings show good resistance and isolation from each other and ground.. Removed rotor, observe two coils, coaxial, with 2 rectifier diodes ZY30-16 observed to have been connected in opposite current directions, one disconnected. Internally one diode is open both directions, one is shorted both directions.
ReplyDeleteConclusion (asking for verification/sanity check..) someone tried to replace rotor diodes once before, botched job several ways.
Question.. when rotor's single pole has two coax coils, each wound from center, counterclockwise (looking from its side of the rotor shaft) out to end of pole then shorted through diode.. such that the unit looks exactly the same when rotated 180°.. are the two diodes each connected to allow current flow from coil outer end back to inner start, or should one be connected this way and the other in reverse? Searching 3 days now, I can't find a reference for orientation of 2 rectifier diodes on a single rotor pole of two coils - please help!