Capacitor-Excited Generators

Hi All,

Has anyone out there got a definitive explanation of how a capacitor regulated alternator (generator) works? I have repaired dozens of these little beasts, the biggest being about 20kW but I have never seen any explanation of their actual working principle.

I have always thought that the residual magnetism generated a current in the capacitor circuit, phase shifted from the main winding, and this resulted in the current being induced into the rotor. When more load is drawn from the output then more current is generated in the rotor, I presumed by a shift in phase, similar to that seen in the current in a loaded and unloaded induction motor.

(The member who's signature line is " if you cannot explain it simply you don't understand it well enough" is never more apt!)

Have I got it right, if so is there a better explanation? Or, if totally wrong then please explain.

I am actually having problems with one at this moment. The unit is a 3600rpm, 2p 60Hz 10kVA belt driven shaft generator.(see diagram)

tests carried out:-

Output windings______Pass 4Amps through both windings in series, Vgrep across each is 5.9V and 6.2V. Is this sufficient imbalance to conclude shorted turn(s)?

Diodes, OK, tested in situ with a 12V 50W lamp, as an ohm meter cannot read them in parallel with the windings.

VDRs______Untested, but these do not come into play until the rotor has current in it and are there to dissipate the reverse voltage spikes. They are not short circuit.

Rotor windings,______ about 6 Ohms each, taken as being good due to equality.

Capacitors, 2 x 30uF, tested ok

Capacitor winding,_______ 0.6 Ohm

spin to 3600rpm,_ 1.7V residual from each of the 110V terminals. conclusion, output windings are OK (It is improbable that they have equal faults)

Add 60uF to output terminals, unit loads prime mover (in our test case a powerful electric drill). Output rises to 40V

Attempt to add a resistive load of 500W, output reduces to zero

remove capacitor, voltage drops to residual and loading ceases.

All windings megger clear of contact with the frame.

Visual inspection, no signs of burning, water damage or other cause of failure.

Having written the above I have come to the conclusion that the output windings MUST have some shorted turns. Can anyone confirm my diagnosis?

Regards

Chas