The MGR ( Main Generator Relay ) is located on the F/E panel.

The AGR (Aux Generator Relay) is located on the APU panel.

There is also an Essential Power Selector switch, which powers critical instruments and has multiple ways to get its power.

On the F/E panel each engine driven generator is driven by a CSD (Constant Speed Drive) which allows the generator to spin at 6,600 rpm regardless of engine RPM.

So, looking at each generator on the F/E panel from bottom up, you have the CSD knob to adjust the generator cycles to 400, then you have the GFR (Generator Field Relay) which turns the generator on and off by taking the PMG (Permanent Magnetic Field) voltage and feeding this into a transformer to build up the voltage to 115 volts A.C. With the GFR open you would get just the voltage of the magnet spinning which gives maybe 15 or so volts. But, by feeding this small voltage into a transformer the voltage is increased to the 115 volts.

Next in the stack, you have the MGR (Main Generator Relay) which connects the generator to its respective load bus. Each generator can power its respective load bus. However, one generator can power the whole aircraft through the next relay called the BTR (Bus Tie Relay).

With just one generator powering the plane via the BTR there is no magic, you are just plugging in more load buses to the single generator.
When you tie two or three generators together they must be in sync. Thus the BTR's become a Sync Bus with many possible faults associated with putting multiple electrical sources together.

That's the down and dirty of the 727 A.C. system.

D.C. is made from the A.C. using TR's (Transformer Rectifiers) which take the 115 volt 400 cycle A.C. and turns it into 28 volt D.C. The D.C. is used to run relays to control bigger load A.C. motors and various small load items such as lights, instruments and radios.

Does this help, or is it too much data?

Lou