When two or more generators are operated at the same time to supply power for a load, they are operated in parallel; that is, each supplies a proportional part of the ampere load. Successful multigenerator operation requires that each generator share the load equally, since a very small increase in the voltage output of one generator will result in that generator's supplying the greater part of the power needed by the load.
The power supplied by a generator for a load is often referred to as ampere load. Although power is actually measured in watts - the product of voltage and current - the term "ampere load" is applicable because the voltage output of a generator is considered constant; therefore, the power is directly proportional to the ampere output of the generator.
Negative Lead Paralleling
To distribute the load equally among generators operated in parallel, a special coil is wound on the same core as the voltage coil of the voltage regulator. This is part of the equalizing system shown in figure 9-28. A calibrated resistor is located in the lead from the generator negative terminal E to ground. The value of the resistance in this lead is such that when the generator is operating at full current output, there is a 0.5 volt drop across the resistor. This resistor may be a special resistor; it may be a ground lead long enough to have the required resistance, or a series winding of the generator.
The equalizing system depends upon the voltage drop in the individual calibrated resistors. If all generators are supplying the same current, the voltage drop in all ground leads is the same. If the current supplied by the generators is unequal, there is a greater voltage drop in the ground lead of the generator supplying more current. Thus, when the No. 1 generator is supplying 150 amperes and the No. 2 generator is supplying 300 amperes, the voltage drop in the negative lead of the No. 1 generator is 0.25 volt and that in the negative lead of the No. 2 generator is 0.5 volt. This means that point E of the No. 1 generator is at a lower voltage than point E of the No. 2 generator, and current will flow in the equalizing circuit from E of the No. 2 generator to E of the No. 1 generator. The equalizing coil will aid the voltage coil in voltage regulator No. 2 and oppose the voltage coil in regulator No. 1. In this way, the voltage of generator No. 2 will be lowered and that of the other will be increased.
Positive Lead Paralleling
The diagram in figure 9-29 shows two generators carrying a total load of 300 amperes. If the generators were sharing this load equally, the ammeters would each indicate 150 amperes. The generators would be "paralleled" and no current would flow in the equalizing coils between the K and D terminals on the regulators. Note, however, that the ammeter for the No. 1 generator indicates only 100 amperes, but the ammeter for the No. 2 generator indicates 200 amperes. This is an unbalanced load and causes current to flow through the equalizing circuit (dotted lines) in the direction indicated by the arrows.
The reason is as follows: With 200 amperes of current flowing through the No. 2 equalizing resistor (from Ohm's law, E = I x R), there will be a 0.5 volt drop in voltage across the No. 2 resistor. Since there are only 100 amperes flowing through the No. 1 equalizing resistor, there will be a one-fourth volt drop across that resistor, and a difference in voltage of one-fourth volt will exist between the two resistors. Since current flows from a high pressure (potential) to a lower pressure and from negative to positive, it will be in the direction indicated by the arrows. When the load is equal, there will not be a difference in voltage between the two resistors.
The current can be traced through the equalizing circuit and through the voltage regulator coils to show the effect on the electromagnets. With the current in the direction shown, the equalizing coil and voltage coil of the No. 1 voltage regulator set up magnetic fields opposing each other, thus weakening the electromagnet of the No. 1 voltage regulator. This allows the spring to compress the carbon disks, decreasing their resistance and allowing more current to flow in the field circuit of the No. 1 generator. As a result, the voltage output of that generator increases, but at the same time, the current through the equalizing coil and voltage coil of No. 2 voltage regulator sets up magnetic fields that aid each other, thus increasing the strength of that electromagnet. This decreases the spring pressure on the carbon disks, increasing their resistance and allowing less current to flow in the field circuit of the No. 2 generator. As a result, the voltage output of this generator will decrease. With the voltage output of the No. 1 generator increased, the voltage drop across No. 1 equalizing resistor increases; and with a decrease in voltage output of the No. 2 generator, the voltage drop across the No. 2 equalizing resistor
decreases. When the voltage output of the two generators is equal, the voltage drop across the equalizing resistors will also be equal. No current will flow in the equalizing circuit, the load will be balanced, and the ammeters will read approximately the same. The generators are then paralleled.
The purpose of the equalizing circuit is to help the voltage regulators automatically by lowering the voltage of the high generator and raising the voltage of the low generator, so that the total will be shared equally by the generators.