Which method can be used to detect when the direction of the magnetic field of two alternators is matched?

The three lamp method is a widely used technique for detecting when the direction of the magnetic fields of two alternators is matched. This method involves connecting three identical lamps in a specific configuration to the alternators. When the magnetic fields are aligned, the lamps will light up uniformly, indicating that the alternators are in phase and can be synchronized for parallel operation.

In this method, if the lamps shine brightly and equally, it means that the alternators are producing voltage in the same direction, confirming that their magnetic fields are aligned. If the magnetic fields were not matched, the lamps would flicker or show varying brightness, signaling that the alternators would not be in phase.

The other methods listed do not provide the same clear indication of magnetic field alignment. For instance, the single lamp test only indicates the presence of voltage rather than the relationship between the two alternators. A voltage meter test allows for measuring electrical potential but does not directly indicate the direction of the magnetic field. Resistance measurement assesses the impedance of a circuit but does not reflect the alignment of magnetic fields. Thus, the three lamp method is uniquely suited to this specific requirement of detecting aligned magnetic fields between alternators.