One of the following conditions is not necessary when the alternator is operating in parallel Same rating.

When operating alternators in parallel, it is important to ensure that they are synchronized and operate in unison. The most important parameters that must be the same for the alternators are the terminal voltage, phase rotation, and frequency.

The terminal voltage must be the same because if one alternator has a higher voltage than the other, it will feed power back to the other and can cause damage to the alternator. Phase rotation refers to the direction of the magnetic field in the stator of the alternator, and if the phase rotation is not the same, the two alternators cannot be synchronized and will not work together properly.

Frequency is also an important parameter because the alternating current (AC) generated by the alternators must be in phase with each other. If the frequency of one alternator is slightly different from the other, it will cause a phase difference between the AC generated by the two alternators. This phase difference can lead to circulating currents between the alternators and may cause power imbalances.

While having alternators of the same rating is ideal when operating in parallel, it is not strictly necessary as long as the other parameters are the same. However, having alternators of different ratings may cause issues such as overloading the smaller alternator or underutilizing the larger alternator.