# Electricity: Part 3 - AC Systems

Here we look at alternating current (AC) systems and how generating AC often requires an intermediate step of converting to DC to improve the efficiencies of AC generators.

Fig.1. The RMS (root mean square) voltage of an AC power system develops the same power in a resistive load as a DC voltage. Note the peak voltage is 1.4 times the RMS value.

Fig.2 left) A three-phase system having a neutral conductor. If the load on each phase is identical, the vector sum of currents at the star point is zero and the neutral conductor can be removed. right) A three-phase transformer. The net flux between the ends of the magnetic circuit is zero and flux in any one limb is the inverse of the vector sum of the fluxes in the other two limbs.

Fig.3 - In a constant speed drive, the hydraulic variable speed path is added to or subtracted from the fixed speed path in a differential so that as engine speed varies the alternator frequency can be kept constant.

Fig.4 - The invertor generator produces bulk DC using a rectified permanent magnet alternator. The AC output comes from an invertor so the speed of the generator can change. The generator only runs as fast as necessary to fulfill the load requirements.

Fig.5- If the inverter generator includes a battery, a much smaller generator can drive loads that require heavy starting currents. The battery provides the starting power and then recharges.

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