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We know an alternator or in general any source of energy does not generate energy but converts one type of energy into some other usable form.

An electric generator produces both real and reactive power. The source of the real power is super heated steam, diesel engine or running water.

But what is the back source of the reactive power generated by the alternator, say if operating alone without any compensation devices.

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An electric generator produces both real and reactive power.

Not unless there's a reactive load. With a purely resistive load it only produces real power.

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Reactive power is the continuous transfer of stored energy back and forth between two storage locations. The initial energy is generated as real power. After that, real power is generated to supply the energy lost in transfer, but the most of the energy is just transferred back and forth, with one round trip per cycle of the AC waveform. In most cases, the energy is stored in the magnetic field of induction motors on the load side and either wound-field synchronous generators or capacitors on the source side.

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what is the back source of the reactive power generated by the alternator?

The generator converts mechanical torque and RPM into available current and voltage output. The Alternator can regulate the output Voltage by field current being applied.

The output current for a given voltage source depends on the load complex impedance more than the source impedance which must be much lower.

Therefore the reactive power output is determined by the load, not the source or the Alternator.

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  • \$\begingroup\$ Thumb Up: Therefore the reactive power output is determined by the load, not the source or the Alternator. \$\endgroup\$ – ferahfeza Dec 19 '19 at 18:37
  • \$\begingroup\$ @ferahfeza since the vehicle battery is actually equivalent to a million or so Farads and an ESR of 5V/CCA so the resistive impedance dominates the load the 3 phase field controlled current source peaks at peak voltage to charge the battery. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 19 '19 at 18:48
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It's the same real component that changes the behaviour. The reactive energy can't be created without inserting the real energy and the reactive energy can be transformed back into real energy. It's kind of energy accumulator.

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