If the load angle, delta is made to be 0, i.e. the magnetic fields of the rotor and the stator of a synchronous generator are in phase there is no real power delivered but there is some reactive power still present. Now I understand mathematically, that because real power is proportional to sin(delta), therefore, if delta = 0, real power = 0. Likewise, as reactive power is proportional to cos(delta), therefore, if delta = 0, reactive power > 0.

However, in a physical sense, why is it that there is a reactive power when the two magnetic fields are aligned perfectly and no real power? Is it to do with the coiling in the generator constituting to the reactive power? Is there a physical relationship between field alignment and the powers?

Any help would be greatly appreciated!

Thank you!


1 Answer 1


If there is a stator magnetic field, there must be stator current. The stator current maintaining the magnetic field implies that there is reactive power. The reactive power represents energy circulating between the generator stator and the grid.

Since the torque angle is zero, no real power is generated. The prim mover is still providing power for mechanical losses. The excitation power must be supplied by the grid or by some auxiliary power source. The stator losses must be provided by the grid.

  • \$\begingroup\$ I see, that makes sense for the reactive power, but what are the implications for the real power? Thank you. \$\endgroup\$
    – Sputn1k
    Dec 13, 2021 at 1:59
  • \$\begingroup\$ See addition to answer, \$\endgroup\$
    – user80875
    Dec 13, 2021 at 3:11
  • \$\begingroup\$ I understand that no power is generated is when torque angle is 0, but why is this case? Is it because no real work is being done? Thank you \$\endgroup\$
    – Sputn1k
    Dec 13, 2021 at 3:13
  • 1
    \$\begingroup\$ Torque angle = 0 means no torque. No torque means not mechanical input power for conversion to electrical power. \$\endgroup\$
    – user80875
    Dec 13, 2021 at 3:16
  • \$\begingroup\$ I see that makes a lot of sense, thank you very much. \$\endgroup\$
    – Sputn1k
    Dec 13, 2021 at 3:21

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