# P-frequency and Q-Vt

Discussion in 'Homework Help' started by mo2015mo, Feb 16, 2014.

1. ### mo2015mo Thread Starter Member

May 9, 2013
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1
Hello my friends...

Through my studying for Synchronous Generators , we used two characteristics as shown in attached photo,
first frequency-real power charac. and their relation equation is P=Sp(Fnl-Fsys.)
and second one, Terminal voltage-reactive power charac. and their relation equation is Q=Sq(Vt nl-Vt sys.)

My question is Why are we relating frequency with real power and relating Terminal voltage with reactive power ?? What are Reasons?

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2. ### shteii01 AAC Fanatic!

Feb 19, 2010
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May 9, 2013
157
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4. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Voltage is related to reactive power because you can use voltage to calculate reactive power.

Reactive power is Q.

AC voltage is Vrms.

Reactive power can only be in inductors and capacitors because they store energy. Inductor stores energy in magnetic field. Capacitor stores energy in electric field. So we need their reactance: XL and XC, since we don't know ahead of time which element we are dealing with, we will use X to denote reactance.

Therefore: $Q=\frac{V_{RMS}^{2}}{|X|}$

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5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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6. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I guess reactive power reflects stored energy in reactive impedance & real power energy input or output in mechanical (rotational) energy. Excess stored reactive impedance energy tends to boost voltage whereas changes in mechanical spinning energy tend to change rotational frequency.

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7. ### mo2015mo Thread Starter Member

May 9, 2013
157
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shteii01 and t_n_k
thank you for your useful information

Last edited by a moderator: Feb 17, 2014