My question is not for a specific problem, but in general. I don't quite understand how to calcuate emf for alternators. I will give an example or two to give you an idea.(adsbygoogle = window.adsbygoogle || []).push({});

1. The problem statement, all variables and given/known data

Example 1) A star-connected alternator on infinite bus vars supplies 500MW at 23kV at a power factor of .95 lagging. The synchronous reactance is 2.8 ohms. Calculate a)line current, b) reactive VA,c) the corresponding open-circuit emf, d) the load angle

Example 2) The alternator described in 1) is operated at the same current a a power factor of .95 leading. Calculate a) the power output, b) the reactive VA,c) the open-circuit phase emf, d) the load angle

Please refer only to the bolded portions of those 2 examples.

3. The attempt at a solution

For 1), I don't quite understand how to get the open-circuit emf. I have the solution to this, and for 1c., it has:

Eph^2 = (Vph + Xs*Iph*sin(angle))^2+(Xs*Iph*cos(angle))^2

where Eph is emf, Vph is phase voltage, Iph is phase current, Xs is reactance, and angle is cos^-1(power factor).

I am not sure how to determine this with the given info in the problem.Ifa phasor diagram was given, I'd be able to know this, but without the diagram, how can determine this?

For 2c., the solution is:

Eph^2 = (Xs*Iph*cos(angle))^2+(Vph-Xs*Iph*sin(angle))^2

And I don't see how that came about.

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In general (for a leading power factor), I think Eph^2 = (Iph*R)^2 + (Iph*X)^2, hence why the above is confusing me.

Load angle is just the angle between Eph and Vph (Iph*R), so if I can figure out Eph, that won't be a problem.

Any ideas/tips/suggestions on this matter is greatly appreciated. Thanks

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# Homework Help: Calcuating Open-Circuit emf for Alternators

Can you offer guidance or do you also need help?

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