# Power Factor vs Sync. Motor/Generator - Under/Over excited?



## COJeff

Can I get some clarification please:

If a generator is over excited:  Leading PF

If a Sync. motor is over excited:  Leading PF

If a generator is under excited:  lagging PF

If a Sync. motor is under excited:  lagging PF

Thanks for being patient.


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## BebeshKing PE

COJeff said:


> Can I get some clarification please:
> 
> If a generator is over excited:  Leading PF
> 
> If a Sync. motor is over excited:  Leading PF
> 
> If a generator is under excited:  lagging PF
> 
> If a Sync. motor is under excited:  lagging PF
> 
> Thanks for being patient.


If a generator is over excited:  Leading PF  *(an overexcited generator has a "lagging" power factor)*

If a Sync. motor is over excited:  Leading PF  *(correct)*

If a generator is under excited:  lagging PF *(an underexcited generator has a "leading" power factor)*

If a Sync. motor is under excited:  lagging PF *(correct)*


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## COJeff

BebeshKing said:


> If a generator is over excited:  Leading PF  *(an overexcited generator has a "lagging" power factor)*
> 
> If a Sync. motor is over excited:  Leading PF  *(correct)*
> 
> If a generator is under excited:  lagging PF *(an underexcited generator has a "leading" power factor)*
> 
> If a Sync. motor is under excited:  lagging PF *(correct)*


Thank you!


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## Dothracki PE

COJeff said:


> Can I get some clarification please:
> 
> If a generator is over excited:  *Lagging* PF
> 
> If a Sync. motor is over excited:  Leading PF
> 
> If a generator is under excited:  *Leading* PF
> 
> If a Sync. motor is under excited:  lagging PF
> 
> Thanks for being patient.


I corrected the relationships in the quote above. The reference handbook on page 51 gives you the diagram for a lagging PF (over-excited) generator. Just remember that motors are opposite in relation between lead and lag. You may also need to remember that you are comparing the real component of Eo against the magnitude of E. When Eo is larger (like in the diagram) it is over-excited and when Eo is smaller, then it is under-excited.


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## akyip

Here are a few diagrams/illustrations I previously drew up for a similar topic in this board to show over-excited synchronous machines relationships.

*Whenever a synchronous machine (synch gen or synch motor) is over-excited, it always delivers reactive power Q. Re(Ea) &gt; |Vt|: over-excited*

*For a synchronous generator:*


Lagging synchronous generator: I lags V --&gt; power angle (angle V - angle I) is positive --&gt; S = P + jQ: both P and Q are positive. For a generator/source, that means both real power P and reactive power Q are being delivered by the synchronous generator. *Since the synch gen is delivering reactive power Q, it is over-excited when lagging.*

Leading synchronous generator: I leads V --&gt; power angle (angle V - angle I) is negative --&gt; S = P + jQ: P is positive and Q is negative. For a generator/source, that means that the synch gen delivers real power P but absorbs reactive power Q. *Since the synch gen is absorbing reactive power Q, it is under-excited when leading.*

*For a synchronous motor:*


Lagging synchronous motor: I lags V --&gt; power angle (angle V - angle I) is positive --&gt; S = P + jQ: both P and Q are positive. For a motor/load, that means both real power P and reactive power Q are being absorbed by the motor/load. *Since the synch motor is absorbing reactive power Q, it is under-excited when lagging.*

Leading synchronous motor: I leads V --&gt; power angle (angle V - angle I) is negative --&gt; S = P + jQ: P is positive and Q is negative. For a motor/load, that means the synch motor absorbs real power P but delivers reactive power Q. *Since the synch motor is delivering reactive power Q, it is over-excited when leading.*


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## rburns18 PE

I try to remember for a generator, you will have a lagging current to supply a lagging load. That load also needs reactive power which is supplied from the over excited generator. Opposite for the leading pf case.


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## Byk

Out of the curiosity, what kinda problem do you expect/anticipate that you will need to memorize this?

I don't think I have seen any problems needing any of the info above.

@akyip I know you mention in one of the previous posts that you did a lot of practice problems, did you ever use this info?


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## akyip

Byk said:


> Out of the curiosity, what kinda problem do you expect/anticipate that you will need to memorize this?
> 
> I don't think I have seen any problems needing any of the info above.
> 
> @akyip I know you mention in one of the previous posts that you did a lot of practice problems, did you ever use this info?


Zach Stone's Electrical PE Review practice exam questions, Engineering Pro Guides practice exams, and Cram for Exam practice exams.

One very good kind of relevant example off the top of my head is from Engineering Pro Guides. It's a conceptual question.

Basically the question gives you a machine connected to a load, and the machine is sending power to the load. The machine is running at a constant speed n. The question asks which of the possible choices are true.

I remember 2 of the choices are something like: "The machine is in phase with the load." and "The machine is lagging the load." These are false because for the machine (synchronous generator) to send real power to the load, it must be leading the load (angle of Ea &gt; angle of Vt).

The other choices are in regards to the machine's speed. I think the other false choice is that the machine is running faster than the load, or vice versa. That is false, because if the machine is running at constant speed, that means machine torque = load torque.


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## Byk

akyip said:


> Zach Stone's Electrical PE Review practice exam questions, Engineering Pro Guides practice exams, and Cram for Exam practice exams.
> 
> One very good kind of relevant example off the top of my head is from Engineering Pro Guides. It's a conceptual question.
> 
> Basically the question gives you a machine connected to a load, and the machine is sending power to the load. The machine is running at a constant speed n. The question asks which of the possible choices are true.
> 
> I remember 2 of the choices are something like: "The machine is in phase with the load." and "The machine is lagging the load." These are false because for the machine (synchronous generator) to send real power to the load, it must be leading the load (angle of Ea &gt; angle of Vt).
> 
> The other choices are in regards to the machine's speed. I think the other false choice is that the machine is running faster than the load, or vice versa. That is false, because if the machine is running at constant speed, that means machine torque = load torque.


Thanks for the post.

I remember one from Zach's practice test but that one (if I recall correctly) had nothing to do with the power factor.

Unfortunately I do not have Energy Pro Guides exam.


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