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Join Date: Sep 2013
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Alternator Question

04/13/2014 10:12 AM

this is a simble question . and not wok oriented one. in a case of synchronous motor it is said to be over excited if power factor is leading. Is it is true in case of alternator.

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Guru
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#1

Re: alternator

04/13/2014 12:05 PM

i believe that the power factor depends on the load. The alternator will deliver power at whatever the load pf is.

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Guru

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#2

Re: alternator

04/13/2014 4:13 PM

Power factor is affected by overexciting an alternator only in the case where the alternator is paralleled with another source (say another alternator). Increasing or decreasing the excitation controls the output voltage in the stand alone case. If the alternator is in parallel with another alternator, adjusting the excitation causes circulating current between the two alternators which is 90 degrees out of phase with the voltage. Voltage regulators are designed to detect this cross current and automatically adjust the voltage when configured for parallel operation (cross current compensation).

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Guru
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#4
In reply to #2

Re: alternator

04/14/2014 3:23 AM

"...Power factor is affected by overexciting an alternator only in the case where the alternator is paralleled with another source.. Really? So in the case of an isolated generator feeding an induction motor the voltage and current are in phase with each other?; i.e., the power factor is unity, if so where does the magnetizing current for the motor's windings come from?

"...Increasing or decreasing the excitation controls the output voltage in the stand alone case...." The excitation affects the voltage in all cases, not just standalone, though it does have a more direct impact in the standalone case.

"...If the alternator is in parallel with another alternator, adjusting the excitation causes circulating current between the two alternators...", not quite, what will happen though is that the machine whose excitation is being increased will push out more VARs and whatever VARS the system can't take will be absorbed by the other machine. On the other hand if two machines are wye connected and both neutrals are grounded then, yes, there will be circulating currents. It's one of the reasons why you usually find paralleled generators where they can be grounded only individually.

"Circulating Current" is a term reserved for currents in one or more parallel paths where there is a voltage source(s) inside the loop(s). An example is two identical transformers connected to the same source and feeding the same bus, but with different tap settings. The difference between the turns ratios becomes a voltage source that forces a current equal to ΔV/(Z1+Z2) to circulate around the closed loop.

"...Voltage regulators are designed to detect this cross current and automatically adjust the voltage when configured for parallel operation (cross current compensation)...", not always true, many machines are not meant to be paralleled and do not have this add-on. Also it's not the terminal voltage that's affected by the CCC, it's the excitation level which in turn controls the VARs being output by each machine.

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Guru

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#6
In reply to #4

Re: alternator

04/14/2014 6:40 PM

I apologize for being less than clear...

The OPs question was "Does varying the excitation of an alternator affect the power factor?"

My answer is "yes, when the alternator is paralleled with another alternator".

If an alternator is paralleled with another alternator and if it is over-excited or under-excited, there will be current that is 90 degrees out of phase with the voltage flowing between the two alternators. This reactive current will obviously affect the power factor seen from the alternator.

Some voltage regulators have a feature called cross current compensation which detects this reactive current and adjusts the excitation to minimize this cross current. The result will be that all paralleled alternators will see the same power factor.

http://engines-blog.blogspot.com/2013/09/vr6-voltage-regulator-cross-current.html

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Guru

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#7
In reply to #6

Re: alternator

04/17/2014 8:48 PM

Here is a little bit more explanation on varying the excitation on paralleled alternators.

The alternator can be modeled as an AC source in series with an inductor (the field windings), as shown in the diagram below. The voltage magnitude of the AC source is controlled by the excitation.

In the diagram, Alternator A and Alternator B are connected in parallel and driving a load. Only one phase is shown for simplicity. Assume the load is shared equally between the alternators by adjusting the engines so that the kilowatts are equal. This means the voltages Ea and Eb are precisely in phase.

If the excitation for Alternator A is set too high, Ea is a greater amplitude than Eb and the voltage (Ea - Eb) will drive current I around the loop as shown. This current will be lagging the voltage due to inductors La and Lb, producing a lagging power factor.

If the excitation for Alternator A is set too low (Eb > Ea), the direction of the current will be reversed (flipped). The current waveform will be leading the voltage waveform and there will be a leading power factor.

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Anonymous Poster #1
#3

Re: Alternator Question

04/13/2014 9:41 PM

Quit showing it the Playboy pictures, and then it won't get over excited.

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Guru
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#5
In reply to #3

Re: Alternator Question

04/14/2014 9:42 AM

Pack it up, Del.

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