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Terminal (load) voltage changes in single alternator due to change in driving torque
Effect of varying the driving torque of single alternator on load voltage (load connected is purely RESISTIVE).

Hi guys,

I would like to know the effect of varying the driving torque of single alternator (not connected to power grid) on terminal voltage (load voltage). Will the voltage across the load increase or decrease (armature reaction)? The load connected is purely RESISTIVE.

Also, I fail to understand how a decrease in terminal voltage due to armature reaction results in increased load current, because this directly defies Ohm's Law, where increase/decrease of load current is directly proportional to increase/decrease of voltage across load.

By blackstoneboy on 19 September, 2008 - 2:07 am


As we always observed, the frequency is directly proportional to the voltage. If we vary the driving torque, it might have effect on the frequency. If the frequency decreases, also the voltage and vice versa.

Apply Ohms Law in regards with increase and decrease of voltage.

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The correct explanation can be derived from information I provided in my White Paper on "The Physics of... Armature Reaction!" If you want detail, let me know, either via the List or Off-List.

However, please advise me if you have an aversion to the use of equations!


Phil Corso (

Thanks Phil , but i would appreciate phasor diagrams as I would like to obseve the changes to terminal voltage vs driving torque.

Also, alternator considered is PERMANENT MAGNET ROTOR type supplying its own independant single RESISTIVE LOAD.


hi phil,
my email, plz send me some tech. literarure regards this topic.

thanks a million.

By Blackstoneboy on 21 September, 2008 - 5:45 pm

Hi Shahvir,
Where can we find a copy of your Physics of Armature Reaction??

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I don't really understand the question; is it related to a single alternator driving a load independent of other alternators, or is related to a single alternator connected in parallel with other alternators driving a load that is fairly large?

If excitation is held constant, then voltage will vary with frequency. But, most synchronous generators I've worked on have exciter regulators which vary excitation to maintain a constant terminal voltage regardless of frequency (which shouldn't vary very much, on large "infinite" grids especially).

If the "grid" or load is small, and the governors aren't very good at responding to load changes and can't control frequency very well, then, yes, changing the torque input to the generator will have an effect on frequency. And, if the excitation is not set to regulate or maintain a terminal voltage setpoint then as the frequency varies the terminal voltage will vary.

But, these changes we're talking about (frequency and terminal voltage) are usually fairly small on most grids or even small loads, on the order of 1% for even a 0.5 Hz change on a 50 Hz grid, which should only be about 1% change in terminal voltage presuming constant excitation.

So, I don't think I really understand the question or the context of the question, or blackstoneboy's response.

As for armature reaction, the way I understand armature reaction is as
follows: As more torque is applied to the alternator more current
flows in the stator windings, which means the alternator is producing more power (driving more load). This increases the strength of the magnetic field associated with the stator windings, which, if nothing is done to change the strength of the rotor magnetic field causes the rotor magnetic field to "collapse" slightly, or to "shrink". This in turn causes the alternator terminal voltage to decrease. So, the exciter automatic voltage regulator (AVR), if active, increases the rotor field strength to maintain the alternator terminal voltage. If the exciter regulator is being operated in DC, or Manual, mode the operator has to increase the excitation to keep the alternator terminal voltage constant as load (torque, amps) is increased.

That's my understanding of armature reaction, and that's how it's always seemed to work on all the alternators I've worked on.

Increasing torque increases armature current which increase the power output (the load being driven) of the alternator. If excitation is held constant as the alternator is loaded, the terminal voltage will Increasing excitation increases terminal voltage.if torque is held constant. To maintain alternator terminal voltage as torque is increase, the excitation must be increased to overcome the effects of armature reaction (that's what an exciter regulator does when operating in AC or Automatic mode, also called and AVR, automatic voltage regulator).

Too often people associate voltage with torque and amperes with excitation; it's the exact opposite. Amps (power) is proportional to torque; voltage is proportional to excitation. Armature reaction forces one to increase excitation to maintain terminal voltage as load (stator amps, torque) is increased. As load (torque, amps) is decreased, if the excitation were held constant the alternator terminal voltage would increase, so to maintain a constant terminal voltage while decreasing load the excitation has to be reduced.

The alternator considered is driving its own independant RESISTIVE load.

Also, I have considered a PERMANENT MAGNET ROROR type alternator with no control on rotor excitation ( hence no AVR!)


hi csa,
from your reply I would presume armature reaction defies ohms law. I would like to believe as driving torque increases so does generated emf increases hence the terminal voltage should also increase in direct relation as per ohms law.

This should be more so true since alternator considered supplies its own independent RESISTIVE LOAD & is PERMANENT MAGNET ROTOR type as stated earlier.

Could you plz explain changes in terminal voltage pertaining to above driving torque variation conditions for 2 PM rotor alternators in parallel which are NOT connected to local grid (NOT CONNECTED TO INFINITE BUS).

Could u also kindly send me some tech. articles on the same on my email

Thanks & Regards.

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I thought this sounded like a science project. I see you're dabbling in the theoretical. Phil Corso, P.E., (, I believe) is your best source of theoretical information. My bad for missing the single generator independent of a grid, but your bad for not saying permanent magnet rotor before today.

Also, a purely resistive load on an alternator is purely theoretical, as no electric motor is a purely resistive device and only resistive heaters and incandescent lights are typical purely resistive loads.

You know, sometimes things just don't fit the normal definition, and I still fail to understand what it is about Ohm's law that's got you questioning the universe. But, again, Phil's your best source for theoretical questions on this forum.

Best of luck!

1 out of 1 members thought this post was helpful...

Shahvir and Blackstoneboy:

I'll gladly provide any information requested. Just contact me with your full name, company or school affiliation, and your location.

The offer stands for anyone requesting information.

Phil (

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shahvir, There is an excellent website,, which has some really good teaching materials on it. Here is a link to a pictorial presentation that, I'm sure, is supposed to accompany some written materials but I've yet to locate the written materials to accompany the pictures (but I've seen them before).

There are even some vectors in the pictures!

Also, by Googling armature reaction, I found the following link:

It's not much, but it does say "...armature reaction produces a demagnetizing effect, and the generated voltage or countervoltage will be reduced when the armature is loaded...."

So, between this and the document from Phil Corso, PE, you should be all set to conquer the world.

Lastly, I would think that a permanent magnet rotor would not be able to compensate for armature reaction very well, resistive load or reactive load.

But, as far as the Ohm's Law reference, the increase in load current is what causes the decrease in terminal voltage, not the other way around.

The frequency of an alternator is a function of the speed of the alternator. To maintain a constant frequency, the alternator must be maintained at a constant speed. If you increase the load on the alternator the tendency will be for the alternator speed to decrease. To maintain a constant frequency, the torque will have to be increased. The voltage is also a function of speed, but when the load is increased the increased magnetic field associated with the increased current flow in the armature (stator) will cause the terminal voltage to decrease.

You could probably increase the speed of the alternator to increase the terminal voltage, but the the frequency would also increase. And, generally frequency is supposed to be a constant.

So this doesn't sound like a typical alternator. What's the application?

Dear CSA,

Thanks for your response. Even though you have helped me conquer the world, your reply is still too practical for my query. I will contact Phil, but the following just for your info.

An increase of amps can only decrease load voltage if I change VALUE OF LOAD RESISTANCE (i.e. increase/decrease the resistance of heater load). In my case, the value of heater resistance is CONSTANT. I ONLY INCREASE TORQUE INPUT TO ALTERNATOR. Hence, as per theoretical laws load voltage should INCREASE.

I know my query is purely theoretical in nature & it may be out of your pervue to reply to my query but I am anyways thankful for your prompt response.

Thanks & Regards,

1 out of 1 members thought this post was helpful...

Tsk, tsk CSA... had you obtained a copy of my paper most of the questions and complaints, noted in your 23-Sep-08 (09:55) post, would have been addressed some two years ago.

Regards, Phil (Note, no !s)

1 out of 1 members thought this post was helpful...


As I stated earlier, I do not respond to off-line technically-oriented requests for information unless I know with whom I'm communicating. I have a hang-up with a person not willing to identify themselves!

Paraphasing an often used quote... while familiarity breeds contempt... anonymity dismisses common courtesy!

Regards, Phil Corso (

Dear Phil,
Sorry, I have been busy lately . I will contact u thru my Email shortly.

Thanks for ur time & patience.