Power required to run a generator 6

[greycloud]

Hello everyone

it is my understanding that all generators or alternators run at 50 or 60 hz speed so what is the reason for the diffrent energy levels required to run different sizes of generators? does the increased generator size require more power to run at the same speed or is it also related to magnetic field of bigger generators.

thanks in advance

 

[Skogsgurra]

You wouldn't expect a 10 HP ICE to be able to deliver 100 HP - would you? Even if they run at the same speed.

It is about work delivered. And that is equal to speed times torque. In SI units, you can use speed in radians/second and torque in Nm to get the power in watts. In other systems, you need to multiply with a constant.

So, it is not only about speed. It is also very much about torque. I think it should be self-evident once you think about it that way.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Basic physics and the law of conservation of energy. You can't get more out than you put in.
If your generator runs a 5 HP motor, then it must be driven with enough power to produce at least 5 HP PLUS losses.
If your generator runs a 500 HP motor, then it must be driven with enough power to produce at least 500 HP PLUS losses.
Most 500 plus HP prime movers are bigger than most 5 plus HP motors.
As for a frequency of 50 Hz or 60 Hz. Not so.
Le Tourneau ran the alternators in the early earth moving machines at 72 Hz and automotive alternators run at a frequency that varies with engine speed and is higher than 60 Hz at all but slow engine speeds. The high frequency AC that is produced is rectified to DC internally.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[greycloud]

thank you both but u havent answered my question exactly. I understand basic physics but i came here to see what exactly require the additional work done hence additional torque. is it a bigger magnetic field required to produce more energy? or the increased resistance to rotation of the rotor itself due to bigger size, what loads will the generator impose on the mechanical engine trying to rotate it according to their power output? this is what i mean by my question.

waross: if that is so then can i for example run a generator at 100 hz ac and convert later to 50 hz ac or convert from lower frequency to higher frequency?

 

[waross]

The frequency of a generator is related to the speed. Directly in the case of a synchronous generator and closely in the case of an induction generator. Changing frequency is not as simple as changing voltage. One method is to rectify to DC and then invert the DC back to AC electronically. Variable Frequency Drives produce a variable frequency to drive induction motors at various speeds.
Why is a big generator bigger than a small generator??
Most industrial motors will act as generators if the load overhauls the motor and tries to turn the motor faster than its normal speed.
When generating, a motor will generate about the same power as it normally uses.
Why is the transmission in my friends freight truck bigger than the transmission in my car?
Why is the transmission in my car bigger than the transmission in my small motorcycle?
Why can I never get more power out of a generator than I put in?
Well in an electric motor the rotating magnetic field pulls the rotor around. The strength of the field is limited by a quality of the iron called magnetic saturation. If the load exceeds the strength of the magnetic field the motor stalls. To increase the total pull or turning force of the field it must be made longer or have a greater radius. That means it must be bigger.

Getting it into a little deeper, at higher frequencies a motor or generator can develop more power. Some examples of this are:
50 Hz machines tend to be 6/5 larger than similar 60 Hz machines.
When motors, generators or transformers are converted between 50 Hz and 60 Hz, the ultimate rating must be adjusted in a 5/6 or 6/5 ratio.
Some examples:
Aircraft equipment is often 400 Hz. The generators may be about 1/8th the weight of similar rated 50 Hz equipment.
At one time some concrete vibrators were constructed with dedicated, small gas engine driven 180 Hz generators. This allowed a small but powerful induction motor that ran at about 10,000 RPM to be used in the vibrator head. These vibrators were more expensive but more trouble free and more effective than the cheaper vibrators using a high speed universal type motors with brushes and a commutator.
Automotive alternators are much lighter than the old generators they replaced due to the high frequencies that the AC section often runs at.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[xnuke]

greycloud,

In order to understand this, you have to know the requirements for generator action and motor action in electromechanical machines, and Lenz's law. Generator action requires a conductor, a magnetic field, and relative motion of the conductor with respect to the magnetic field. Motor action requires a current-carrying conductor in a magnetic field.

A generator's rotor is usually a magnet (typically an electromagnet carrying field current). When the rotor is turned by the prime mover, the magnetic field lines cut the stator conductor windings, producing relative motion between them, and meeting the requirements for generator action, so a voltage is produced. If an output circuit is connected (i.e., load), current flows through the generator's conductors to deliver power to the load (Power = voltage x current). The generator will then experience what is known as counter-magnetomotive force (i.e., countertorque), a consequence of Lenz's law, which fundamentally says that the effect produced will oppose its cause. This is due to motor action opposing the rotation produced by the prime mover. The motor action occurs because there is now a current-carrying conductor in a magnetic field. Thus, the increased resistance to rotation of the rotor is actually due to the electrical load on the machine. As more current is drawn to support more load, there is more countertorque, and the prime mover must provide more mechanical power (Power = torque x speed) into the rotor to keep the desired speed/frequency.

Regarding 100 Hz, then 50 Hz operation: In general, electrical machines have their mechanical components, electrical circuits, and magnetic circuits designed to allow operation over a small frequency range or damage can occur, so I wouldn't advise trying to run a generator over the frequency range you mentioned. There are mechanical, electrical, and magnetic effects that will likely damage your machine. Stick to the manufacturer's stated specifications, and all should be fine.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ScottyUK]

Higher losses in the larger machine typically occur because of:
[ul]
[li]Windage losses from a larger fan[/li]
[li]Friction losses in larger bearings[/li]
[li]I²R losses in the larger field winding[/li]
[/ul]
On a percentage of machine output basis the losses are usually lower in larger machines, but in absolute values they are larger.

 

[electricpete]

I understand basic physics but i came here to see what exactly require the additional work done hence additional torque. is it a bigger magnetic field required to produce more energy? or the increased resistance to rotation of the rotor itself due to bigger size, what loads will the generator impose on the mechanical engine trying to rotate it according to their power output? this is what i mean by my question.

With respect, your followup questions suggest to me that you don’t seem to understand those initial responses relating to conservation of energy:

As a first approximation, keep it simple and make some simplifying assumptions:
1 – neglect losses
2 – assume steady state at specified constant frequency and voltage.

Then it becomes very simple. Power (and hence torque) throughtout the machine depends only on the load that you hook up to the machine and the power that it draws. It doesn’t depend on the size of the machine. So if you hook that same load to a small generator or big generator (assuming same frequency and voltage and again neglecting losses), the power delivered by the prime move will be the same in both cases.

So why do we even have different size machines? Well then you will get into issues of temperature limits, efficiency, losses etc. Don’t even go there until you have the first part down. I apologize if I have misunderstood your view.


=====================================
(2B)+(2B)' ?

 

[itsmoked]

Size doesn't matter.

Just consider the generator as a converter. (Mechanical energy -> electrical energy) It only converts what is asked for by the electrical load attached to it.

The engine used to rotate the generator has to deliver ALL the energy needed to provide what the generator is being asked to supply by the load.

Neither the size of the generator nor the frequency makes any difference to the engine's demand - it's the electrical load.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[cswilson]

To build on what Pete said, it is easiest to start to get an understanding by looking at the steady state case and ignoring losses for the time being.

In the conversion between mechanical power and electrical power, energy must be conserved. So:

Mechanical power (torque x speed) = Electrical power (voltage x current)

A limit on any of the four of these quantities can limit your power transfer, either motoring or generating. If you want to increase the power transfer capability, you must increase at least one of the quantities on each side. A physically bigger device could provide more torque on the mechanical side, and have room for larger conductors to support more current on the electrical side.

And since the conversion goes "through" the magnetic realm, you must also consider the limits of the magnetic circuit -- how much magnetic flux can be created before saturation.

When you are comfortable with this analysis, you can start to consider the losses. Most of these are "second order" effects, less important than those I mentioned above. One of the important considerations here is how well the device can rid itself of the waste heat from its losses. In some applications, this can be the controlling factor in the physical size of the device.

Curt Wilson
Delta Tau Data Systems

 

[waross]

Don't forget Power = torque x speed.
With most types of motors and generators, speed is related to frequency.
With the torque held constant, increasing the frequency/speed gives increased power at the same torque.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[greycloud]

sorry everyone for not answering for so long, i was too busy.

Waross,xnuke,and cswilson: ur answers are what i was looking for. i'm working on some renewable energy method based on bouyancy and i was wondering if its possible to use 300 mw generator and what are the options for that. now i understand that i can use a low frequency generator provided i can provide extra torque and the generator will be bigger.

thank u all

 

[Skogsgurra]

Why do you want to build something as small as a 300 milliwatt generator?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[greycloud]

sorry, I meanst 300 mega watt.

 

[Skogsgurra]

Yeah, I figured. But, please, do use capital letters where needed. Texting may be OK on phones. But this is a mostly screen/kbd based forum. It is important that we avoid ambiguities. Especially when talking technology.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[genhead]

"kbd" ??? (haha)

 

[itsmoked]

<snicker>




Keith Cress
kcress -

http://www.flaminsystems.com

 

[greycloud]

, anyway, is there any good beginer book on this topic cause ill need some more detailed info on this manner.

 

[catserveng]

I have found this to be a good overall primer for power generation,

http://www.egsa.org/Publications/ReferenceBook.aspx

A Woodward Publication,

http://www.otherpower.com/images/scimages/3409/Governing_Fundamentals_and_Power_Management.pdf

Woodward also has an Application Note 51214 called "Work vs Torque" that you should be able to find on their site.

And a Basler Application Note you might find helpful,

https://www.basler.com/ResourceDownload.aspx?id=132

Hope that helps, Mike L.

 

[greycloud]

thanks alot