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Relation between torque and generator output power
It is about my diploma project. I want to show a relationship between quantity of steam, steam pressure, torque of the prime-mover, torque of the generator and the total output power


As I tried to explain in a few words, my goal is to describe the generator output power variations when the torque is changed.
I tried to search all overt the internet for an answer but unfortunately with no result until now at least.

My project is a Waste to Energy Power Plant (automation and control). When I present the powerplant components, I would like to describe the generator functionality as well as for the other components.
Mainly, I want to compute the counter-torque (when the prime mover does not react to the quantity of steam or when the power plant is connected to the grid but there is no steam generated) and the necessary torque to oppose to that torque.

In the end I want to understand the relationship between quantity of waste and generator output. I do have the chemical formula for waste so I can compute the quantity of heat provided by it, the quantity of steam generated (including pressure and temperature) in a water tube boiler, but I cannot figure it out this problem presented above.
I know that I have to spin the generator at synchronous speed to obtain frequency and if that speed is exceeded by the primemover, the difference is converted into torque. How can I compute that torque and how can I convert that torque into electricity?

Thanks in advance,

I will be happy to answer to any question about my problem, in order to clarify my scope.

Unfortunately I have no "on site" background, so I know processes only from theoretical point of view.

If there is an article / book that covers my subject, please let me know and I will be happy to study it and after that I will post here an overview / summary.

Thank you,
With regards,
Marius Jantea

2 out of 2 members thought this post was helpful...

Hi Marius,

You are dealing with a system where there is an energy or power balance. The mechanical power in to the shaft must be matched by the electrical power out if the system is to remain at the same speed.
In mechanical terms, the power is given by torque x velocity, and if the speed remains constant power in is proportional to torque.

In electrical terms, the power is the product of voltage and current. Voltage is developed by the interaction of the rotor magnetic field with the moving conductors, and is also proportional to speed.
Since power in is proportional to (speed x torque) and power out is proportional to (speed x current) the mechanical torque applied is proportional to the current being drawn from the generator.

As the esteemed CSA says, a generator is a device for converting torque into current, while a motor converts current to torque.
Changes in input torque must be matched by changes in output current - the exact way in which this comes about depends on the detail of how the generator is connected to the load, but in a machine connected in parallel with others involves a negative feedback mechanism.



This statement I find very confusing:

> Mainly, I want to compute the counter-torque (when the prime mover does not react to the quantity of steam
> or when the power plant is connected to the grid but there is no steam generated) and the necessary torque to
> oppose to that torque.

The amount of electrical power produced by a generator is directly proportional to the amount of torque supplied to the generator by the steam turbine. So, if you know how much torque is being produced by the steam turbine, you can pretty much easily calculate how much electrical load will be driven by the generator. There are some losses, and if you want to find out more or less precisely how much they are you could refer to any textbook or reference about electrical generators to determine some typical numbers.

But, it's that one sentence that confounds me. If there is no steam being produced, then there is no reason for the steam turbine-generator to be connected to the grid. The plant can still be connected to the grid for the lights and pumps and fans and air conditioners (operators would NEVER operate a power plant without air conditioning!) and computers (the bean counters need their computers--and air conditioning, too!). That's typically called "house load" or "hotel load"--the amount of power consumed by the plant when it's not generating electricity.

And, when it is generating electricity, the "house load" will be even higher, because more pumps and fans and equipment will be running. There are usually some very strategically placed watt-hour meters in combined-cycle plants to measure the amount of energy consumed as well as the amount of energy produced.

I'm confused about "...the prime mover does not react to the quantity of steam...." also. Many of these types of plant operate with the steam turbine control valve wide open (100%) and so the amount of steam being produced all goes into the turbine. As the steam production varies, so does the pressure, and so does the torque produced by the steam turbine. Even if the control valves aren't operated in the full open position, they are modulated to control steam pressure (usually) and so affect the torque produced as they limit the steam flow to maintain pressure.

It's this whole "counter-torque" thing that's got me really confused.

I think you also need to know that when a synchronous machine is being operated in parallel with other synchronous machines on a large or infinite grid the speed of the synchronous machine and it's prime mover (if it's directly coupled, as most are) is fixed by the frequency of the grid. So, any change in steam flow will only result in a change in torque. In fact, if there is no steam flow and the generator remains connected to the grid the generator will become a motor and spin the steam turbine at the same synchronous speed as it would be spinning at if it was running as a generator. (By the way, spinning a steam turbine with the generator acting like a motor is EXTREMELY hazardous to the turbine and anyone within 20 meters of the unit-even behind walls!)

I don't know if this helps at all.

I think the original poster would benefit from doing a plant heat balance for the intended system. If you have your steam flow and conditions, you can theoretically calculate how much power you will get from a steam turbine.

The depth that you are trying to dive into this project is not stated, but "Power Plant Engineering" by Black and Veatch would probably answer all your questions and give you guidance in the calculations and systems.

But to keep it simple, you can take your energy in the steam and then use some standard efficiencies (often referred to as heatrate) for steam turbine-generators and figure out what the electrical gross output will be from your plant.

Fixed pressure and sliding pressure and all that stuff is probably outside the scope of your project. These topics are primarily driven by operational concerns and how the plant is originally designed.

Though seems to usually be about gas turbines (and mostly GE heavy duty gas turbines at that) and not much steamer stuff, there are a few folks on the forum that can help with the steam stuff.

Thank you for your answers! It was very helpful! I have started to read the book recommended, Power Plant Engineering and indeed it is very useful, for me at least!

Thanks a lot and I do hope that I will manage to realize this project in time!

Glad we could help you out. Another industry standard book that is very in depth and specific on boilers is "Steam, Its generation and use" by Babcock and Wilcox. However, it only discusses boilers (design, construction, compromises, etc).