from the Electrical department...
Conversion of torque into amps in synchronous generator
 Posted by Chandru on 26 October, 2012 - 12:15 am
Hello,

In synchronous generator, while increasing the load, i want to know how the torque getting converted to amps. Instead of simple theoretical way, can anyone please explain in clear?

 Posted by CSA on 26 October, 2012 - 11:35 am
Chandru,

http://services.eng.uts.edu.au/cempe/subjects_JGZ/eet/eet_ch6.pdf

(Remove any spaces inserted by the forum software.)

There is NO difference between an electric machine used as a motor or one used as a generator--except the "direction" of torque and current. An electric machine used as a motor converts amperes into torque to drive a load. An electric machine used as a generator converts torque into amperes.

The amperes produced by the generator are just a means of transmitting the torque produced by a prime mover (a turbine or a reciprocating engine) in some location to motors and other loads which may be a long distance away from the source of the energy used by the prime mover. The torque "transmission" is done using wire.

Why do people never have a problem with the fact that motors convert amperes into torque, but can't get their head around the concept that generators convert torque into amperes? The work that is performed by electric motors is actually done by the prime mover driving the generator. Amperes don't really get "used" anywhere in the sense that they are produced by generators and consumed by motors and loads; that's just for simple theoretical understanding. Amperes are like hydraulic fluid--a transmission media, a means of converting power or work in one area to power or work in another area. Hydraulic systems use pipes and hoses; electric systems use wires. The hydraulic fluid doesn't get "produced" or "consumed", it doesn't have to be replaced as it's used at the ram or cylinder or hydraulic motor end; it's recirculated back to the hydraulic pump.

Electricity is similar; it's just a means of producing power in some area and transmitting it to another area to do work in the "remote" (from the prime mover driving the generator) area. Reactance (capacitance and inductance) impedes the "flow" of electric power and reduce the efficiency of transmission. I guess you could consider the losses in a pipes, elbows, and hoses and fittings to be similar to reactance in electrical systems--they impede the flow of power being transmitted.

Have you encountered the idea of regenerative motor applications, such as are used in electric automobiles, or in variable speed drives? In these cases, the electric motor which normally drives a load (the automobile or an elevator car, for example) and when the automobile is braked the inertia of the automobile is converted to amperes by the same motor used to drive the wheels at other times. In the case of the elevator, as the car moves down the shaft the motor driving it becomes a generator and the variable speed drive actually pumps power back on to the grid from which it normally draws power to raise the car and loads.

There are also examples of pumped hydro storage applications, where a single electric machine is used as a generator driven by a hydro turbine when electric power is required, and then when that need is not there the same electric machine is used as a motor to drive the hydro "turbine" as a pump to move the water back uphill to a storage area, from where the process can be reversed again the next day. In this case, the electric machine is the same--it's just a motor in one usage, and a generator in another application.

Use your preferred Internet search engine and comb through the results to find the "clarity" you seek. Or, tell us precisely what about the flow of torque and current in an electric machine you don't understand. Do you need maths? Vectors? Derived equations?

Just remember: There is no difference between an electric machine used as a motor or a generator--except the directions of torque and current flow. Some heavy duty combustion turbines use the generator as a starting motor (through a variable speed drive). Any generator can become a motor if the torque being applied to it is less than that required to keep it spinning at synchronous speed.

 Posted by CSA on 28 October, 2012 - 12:10 am
Chandru,

The speed of the prime mover driving a synchronous generator is locked into what's called "synchronous speed"--the speed that is proportional to the frequency of the AC in the generator stator windings.

If the torque being provided to the generator rotor is exactly equal to the amount required to keep the generator rotor spinning at synchronous speed, the load is zero.

If the torque being provided to the generator rotor is increased from the preceding paragraph--which would normally increase the generator rotor speed if the generator were not synchronized with other generators and their prime movers on a grid--the generator, being unable to increase the speed converts the extra torque into amperes.

If the torque being provided to the generator rotor is decreased below that required to keep the generator rotor spinning at synchronous speed, the generator becomes a motor and current flows from other generators and their prime movers on the grid to keep the generator rotor spinning at synchronous speed--actually providing power TO the prime mover, which is very bad for some types of prime movers.

You can find all kinds of maths and vectors and descriptions of load angle and armature action and -reaction and MMF and EMF and counter-EMF and all those kinds of things using your preferred Internet search engine.

I think the thing that many people don't really consider is why electricity exists: To be able to transmit torque from areas where a "source" exists to areas where it's needed or can be used. In North America there were huge sources of hydro power available but it wasn't considered feasible to make a long shaft and drive the shaft with a water wheel on one end and then have pulleys and belts along the shaft to use the torque from the water wheel at factories many tens or miles from the hydro power. So, generators were used to convert the hydro power to amperes, and wires were used to transmit the amperes to electric motors, and electric motors were used to convert the amperes into torque to be used in the factories.

To get torque from amperes, you have put torque into a device that converts them to amperes. An electric machine can be used to convert torque to amperes, and a very similar machine can be used to convert the amperes into torque.

In today's world, many electric machines are used as both motors and generators. Electric cars are one example. When an electric car is driven down a steep hill it's very common for the drive motor to be "over-driven" by the inertia of the car going down the hill and become a generator, returning energy to the battery. If the driver needs to slow that car down and puts his foot on the brake, there are electronics that further increase the "load" on the drive motor that's become a generator to slow the car down, increasing the amount of energy being returned to the battery.

Variable frequency dives are quite commonly used to power electric motors used in elevators in high-rise building. When the elevator car is descending the inertia will "over-drive" the motor and electronics in the variable frequency drive will go into what's called "dynamic braking" mode to control the rate of descent, and that actually pumps energy back onto the grid in the process.

Those are two cases of electric machines being used as both motors and generators. The only difference is the "direction" of torque and current "flows".
`Motor:     Amperes "IN"  ----> Torque  "OUT"Generator: Torque  "IN"  ----> Amperes "OUT"`

Again, if you can tell us how you need to see your proof we might be able to help. But, for many detailed explanations, you can find many excellent papers and explanations on the World Wide Web (Internet). They can provide formulas and drawings that we can't here on control.com. There even many videos on many places on the World Wide Web; I know YouTube isn't available in every place, but there are many other sites with videos as good or better in some cases.

Some very prestigious universities are now putting many of their courses and lectures on the World Wide Web for FREE!!! If you can access Google Books, they have digitized many references and text books and made them available for free.

 Posted by Chandru on 29 October, 2012 - 1:31 am
Hi CSA,

Thanks for your valuable answer CSA. As you explained, i understood that motor will convert amps to torque and generator will convert torque to amps. But can u please explain me in detail with derived equations..? how the torque developed by increasing the fuel in gas turbine getting converted to amps..? Please reduce the theoretical explanation...already i got bored with them from my college lectures...whenever i ask this question..they are telling its just simple conversion...but how?

 Posted by CSA on 29 October, 2012 - 12:37 pm
Chandru,

By the way, you are absolutely right to challenge anyone who ever tells you, "It just works that way," or, my favorite (NOT), "That's the way we've always done it!"

Neither are good explanations or justifications for anything. Even if it's always been done that way, it doesn't mean it should be done that way. And helping to understand why it's always been done that way can be VERY beneficial to understanding why some things work the way they do, and why they don't work the way they should, or how they could be made to work better.

 Posted by CSA on 29 October, 2012 - 1:30 pm
Chandru,

I was thinking this was the case!

This is one of several concepts you are going to have to grapple with in your own way and in your own mind. One knows it's true, it just seems that it shouldn't be true.

When I was in university I used the library quite a lot for these kinds of "dilemmas." Sometimes all it takes is a different spin or order or association to help with understanding. We didn't have the Internet and World Wide Web these days, with so many resources and so much information presented in so many different ways. When I was researching your query I even found some very good Flash animations for visualizing "rotating" magnetic fields and rotors. But nothing for the torque-to-amperes thing. Since motors and generators are virtually identical, perhaps it's worth searching for electric motor torque production and just realizing that generation is the opposite???

There might be other contributors to control.com who will gladly use derived equations and vectors and explanations of load angles and MMFs and emf's and counter-emf's and on an on. I know those things are important for equipment designers and for those who troubleshoot the equipment, but for operating the equipment it's not necessary.

One of the formulas I frequently refer to is this one:
`Torque = Kt * If * Iawhere Kt = Torque Constant (how the machine is           designed; the diameter of the rotor;           the air gap; the number of windings;           the number of turns per winding;           etc.--and none of these are variable           when the machine is operating)      If = Magnetic Field Strength (since this is           directly related to generator terminal           voltage, the usual variation is           restricted to less than approximately           +/- 5%, and to keep voltage constant           and stable this value needs to remain           constant and stable)      Ia = Armature Current, which is really the           only "variable" in this equation           during normal operation.`

If you solve this equation for Ia, you will see that since the other values are fixed or must remain constant, the only variable that can be changed is Torque. Increase the torque, increase the armature current; decrease the torque, decrease the armature current.

That's about all the maths and vectors I'm willing to provide. Again, use the resources available to you; look at many different explanations and roll them around in your mind as you grapple with understanding this. And in the meantime, at a minimum learn the formulas your instructors are using--because those are the ones that are going to be used on the exams they give!

 Posted by Chandru on 29 October, 2012 - 10:23 pm
Hi CSA,

Once again, thanks for your immediate reply. Actually, i have so many doubts, i will ask u with some time gap. otherwise, u will get bored with me.

 Posted by CSA on 30 October, 2012 - 1:08 pm
Chandru,

I'm not bored by questions. I, too, learn a GREAT deal from contributing to control.com. I worked with a colleague years ago who said, "I don't understand something until I can explain it to someone so they understand." And I've adopted the same philosophy. When I'm unable to find the right way to present a concept or fundamentals it can be frustrating for me; I'm sorry if that showed in my posts.

I'm just a simple field engineer who has benefited from the knowledge and experience of many people over many years who contributed to my understanding. They "allowed" me to ask a lot of very basic questions to help me understand some very basic concepts so that I could understand some very difficult concepts. Did I frustrate some of those people? Yes. The people who really understood the concepts and principles just tried to find a different "tack" (to use a sailing term) to get the point across to my simple mind.

I have found, and find it difficult to remember, that sometimes the most basic assumptions about another person's knowledge or experience can be very important to getting a point across. That's why my posts tend to be a little more verbose--I try not to make too many assumptions, and I know that many people may eventually read the replies and have very different knowledge and experience and so try to cover as many salient points as I think are necessary to a proper understanding.

As Bruce Durdle has so eloquently reminded me, the simplest things (balance in this case--outputs have to equal inputs minus losses!) are sometimes the most important things.

I'm also reminded of a passage in one of my favorite books: "Learning is finding out what you already know." When someone explains something to you so that you really understand it, don't you say to yourself, "Oh, yeah! I knew that!" It seems obvious--when it's explained properly.

And, although I can't see everyone who reads these threads, I know what it's like when people "find out what they already know." It's very rewarding when you see that recognition in people. (And, it's kind of frustrating when I don't--and I really want to!)

1 out of 1 members thought this post was helpful...
 Posted by Bruce Durdle on 29 October, 2012 - 10:31 pm
Hi Chandru,

I take a slightly different approach to these things than CSA, so you might find my way of looking at things to be useful.

I have always found it useful to consider the balance effects inside systems to be worth looking at. In an alternator, the main balance is the power balance - the mechanical power in must equal the electrical power out (+ losses). Mechanical power = torque x speed, and electrical power = voltage x current.

At the mechanical end, the speed is more or less fixed, so the mechanical power is proportional to the torque. At the electrical end, the voltage is more or less fixed, so the electrical power is proportional to the current. Relating the two power terms, you can see that the mechanical torque is proportional to the electrical current.

The details of how all this happens are quite complex, but effectively you have an electromagnet produced by the field windings interacting with the electrical field produced by the stator current. If the stator current increases, the stator field becomes more intense, and the force tending to align the two magnets becomes stronger.

 Posted by CSA on 30 October, 2012 - 12:24 pm
Bruce Durdle,

Thank you very much (again) for helping to clarify a situation with a pair of fresh eyes and some clear(er) thinking!

 Posted by chandru on 30 October, 2012 - 11:32 pm
Hi friend,

Thanks for your explanation. In that,at one point you mentioned,"The details of how all this happens are quite complex". Exactly that's the point, i want to know for.

1 out of 1 members thought this post was helpful...
 Posted by Bruce Durdle on 31 October, 2012 - 3:49 pm
You asked for it ... I've also had a look at your post about pole position detection, and will have a go at helping you sort what is going on. I'll start at a very basic level.

If you take a standard three-phase stator winding, (such as a squirrel-cage induction motor outer casing) and lie it horizontally on a bench with the motor axis vertical, and supply it with a three-phase voltage, a rotating magnetic field will be set up inside the stator. The strength of the field will be constant but its orientation will rotate smoothly.

The rotating magnetic field moving past the stator windings develops an emf - this is just enough to match the applied external voltage. Electrically, the setup can be represented as a voltage source (the developed emf) connected to an external voltage (the supply) through an impedance which is highly inductive.

If instead of powering the stator and putting a compass needle in the centre, we leave the stator unpowered but put a strong magnet (can be permanent or an electromagnet) in the centre and rotate it, the field of the rotating magnet will develop an emf in exactly the same way as the rotating magnetic field.

With the stator connected to a supply AND a fixed magnetic field rotating inside it, there is interaction between the two fields. To make sense of anything, these must both be rotating at the same speed - or synchronised. In this case, we can look at what happens by analysing the circuit made up of 2 AC voltages of the same frequency but different phases. connected through an inductance.

If the two voltage amplitudes are the same (V) but there is a phase difference of T degrees between them, with a reactance of X ohms connecting the two, a current will flow in the inductance. This current lags the DIFFERENCE in voltage between the two sources by 90 degrees, so has a phase half-way between the two. The result is an electrical power flow from the leading voltage source to the lagging one of (V^2) sin(T)/X watts. As I mentioned in my earlier post, this electrical power flow must be matched by a mechanical power which is proportional to the torque. If we apply more torque to the rotating magnet, the angle between the two fields will increase and sin(T) will increase (but only up to a maximum of 90 degrees - if more torque is applied at this point, pole slipping takes place and the two fields fall out of synchronism.)

As the angle difference between the two fields increases, the voltage difference across the inductive element also increases - so more current flows. Since the angle difference increase the torque and the current at the same time, torque and current are proportional.

Note that my explanation has not so far looked at which way round the two voltages are related. If there is no torque applied, the two fields are in exact step and the angular difference T is 0 - no power is produced or consumed. If the rotating element is loaded in some way, its field will lag behind the stator field and power will flow from the external source into the stator - this then becomes a motor. On the other hand, if the rotating element is driven so that its field moves ahead of the stator field, the torque will oppose the driving torque and power fill flow from the stator to the external source - this is then generator action.

In relation to your question about pole position detection, there is no need for external measurement of pole position - the physics of the interaction between the two fields takes care of that.

I hope this helps - any more detailed explanation will need some pretty heavy maths which this forum isn't really equipped to deal with.

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