induction generator torque-speed relation when not sync'd to grid? 6

[electricpete]

This question arises from discussion in the thread on degraded voltage. Please be assured that if you respond I will not argue with you ;-)

An induction generator can apparently operate without being interconnected to another power source, provided that capacitors are attached to the terminals.

I assume that voltage can be controlled by adjusting the terminal capacitance.

What determines and/or controls real power and torque of the generator? It seems like the induction machine torque-slip characteristic is irrelevant if there is no "syncronous" frequency and therefore no slip. What determines the generator torque-speed characteristic?

 

[rhatcher]

hey pete,

I have some links I will share with you tonight or tomorrow, I do not have them with me now. Anyway, an induction motor will not operate as a generator without being attached to another AC power source. I believe that I am the one that brought up the example of rotary phase converters in the other thread. For these devices a single phase is connected to two of the motor legs (L1 and L2) and the third leg is the generated leg (T3). The capacitors are connected between one (or both) of the energized legs and the generated leg (L1-T3 and/or L2-T3). Presumably this provides excitation to the generated leg. The "3-phase" output is taken from L1, L2, and T3 respectively.

I believe that you set the output of the generated leg by choosing the amount of capacitance.

I will send you the links later and will be returning to the other thread when I have enough time to write a detailed post.

 

[gjones33]

Hi again electricpete,

No! Voltage regulation in this condition is very poor if not erratic, this is why an induction generator is rarely used as a stand alone generator.

Induction alternators are ideal for wind generators connected to a solid grid because they can adjust automatically to the wind conditions. When the wind increases say to a gale force, the machine will try and increase the frequency and the voltage on the grid and thereby grab more load, which, in turn, exerts a resistive torque on the rotor and prevents a potentially catastrophic overspeed condition. When the wind drops the reverse happens.

The point I'm making in answer to the good question is that there is no control of torque or speed as such. If you are experienced in running fossil fuel gen sets then you cannot decide to take half load today and reduce the throttle, the best you can do is feather the wind vane, not quite the same

Cheers,
G

 

[electricpete]

rhatcher - I think probably you're right. An induction machine requires a stator field at different frequency than rotor to induce slip frequency currents into the rotor. Without a separate power source I have a hard time imagining how it could work.

I did hear a few people claim on alt.engineering.electrical that an induction machine could operate without being connected to an another power supply, but I can't see how that is possible as discussed above.

Here was one example

http://groups.google.com/groups?hl=en&lr=&safe=off&th=343513e3f6509db9&rnum=4

Has anyone ever heard of an induction generator operating without being connected to another power supply?

 

[rhatcher]

pete...i thought i already responded to this post but it obviously did not go through. Here are the links that I mentioned:

http://www.3phaseconverters.com/u&s_index.htm

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

I stand by the position that for an induction generator to work that a power source must be applied to the stator for excitation.

 

[gjones33]

to ratcher

I quote verbatim from "Electrical Machines and their Applications" - Hindmarsh, Pergamon Press (1977)
"To operate as an induction generator then, it is necessary to have a system connected across the line that can provide a lagging current, or what amounts to the same thing, can accept a leading current. This is not usually a problem, because induction generators, which have a limited application, are normally connected in parallel with synchronous machinery for which the reactive volt-amperes are readily controllable.

However, it is possible to employ self-excitation using a capacitor of suitable value to provide the necessary current/voltage relationship. ....." end of quote

The text then goes on to describe methods of determining the conditions for stable operation!

Cheers,
G

 

[jwerthman]

I don't believe there is any way to run an isolated induction generator in a stable condition using capacitors for the excitation - at least not without some very sophisticated controls. It is not uncommon to use capacitors to provide the reactive excitation on an induction generator connected to a grid to reduce the reactive load on the grid.

 

[electricpete]

gjones - One way to interpret your reference is that the generator requires vars to be supplied to it either from caps or from the system. That much seems very reasonable. But it doesn't come out and say whether that is the only requirement....ie does the machine also require connection to an independent voltage source even if the vars are supplied by a cap.

As mentioned above, the rotor current is generated by "slip" between rotor speed and the power frequency...if there is no other frequency applied then there is no slip and it seems like there will be no rotor current and therefore no ability to deliver (motor) or absorb (generator) torque.

 

[busbar]

Notable research commentary at:

http://www.qsl.net/ns8o/Induction_Generator.html

 

[electricpete]

Well, I vote stars to everybody for a good discussion.

busbar's link certainly looks like an induction generator driven by lawnmower engine with no interconnect to any outside power source. He talks about getting a reliable engine so you don't have a problem starting it when the lights go out... so I assume it's intended for use when there is no outside power.

He also talks about powering about 1kw of lights with a 1hp indcuction generator (driven by 3hp engine) for several hours I think.

I'll have to think about it. Generating a voltage using residual magnetism of the rotor with caps connected to the terminals (at no load) makes sense. How the generator can absorb torque and generate power is perplexing. As discussed above there must be current in the rotor to create torque. If there is no slip to induce current in the rotor... maybe it is not the difference in speed but the difference in phase that creates the current. I'll think some more.

What do you guys think now?

 

[electricpete]

I'm trying to keep an open mind to all possibilities on this one.

One thing that makes me a little skeptical of busbar's link is the statement that he has to get the machine a little bit above syncrounous speed (1800rpm or 3600rpm) to generate voltage or power.

1800rpm and 3600 rpm should have no special significance to the generator itself without any connection to outside power source.

 

[UKpete]

Forgive me for not being able to provide a technical explanation, but I do know that induction generators have been used in "island" mode, i.e. in isolation, frequently in micro-hydro projects in 3rd world countries.

They are typically rated at a few kW, and the primary reason for using the induction machine is its simplicity/repairability. I read an article (which I didn't keep!) in one of last month's IEE publications in the UK.

 

[gjones33]

to electricpete

The text I quoted relates to an isolated induction generator. It is followed by a circuit diagram which I could not reproduce on here and there is only a capacitor connected to the motor terminals and it is followed by a graph table showing the stable points and how to achieve them.
Cheers
G

 

[busbar]

Proposed is the concept of 'alternating-current flashing' for the initial semi-resonant induction-stator/power-capacitor interaction.

 

[jbartos]

Suggestion: Reference:
1. M.G. Say "Alternating Current Machines," John Wiley & Sons, 1978
Sect. 8.20 Induction Generator.
It treats the induction generator what is its worth including excitation, electrical equivalent circuit, and it correlates it to the motor circle diagram (which is very helpful). It also states the drawbacks, e.g. the need for ac magnetization, moderate efficiency and (for mechanical reasons) the short air-gap.

 

[electricpete]

I don't know why I was thinking that there can be no slip in the absence of connection to an external power source. There certainly can.

I was stuck in the mindset that syncrounous speed is fixed and machine speed must vary according to the load.

But if we have a constant-speed engine driving the rotor and Wr, and we hook up an electrical load P, then the rotor will be under torque T=P/Wr, and the electrical frequency will droop below Wr by an amount given by the T-s curve.

Another way of looking at it....if the induction generator can supply a load while connected to both caps and an external power source, with no real or reactive power flowing to the external power source, why can't the generator do the same thing when we remove the power source. But as mentioned by gjones there are control challenges.

 

[electricpete]

busbar - I don't know what you mean by flashing... but I agree it looks like they selected the cap to be in exact resonance with the magnetizing inductance (calculated from no-load current).

In my message above I was perplexed at why the machine would not generate any voltage until it reached 3600rpm (after all what is special about 3600rpm if not connected to the grid). The answer to my question is that the resonant amplification only words near the resonant frequency..... And we computed capacitance to give resonance at 60hz (duh).

 

[Lewish]

Hello all,
If you can't operate induction motors as generators without being tied to an external power source, please don't tell all those people who are doing it. Their lights might go out.
For a good reference on doing this, albeit it is done for 50Hz only, please see the book "Induction motors as Generators" by Nigel Smith. My copy is out on loan, so I can't give you the ISBN number right now. The book is only about $10 U.S.
The book treats the aspects of sizing the excitation capacitors, different ways of connecting the capacitors, and covers various simple control schemes.
Frequency out is a function of number of poles in the motor and the RPM it is being turned at, plus slip. Voltage out is only a function of RPM.

 

[electricpete]

Thanks Lewish, that's good info.

Is voltage a linear relation with speed or does it jump up as we appropach operating speed? (resonant type behavior).

Doesn't voltage out also depend on the capacitor values?

 

[electricpete]

I posted the same question in another forum and just checked back. It gathered 25 responses. Check 'em out:

http://groups.google.com/groups?hl=...24GG6.10366017%40news3.calgary.shaw.ca&rnum=1