Can a generators resistance be variable to maximize efficiency?

[gar]

181203-2402 EST

If the generator voltage is less than the grid voltage, then energy will flow from the grid to the generator, and then the generator becomes a motor instead of a generator.

In an automotive DC generator the voltage regulator unit has a relay called a cutout relay that prevents reverse current flow. In an automotive alternator the diode rectifiers used to produce DC from the alternator AC output perform the cutout function.

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[winnie]

Finally got approval for this post:

I am going to toot my own horn here, since I think it is relevant.

https://patents.google.com/patent/U...=Edelson&oq=Inventor:+Jonathan+Edelson&page=1

In your application, the generator is a mechanical load on the turbine. It is useful to think of both the turbine and the generator as having a 'torque vs speed' characteristic curves. The operating point of the system is where these curves intersect, the speed where the torque absorbed by the generator matches the torque produced by the turbine. These curves will _change_ depending upon the characteristics of the equipment and the operating conditions. The torque/speed curve of the turbine will change with different pressure and temperature, for example.

What you want to do is change the torque/speed curve of the generator to control the rest of the mechanical system in some fashion that you desire. This is entirely possible.

Different types of generator will have different native torque/speed characteristics, so the particular electronic control that you use will depend on what you need to tweak.

For example, a synchronous generator connected to the grid will pretty much operate at a fixed speed. You could tweak things slightly by adjusting excitation, but basically you will spin at some speed related to the grid frequency unless you lose synchronism, at which point you are not generating electricity.

Induction and synchronous generators can be speed controlled if you have electronics to provide a variable frequency. You could change the frequency going to the generator in order to control the speed of the turbine/generator system; generated power would end up on a DC bus, and you would then have a line interactive inverter to get this power to the AC grid.

DC generators (including AC synchronous machines feeding a rectifier) will have torque/speed characteristics that depend on the voltage/current characteristic of the connected load. Voltage/current characteristic is essentially resistance, however you wouldn't want to have an actual resistor; you would want some sort of electronic converter that controls it apparent input resistance while coupling power output to the mains. By changing the apparent input resistance of this electronic converter, you will control the torque/speed characteristic of the generator, and thus the power output of the system.

There are lots of variations on this theme, depending on the specific goal.

-Jon

 

[gar]

181206-2244 EST

winnie:

Interesting. I read portions. There were a very large number of references. Must have taken a lot of examiner time, and the examiner had to have a broad background in the subject.


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[gar]

181207-0923 EST

winnie:

I have still only read a portion of your patent, but your first claim is intereseting.

1. A method for controlling a power output of a prime mover, comprisinga) receiving a signal indicating a changing power output requirement of the prime movers, comprising sampling a difference between a power output of the prime mover and a power output requirement of the prime mover;
b) adjusting a torque load on the prime mover according to the steps:
i) increasing the torque load when said signal is indicating a decreasing power output requirement,
ii) decreasing the torque load when said signal is indicating an increasing power output requirements,
iii) equalizing the torque load on the prime mover with the power output when said signal is indicating a maintaining of power output.

So we are controlling power output of a source by some signal that says I want this amount of power. As I read this the signal could be the load itself, or it could be anything else, like a potentiometer that I manually set.

The words "a power output requirement of the prime mover" seems to mean I want X power from the prime mover.
While the words "a power output of the prime mover" means the actual power output of the prime mover.

I have to leave.

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[winnie]

Correct. The point is to control the fuel consumption and mechanical power output of the prime mover by electronically adjusting the load characteristics of the connected electrical machine.

In the most general case this means understanding that if you wish to lower the power output over the long term, you must transiently increase the mechanical power absorbed by the electrical machine, to cause the system to slow down, and vice versa to increase power output. (This of course with the assumption that the characteristics of the prime mover are such that power output tracks speed, which is true for most prime movers over their normal operating range.)

The patent then went into specific implementations of the general concept, using different types of electrical machines

-Jon

 

[Noswad4]

Winnie,

Thanks for sharing. This is exactly what I was proposing. Would you be able to elaborate a little on what kind of controls could be put into place for a DC generator coupled to a heat engine that is feeding into the grid?

In your patent you reference a DC-DC converter. With the heat engine at WOT or (where ever maximum efficiency is) you could change the power feeding into the grid and the speed of the heat engine by simply changing the torque load on the generator. It is unclear to me how the DC-DC converter would accomplish this.

Appreciate the help!

 

[Russs57]

Hard for me to type. Got run over by a car and I’m laid up in a hospital bed.

I’d suggest you get up to speed on how standard generator controls operate and then see what changes as one moves to parallel operation and/or feeding the grid.

Typically one has tight control over the prime mover and forces it to operate at a certain RPM/hertz. Voltage is usually also maintained within a narrow range.

Your conditions are very different. For example, winds might become strong enough that self-destruction is going to happen even if maximum generator load is connected.

 

[gar]

181209-2115 EST

Noswad4:

I am curious as to what kind of project you have, even though it probably doesn't really matter for the discussion. Is it a class project, some idea you have, something at work, or energy storage project.

To get experimental data on your concept you can model the test at the 1 kW or less level. My suggestion of using an old automotive DC generator provides something on a small scale where you can work with moderately small components. This can hold experimental cost down. Even starting with a DC motor driving the generator instead of a turbine can allow you to see how torque and load power can be adjusted. If you have DC power it can be converted to AC to connect to the grid so this is not an issue. The big issue will be the characteristics of your turbine. To some extent the turbine can be modeled down to the 1 kW range. Scaling electrical components is probably easier than fluid flow components. Long time since I had a fluid dynamics class.

To see a hydraulic jump experiment was quite interesting. Theory and the real world can correlate.

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[gar]

181209-2456 EST

Noswad4:

I think that if you study your problem from the perspective of the control of a DC generator (dynamo
in Edison's day) with a battery as the grid you will be able to get an intuitive feeling of what happens.

I had a great teacher for both DC and AC machinery, J. G. Tarboux. He was both a full professor and a PHD, but these two titles are not what made him a great teacher. His basic intuitive understanding and his simplified technique of teaching were what made him good.

The reason for you to approach your problem using a DC generator is because it is a simple model to understand. Basically a DC generator is a variable voltage source with some fixed internal resistance. The source voltage is relatively linear with RPM at a fixed field excitation, and at a fixed RPM the the source voltage is relatively linear with respect to field excitation.

Note: The terminal voltage is the generated voltage minus the load current times the internal resistance.

Generated power is the internal voltage times the load current. Input power equals generated power plus some frictional losses that can be neglected as a first approximation. Thus, assume generated power is equal to input power.

Input power is a constant times torque times RPM. Other than the use of some fixed gear box, and therefore not important from a concept perspective, you have no direct control of RPM. The only thing you can control is field excitation to the generator.

Over some range you can create an electronic control to adjust field excitation so that some signal you want as your control can adjust power generated, and therefore, drawn from your prime mover. I believe you are thinking of a turbine powered from compressed air in some tank, and as power is drawn from this air tank the air pressure drops.

This tank is possibly some big underground cavity, such as we have in Michigan for storage of natural gas. Our natural gas prices are low because we have these huge cavities to store gas in the summer for use in the winter.

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