Electronic Speed Control of a Generator

[Noswad4]

In an effort to refocus and hopefully wrap up this discussion I will propose two general questions.

Question 1: Are there other electrical methods for controlling the torque on a generator besides controlling the load the generator sees?

Question 2: What type of devices can be used to control the load a generator sees? (e.g. phase angled controller? field oriented controller?)

Scenario:

The goal is to affect pressure and flow of air in a pipe down stream of the turbine/generator. The proposal is that by controlling how fast the generator spins the connected turbine/expander can put backpressure on the air flow and control the pressure downstream. The situation is that there is a constant air flow from a reservoir that varies slightly ~10% in pressure and is upstream of the generator. Downstream of the generator are users that have priority. Those users require varying air pressure, but always less than the source. Currently there are choke valves that are controlled with via PID to adjust the pressure based on the needs.

I understand that there are many (more practical ways) of going about this, such as mechanical braking, bypass valves, etc. My focus is on controlling the speed of the generator through electronic means rather than mechanical. I am only looking at this from a theoretical stand point for the time being and don't necessarily need to control the load.

 

[gar]

230412-1132 EDT

The reason I described a DC generator feeding into a constant voltage load with diode isolation between the generator and the load is that it may do what you want and is simple to analyze. From a practical point of view it is also likely to do what you want. There are many different electronic means that can accomplish the same result.

The diode isolation is simply to prevent back feeding the source and turning it into a motor instead of being a generator.

A battery is a reasonably constant load voltage for the purpose at hand. and provides a relatively constant load voltage ( the threshold point of control ).

A field excited generator, by adjusting the field, provides a means to adjust the generator RPM where it starts to provide an electrical load.


If you do this same function, but use electronic circuits, then you provide greater control, and a tighter control of the generator speed after the threshold is reached.

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[Noswad4]

Thanks Gar. A battery and diode isolation seem like they will be part of the solution. However I am looking to find something that has greater control than simply a battery.

Could you elaborate on what type of devices or methods these electronic circuits would be? I assume they would include some type of PID controller, but not sure what types of devices would be able to influence field excitation or load.

 

[gar]

230412-1659 EDT

Noswad4:

The simplest basic electronic approach might be some adequate load resistor, fed from an electronic switch where the switch is pulsed on at a high enough rate, the percentage of on time being adjustable to vary average load current, an inductor in series to smooth the average load current as seen at the generator, and a voltage sensing circuit and control circuit to adjust the pulse duty cycle. And included in this is an adjustable voltage reference that that is adjusted to a level that sets the limiting of the generator speed.

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[Noswad4]

Thanks Gar - very helpful.

Through my research I have found that it may be possible to also control the speed of a generator by adjusting frequency. Would the people here be able to confirm if this is true. If it is, would I still need to adjust the load or have some type of load off take?

 

[rambojoe]

Depends on when your thesis is due.....

 

[gar]

230413-0553 EDT

Noswad4:

What is the maximum power you need to extract at 1800 RPM for an example.

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[winnie]

Conservation of energy.

If you are removing energy from the velocity of the airflow, then it has to go somewhere.

That energy can be returned to the airflow itself in the form of heat, or it can be absorbed by an external system. But it must go somewhere.

You can vary the energy absorbed by a generator by adjusting the mechanical supply to the generator (eq with some sort of variable pitch turbine), by adjusting some characteristic of the generator such as excitation, or by adjusting some characteristic of the load, such as resistance.

There is a huge space of possible ways to vary each of these.

Is this an academic exercise? Are you trying to develop technology where you want to control IP? Do you want to modify an existing system or is this a clean sheet exercise?

And yes, the speed of some types of generator can be adjusted by changing frequency. This is not true for the DC generators which @gar has been describing. But it is true for AC induction generators and AC synchronous generators. AC synchronous generators probably provide the vast majority of baseload power on the electrical grid.

Jon

 

[gar]

230413-0821 EDT

Noswad4:

If the maximum power you want to remove is in the low watts to low KW range, then there are some moderately inexpensive items that you can play with. An automotive generator can be in the KW range, and good size car battery is a reasonable size load. Etc.

.

 

[Noswad4]

This is more a starting point for technology development. Just trying to get an understanding of all methods. The initial thought would be for a 3 kW unit and expand from there.

The possible solutions seem divided into the two groups Winnie motioned (DC and AC induction/synchronous). DC seems to be controlled by load while AC might be able to use load as a control, but also frequency.

I am starting to get a beginners handle on how to control load to a DC generator thanks to this forum. Would anyone be able to comment on methods to control frequency on AC induction/ synchronous generators and if you can also control these generators with load?