Electronic Speed Control of a Generator

[Jraef]

I'm still on why you want to "control the speed of a generator"- that makes no sense to me, especially in the realm of energy recovery. Also, please separate the concepts of generation control (output/frequency) from over-speed protection (most often done by removing the available energy hitting the blades (valve/gates/etc) or the amount they capture (varying the blade pitch)).

I interpreted his need as being to prevent the mechanical failure of the generator from over speeding, due to his "pipe" setup being there for something ELSE, and he is wanting to harvest power from it without the ability to change the flow through the pipe. I was envisioning an exhaust vent of some sort and he wants to take advantage of that air flow by running it through a wind turbine.

 

[retirede]

I interpreted his need as being to prevent the mechanical failure of the generator from over speeding, due to his "pipe" setup being there for something ELSE, and he is wanting to harvest power from it without the ability to change the flow through the pipe. I was envisioning an exhaust vent of some sort and he wants to take advantage of that air flow by running it through a wind turbine.

It may be worse than that. I suspect the primary user of the air flow is downstream of the generator turbine. When the air is needed for its primary use, the generator isn’t supposed to absorb energy from the air. If the generator is electrically unloaded, it will overspeed. Any attempt to slow it will necessarily impede the flow. No different than loading the generator back up.
If this is the scenario, the only options are dynamically altering the blade pitch or bypassing the turbine.

 

[Noswad4]

230406-1041 EDT

Noswad4:

The generated output voltage of a DC generator is primarily controlled by (1) physical structure of the generator, (2 ) the generator RPM, and (3) the field magnetic intensity. So for a given generator the easiest parameter to electrically change is field excitation. The output voltage at a fixed RPM will increase as field excitation is increased. The ultimate limitation is set by core saturation.

Starting some time in the 1930s automotive generators had their output voltage controlled by adjusting the generator field excitation.

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Thanks Gar. Could you elaborate on the types of methods used to adjust DC field excitation?

 

[Noswad4]

It may be worse than that. I suspect the primary user of the air flow is downstream of the generator turbine. When the air is needed for its primary use, the generator isn’t supposed to absorb energy from the air. If the generator is electrically unloaded, it will overspeed. Any attempt to slow it will necessarily impede the flow. No different than loading the generator back up.
If this is the scenario, the only options are dynamically altering the blade pitch or bypassing the turbine.

This is the correct scenario, but it the generator is allowed to impede the air flow. Therefore, I believe some time of field oriented controller or phase angle controller could control a DC generator's speed. if I am wrong I would love to better understand.

 

[Noswad4]

The controller on this setup seems to be doing exactly what I am talking about.

Primus Wind Power Air 40 + Control Panel

AIR 40 TURBINE + CONTROL PANEL The proven choice for remote energy AIR 40 is the premier micro-wind turbine for land-based applications. It operates efficiently across a wide-range of wind speeds, providing energy for telecom, water pumping, lighting, SCADA, off-grid homes, or other low energy...

www.thecabindepot.ca

 

[zbang]

Depends on whether you need manual or automatic control. Manual control can be as easy as a rheostat between the excitation supply and the field. OTOH if it's a permanent-magnet field, that's not an option.

Looks like the Primus control panel is only an interface to the batteries, it doesn't even connect to a brake nor have any field control. The over-speed control seems the be entirely in the turbine-

Over Speed Protection: In gusty or sustained high winds, over 22 m/s (50
mph), AIR enters overspeed protection where the blades come to a near
stop. The turbine stops the blades for 30 seconds then allows them to
begin spinning. If the wind is still high or gusty, the cycle is repeated until
the wind speeds drop below 22 m/s (50 mph).
Overspeed protection is stressful on the turbine. Primus Wind Power
recommends taking measures to protect the turbine in excessively high
wind situations.

It sounds like the OP needs to do some more research about both alternator/generator control schemes and power generation from variable sources (i.e. wind). I think most wind turbine controllers ought to be able to do all this, or it could be programmed into a small PLC or microcontroller with the appropriate sensors and I/O, but that may not be cost effective.

 

[Noswad4]

There are a few reason why I want to control the speed of the generator. By this I mean I would like to manipulate the rotational speed of a generator. The two main reasons would be to (1) affect backpressure on the air flow and (2) ensure that the maximum rotational speed of the generator or expander is never reached.

 

[retirede]

This is the correct scenario, but it the generator is allowed to impede the air flow. Therefore, I believe some time of field oriented controller or phase angle controller could control a DC generator's speed. if I am wrong I would love to better understand.

If the airflow doesn’t change and the impeller pitch doesn’t change, the only way to limit the speed is to effectively load the generator.
If you want the generator to take less power from the airstream, it has to speed up.

 

[gar]

230409-2350 EDT

Noswad4:

Suppose I have an ideal battery. This is a battery that has a terminal voltage that is independent of whether current is flowing in or out of the battery. Actual batteries somewhat approximate this.

This battery is connected to something with an internal impedance. There is an energy source or receiver in this said something.

That source has to develop a voltage that is above or below the constant load voltage such that the battery potiental remains constant.

That will required the source voltage to adjust to achieve balance.

In a generator the output voltage is determined by the magnetic field in the generator and the speed of rotation of the generator. So by changing the magnetic field intensity one can control the generator speed.

.

..

 

[Noswad4]

230409-2350 EDT

Noswad4:

Suppose I have an ideal battery. This is a battery that has a terminal voltage that is independent of whether current is flowing in or out of the battery. Actual batteries somewhat approximate this.

This battery is connected to something with an internal impedance. There is an energy source or receiver in this said something.

That source has to develop a voltage that is above or below the constant load voltage such that the battery potiental remains constant.

That will required the source voltage to adjust to achieve balance.

In a generator the output voltage is determined by the magnetic field in the generator and the speed of rotation of the generator. So by changing the magnetic field intensity one can control the generator speed.

.

..

That makes perfect sense to me. Thanks for simply explaining it. My follow up questions would be what type of methods are used to control the magnetic field intensity while also allowing voltage to flow from the generator (said something) to the battery? How does it change for DC and AC machines? I originally though that a VFD would solve this because it can adjust frequency. I learning that it is not a solution. Instead, others have recommended a phase angle controller or a field oriented controller.

Open to any suggestions!

 

[gar]

230410-0901 EDT

Noswad4:

A DC generator with a wound field coil has the field intensity adjusted by the DC current flow thru that field coil.

Increasing the field intensity of a genertor with a constant input RPM produces a higher output voltage

Or if the generator output voltage is held constant by the generator's load, then the generator must run slower if field excitation is constant.

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[Noswad4]

Thanks Gar. Just to make sure I understand the last sentence. If you increase the current a wound DC generator experiences while holding the generator's load constant then the generator will rotate at a slow speed?

 

[zbang]

Sounds like we're getting into the basics of rotating machines, might be helpful for @Noswad4 to read up on that. Maybe start with the wikipedia articles-

Electric generator - Wikipedia

en.wikipedia.org

Alternator - Wikipedia

en.wikipedia.org

Dynamo - Wikipedia

en.wikipedia.org

Those will cover the basic principles and terms.

 

[Noswad4]

Thanks zbang. Agreed that I need to learn more of the basics. I am researching these things as well.

I was hoping that some one might help me identify a controller that is capable of modifying the generator's speed.

 

[gar]

230510-1507 EDT

Noswad4:

Consider only a DC generator with a diode in its output connected to a storage battery. The DC generator has a field winding to which I can adjust the current. The battery voltage is relatively independent of current flow into the battery. When the generator reaches a speed where its generated voltage is equal to or greater than the battery voltage, then the generator starts to mechanically load its input shaft.

At a generator speed below the point where current flows into the battery there is little load from the generator back to its input. Thus, the input can do whatever it wants to do.

When the generator reaches a speed where it gets loaded from the constant battery voltage, then power is drawn from the mechanical input to the generator. How much the generator speed increases after this point is a function of the internal impedance of the generator.

I can change the generator speed where this starts to happen by adjusting the magnetic field intensity of the generator field coil. The greater the field intensity the lower is the speed where the generator starts to load its input shaft speed.

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[Noswad4]

found this and thought it was useful.

 

[gar]

230411-2047 EDT

Noswad4:

The audio was so bad on the video you referenced I could not justify trying to understand what was being said. Sorry.

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[LarryFine]

The audio was so bad on the video you referenced I could not justify trying to understand what was being said. Sorry.

Watch with sound off and closed captions on.

 

[gar]

230411-2419 EDT

LarryFine:

That discussion is nothing more than several ways to provide adjustable dynamic braking on a generator of electrical energy.

..

 

[Noswad4]

Thats what I gathered as well. So really they are just adding adjustable dynamic braking to a VFD controlled motor?