Electronic Speed Control of a Generator

[Noswad4]

I am trying to better understand methods that can electronically control the speed of a generator (specifically). The generator will be spinning using mechanical energy generated by air through a pipe. The electricity will be sent to a load or battery bank.

I thought a VFD might be the solution, but have begun second guessing if it is feasible. Another solution that may be adjusting the resistive load using a phase angle fired. A third option might be to use a controller such as ODrive (link). I am open to any input or suggestions.

For clarity sake - I know there are mechanical ways to control speed used on most large geneators (e.g., brakes, blade pitch, control valves), that are the preferred method due to efficiency. I am looking for electronic solutions due to surplus of energy and no control over the mechanical input.

Thanks in advance!

 

[zbang]

I'm not clear what you're trying to do- control the output of the generator into a specific load or how fast it spins. Is this a test setup for for power generation?

There are mechanical ways to control speed used on most large geneators

Sort of, but those control the power input to the generator; which has the side effect of controlling the speed*. And some, like variable-pitch turbine blades, change the amount of power extracted from a water flow without changing the actual flow. (Brakes????)

*(I'll leave out grid-tied generation, that really changes the discussion.)

AC (single/three-phase) or DC generation? How is it excited? What's the power level?

For a given available energy, varying the load will perforce vary the rotation speed. It sounds like you're trying to control the load (yes?). If that and dumping all the generated power is the goal, then the simplest way into a bank of resistors, you can switch various fixed resistors to get the correct loading, use power-activated variable resistors, use a water rheostat, or maybe use some sort of inverter thing like the OLink product. Devices like a VFD or phase-angle firing controller are usually limited in the input frequencies they can handle.

Depending on what's going on and the power levels, the control system becomes non-trivial.

 

[synchro]

What is the reason behind needing to control the speed? Is it to prevent it from mechanical damage due to overspeed, or are you trying to control the output frequency of an alternator?
If the output frequency is the concern, rectifying the output and using an inverter to supply any AC loads could provide a very precise output frequency.

In principle it would be possible to use a VFD with a braking resistor to control the generator speed. But the devil is in the details. You might need another power source for its operation.

 

[Noswad4]

Thanks for the clarifying questions. I should have included them in the original post.

I am trying to control how fast it spins in order to prevent overspeed. The prime mover (air through pipe) is variable speed/pressure and can not be regulated. I don't necessarily need to control the load, but from my understanding that is the best (only?) way to control the rotational speed of the generator electronically.

From my research the VFD would need to be tied to another motor in order for it to control the speed of the generator. Is that true?

Any experience with phase angle fired controllers? That seems to be an alternative.

 

[zbang]

Still a bit shy on the details-
If nothing else, what's the power level involved? milliwatts for a measurement transducer or kilowatts for power generation.
DC or AC generator?
How much is overspeed an actual problem? (If you brake the device somehow, it will still extract energy from the airflow; a free-spinning device with no load will cause the least disruption.)
(It's always helpful to describe the entire setup, not just the exact piece in question.)

You can slow both of them by injecting power into the rotating machine (electric braking) or by using a mechanical brake, the latter may not be practical over a given smaller size unit.

Phase-angle heating controllers and resistive heaters can make a nice load, but those devices can be frequency-sensitive.

 

[gar]

230406-1121 EDT

Noswad4:

It is not clear exactly what is your goal.

You have air flow thru a pipe. In this pipe is a transducer ( air flow to an electrical output ). There is an internal impedance from the input air flow to the electrical output And there is some electrical load ( resistance ) or energy receiver ( battery ).

Apparently you want to prevent overspeed of the generator. Is that correct?
What is the purpose of this system?

The impedance of the system from air flow input to generator electrical output determines how easily you can control the generator speed.

Any means by which you can control the current load on your generator will provide the ability to control maximum generator speed.

Can you define the purpose of this system?

..

.

 

[Noswad4]

230406-1121 EDT

Noswad4:

It is not clear exactly what is your goal.

You have air flow thru a pipe. In this pipe is a transducer ( air flow to an electrical output ). There is an internal impedance from the input air flow to the electrical output And there is some electrical load ( resistance ) or energy receiver ( battery ).

Apparently you want to prevent overspeed of the generator. Is that correct?
What is the purpose of this system?

The impedance of the system from air flow input to generator electrical output determines how easily you can control the generator speed.

Any means by which you can control the current load on your generator will provide the ability to control maximum generator speed.

Can you define the purpose of this system?

..

.

Your explanation is correct. The purpose of the system is to scavenge free energy when possible. The primary purpose of air flow is for other uses that supersede energy generation. If the end use of the air flow is in high demand the generator must spin as free as possible. The air flow speed/pressure fluctuates quite a bit both from supply and demand sides. I understand a smart valve system with bypass could also be put in place, but I wanted to explore the option of controlling generator speed.

 

[Jraef]

But still, you haven said how MUCH power you are talking about here. Milliwatts, kilowatts or megawatts? And what type of “generator”, DC, AC, AC synchronous with a DC excitation system, AC grid connected? Makes a HUGE difference in the practicality of various methods.

 

[Noswad4]

Sorry. This part is flexible. Lets say 1 kw, 20 amps. The generator can be any of the above, but probably lean towards DC since it will be charging a battery bank.

I would like to learn how the solution would change if the generator was connected to the AC grid.

 

[gar]

230406-1400 EDT

Noswald4:

If you are trying to transfer energy from some source to an existing system where that load is something like a voltage source ( the load ), then you need said source to produce a voltage that is greater than the load voltage, and you will need a means that prevents reverse current flow from that load when the said source voltage is less than the load voltage. Thus, the load is not a linear load. Think of this like an automotive generator charging a battery.

Doing what you want is more difficult in an AC system.

.

 

[Noswad4]

Thanks gar. I guess I will focus on a DC system first.

How would you accomplish what you laid out above?

 

[hillbilly1]

If you want to do it electronically to control a mechanical device, you still will need a mechanical means to do it. Sure, you could use an electronic speed monitor to control a mechanical valve to bypass excess air flow, but if you are wanting to load the generator to control the speed, I think you are going to have heating and wear issues.

 

[gar]

230406-1440 EDT

Noswad4:

In a DC system you use a diode between the energy source, and the load.

There will be no power load on your generator until the generator's output voltage exceeds the load voltage ( the battery voltage ). When this threshold is exceeded, then power flows from your generator to the load. Consider a very stiff load voltage, then the speed of your generator will depend upon the impedance between the generator and the battery. This impedance consists of the sum of any external resistance ( cables ) and the internal impedance of the generator.

.

 

[Noswad4]

If you want to do it electronically to control a mechanical device, you still will need a mechanical means to do it. Sure, you could use an electronic speed monitor to control a mechanical valve to bypass excess air flow, but if you are wanting to load the generator to control the speed, I think you are going to have heating and wear issues.

Agreed, but couldn't you dissipate heat if needed?

230406-1440 EDT

Noswad4:

In a DC system you use a diode between the energy source, and the load.

There will be no power load on your generator until the generator's output voltage exceeds the load voltage ( the battery voltage ). When this threshold is exceeded, then power flows from your generator to the load. Consider a very stiff load voltage, then the speed of your generator will depend upon the impedance between the generator and the battery. This impedance consists of the sum of any external resistance ( cables ) and the internal impedance of the generator.

.

I follow - but when the load voltage exceed the generators would it turn the generator into a motor? Is there a way to avoid this?

 

[Jraef]

The only way to “electronically” control the speed of a DC generator it to make it into a motor, but it cannot be both at the same time. You can electronically LOAD a generator by dumping excess power that it is generating into a load bank and burning it off as heat. But that is only good up to the thermal capacity if the windings. Beyond that you must control the mechanical speed.

What is typically done is to control the mechanical force being expressed on the generator. In wind turbine applications, this is done with pitch control on the blades. If your application is more of a Venturi effect situation, some sort of throttling capability would likely be in order, like a damper or iris to control the air volume across the blades.

In an AC induction generator that is grid tied, the grid frequency is stable enough that any excess air flow cannot force the frequency of the entire grid to change, so that creates a defacto means of retarding the speed of the rotor, up to the negative torque capabilities of the induction machine. Above that the air flow will “spill off” of the blades in some designs, or they use pitch control and /or mechanical brakes.

 

[gar]

230406-1630 EDT

Noswad4:

I already told you that a diode, in a DC system, prevents the back flow of current from a battery load into a generator. In the old days of automotive generators this was done with a cutout relay.

How fast a generator will rotate when supplying current to a load will depend on how much the load voltage changes with current ( the load's internal impedance ), I was assuming no voltage change. With this assumption, then generator speed is is determined by the total impedance between the battery, and the voltage source within the generator.

.

 

[Noswad4]

Focusing only on the DC generator for now. It seems to be that the only way to electronically control the rotational speed is to electronically load the generator. Getting into the details - can you both load and generate at the same time. or example, if the generator was feeding the load at 1,200 rpm, but you wanted to slow the generator down to 1,000 rpm, while still supplying power to the load (batteries)?

Side note. I came across this today and still need to research it a bit. https://www.thecabindepot.ca/produc...05857&pr_ref_pid=1995789500513&pr_seq=uniform

 

[gar]

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.

.

 

[gar]

230407-0629 EDT

Nowsad4:

I have reread your first post. I believe it says you want to control the speed of a generator.

Thru the various posts it seems you would like to control the generator speed electrically by varying the load on the generator. This is an unusual approach.

A DC generator is the easiest way to describe a method.

The basic electrical-mechanical characteristics of a DC generator can be described as:

(1) Output voltage is proportional to generator RPM.

(2) The generator has an internal impedance that drops the generated voltage to an output voltage an amount proportional to load current.

(3) If the load is a constant voltage, then the generated internal voltage is some amount greater than the load voltage. This amount is a function of the load current.

(4) From some mechanical input force there is energy loss from that input to the generator output. This might be described as an impedance from mechanical input to electrical output.

(5) If you want the generator output terminals to have a constant voltage independent of load current at constant generator input speed, then you put an addition to the field winding that changes field flux density to compensate for electrical load.

Doing all this, then by connecting to a constant load voltage ( a battery ) you can extract a variable amount of energy from some source.

Fundamentally it means you need a fair amount of power loss capability from your fluid source to your electrical load. The electrical load ( battery ) has to have a low internal impedance, and a capability to store a large amount of energy, or dissipate the energy some way.

Fundamentally the battery voltage defines the operating speed of the generator.

.

 

[zbang]

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)).