Electronic Speed Control of a Generator 3

[Dawsonh4]

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!

 

[edison123]

This is a DIY project. Should have been RF'ed already.

Muthu

http://www.edison.co.in

 

[crshears]

OP doesn't seem to be providing a wealth of information for us to work with; you shouldn't make us drag it out of you . . .

Are you building this whole rig from scratch, or are you constrained by adding to or modifying an existing installation? The answer will alter the preferred solution . . .

Quick and dirty overspeed control: employ a directly coupled "fly" mechanism that uses centrifugal force to extend a pair of wings that add mechanical load to the air motor. Rotation of the air motor blades to reduce the amount of energy extracted from the air stream would also work. Both of these methods were/are used on some traditional designs of wind-driven rural water pumps [ google "Aermotor" ].

Alternatively to both of the above, waste the extra energy driving a "ghost load" fan with a motor controlled with an appropriate chopper circuit; since the - energy? power? required to drive a fan varies as the cube of its speed, it should not be insurmountable to make this work. Note however that if your site is not remote there may be some objectionable siren-like noises that could/would be very annoying to the neighbours.




CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[LionelHutz]

Generally for 1kW you design the prime mover wind turbine to match with a 3-phase permanent magnet generator and then rectify the 3-phase to DC. The DC can go to a controller or batteries or whatever where it can power something directly or be put through a (grid tied) inverter to create 60Hz AC.

Whole books are written on this subject so trying to give help on a "tips" website to someone who doesn't seem to know how to do it at all seems rather pointless. Go do some research. Try otherpower.com for some info on how to do this.

 

[Dawsonh4]

Waross - thanks for the insight. I am aware that individuals on this site have studied these concepts. The reason I am here. My background is in chemica121 engineering and am very familiar with PID controllers. I do struggle quite a bit with the electrical side of things, but I am learning. The Odrive link is here. My understanding is that it is a field oriented controller. The forum there has been very helpful.

Crshears - I apologize for not giving all the details. I will attempt to do so. This rig would be built from scratch.

There are a lot of similarities to Waross's hydroelectric example. 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.

I hope that helps.

 

[LionelHutz]

Regenerative capable VFD. No generator, 3-phase motor run at whatever speed causes the back pressure you want. If you run it slow enough it will generate power back through the VFD. If you run it too fast then you start motoring it making the contraption a pump.

The thing is that a regen VFD at 1kW is rather difficult to find and expensive.

 

[Dawsonh4]

Thanks LionelHutz. Is finding a 1 kw regen VFD difficult because it is too small or too big?

Are there other options besides a regen VFD?

 

[waross]

The regen control circuits and algorithms are basically the same regardless of the rating of the drive. That is a fixed expense per unit.
There is little call for regen VFDs as small as 1 kW. So, there is little economy of scale for the major manufacturers and little incentive for the offshore knock-offs.

You may be able to extract energy from the braking circuit of a standard VFD.
This is subject to further investigation as to the practicability of the scheme.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

I suggest that you contact Odrive.
You want to extract energy by using a motor in regenerative mode, in response to a signal from the output of a PID controller.
Is it possible to configure a motor and an Odrive to control the regenerated current of the motor?
Possibly torque control mode?


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dawsonh4]

Here are ODrive's comments. It seems like a decent solution.

An ODrive that has regen (=generator mode) enabled sort of assumes your DC bus is connected to a rechargeable battery. This bus is used to power up the electronics of the ODrive and potentially to drive a motor, but as @madcowswe has mentioned, if a braking torque is applied (regen/generator mode), the power direction is reversed - power is taken from the motor and pushed to the DC bus, raising its voltage. The battery counteracts this raising - current starts flowing into the battery, so it is now charging. If you keep doing this - something spins your motor and the ODrive applies braking torque - the battery will keep charging until it is 100% charged. When this happens, there would be no where for the energy to go. This is where you need some external logic (the external Teensy/Raspberry Pi/PC mentioned), which needs to act to prevent battery overcharge. This will e.g. disable the regen, or add an external load, likely a properly-sized resistor, to turn the excess energy into heat, as you’ve suggested.

 

[waross]

Switching in an appropriate resistor to the Primus unit will do the same thing.
Your choice.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dawsonh4]

Are all these solutions based on varying the amount of load the generator sees. Is that correct?

The video below is a discussion on slowing a generator/motor. They break it into a few different methods, but the two main ones are "regenerative" and "dynamic" breaking. I am still trying to determine the difference, but it seems like the both employee an "active front end" to the VFD. I am not sure what the active front end entails. Would any one have insight on this?

 

[waross]

Regeneration: Returning energy to the source.
Dynamic: Wasting energy internally to the motor.
A common dynamic scheme is used when a high inertia load must be stopped faster than coasting down.
For an industrial AC induction motor, the supply is disconnected and DC is injected into the windings.
Spinning the rotor in a fixed magnetic field sets up eddy currents in the rotor.
The the eddy currents generate heat in the rotor.
Thus the energy of inertia is wasted as heat in the rotor.

Active front end.
The basic front end is a bridge rectifier, which will pass current only one way, from the source to the load.
An active front end employs switchable elements with or in place of the basic bridge rectifier, so that energy may be switched from a re-generating load, back to the source.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dawsonh4]

Very helpful! Thanks Waroos. It seems that I am looking for the regeneration solution. I will dig deeper.

 

[Dawsonh4]

Follow up question - is the active front end really what makes it a regeneration VFD? Is there something else that is needed?

 

[Gr8blu]

Dawson An active front end drive does not equate to one with regeneration capability. All drive topographies can be either 2-quadrant or 4-quadrant: 4-quadrant is what is required for regeneration. Expect to pay roughly 1.5x to 3x the price for a 4-quad drive compared to a similarly-rated 2-quad drive.

Converting energy to motion for more than half a century

 

[Dawsonh4]

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.

 

[waross]

How impractical do you want to be?
You could go for a doubly fed induction generator.
In that size that would mean a complete design and build from scratch.
Probably at least 10 X the cost of an Odrive in torque control mode. Maybe even more expensive.
It may work as well as an Odrive, but not better.
You may have over thought this until you fell down the rabbit hole.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dawsonh4]

Haha. It seems that way. I am just trying to come up with a general term at this point for devices such as Odrive that can regulate the load. Also want to ensure there aren't other ways to control speed such as field excitation.

 

[Dawsonh4]

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?

 

[waross]

If you are wasting energy in a resistor bank, you may control the load by controlling the excitation and thus the output voltage and so also the current.
That doesn't work quite as well for battery charging but may work.
However, controlling the excitation implies brushes.
Many years ago a was in charge of electrical for a large sawmill.
As I remember, we had 4 DC generators, 4 dc motors, 4 amplidynes, 1 exciter and 2 wound rotor motors.
Every Sunday, one man spent most of the day, checking brushes.
Miss a Sunday cleaning and by Wednesday we would have sparking and burnt commutators.
I admit that the environment was extremely dusty, but ever since, I have tended to avoid brushed machines if there was a suitable alternative.
The Odrive motors appear to be either three phase induction motors or permanent magnet synchronous motors.
The field is not controllable.
The Primus is a permanent magnet AC generator.
The field is not controllable.
While I have said that a permanent magnet field is not controllable, that is not strictly true.
In my early teens, I had a bicycle generator with a controllable permanent magnet field.
There was a flyweight on the shaft.
As the rotational speed increased, the opposite and opposing force to the centripetal1 force would act against a return spring and move the rotating permanent magnet out of the center of the stator core.
That was over 60 years ago and I have never seen another such control.
Note[sup]1[/sup] I saw on another forum the statement;
Never say "centrifugal" to a physicist.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!