I am hoping to understand and get a full pictures of different devices that can control the speed of a generator (specifically). I have started with a VFD, but am unsure if there are other methods. If I use a BLDC motor would it be possible to use a controller such as "ODrive" to replace the VFD? If so, what are the pros and cons to switching. Are there other solutions I could use?

For clarity sake - I know there are mechanical ways to control speed as well (e.g., brakes, blade pitch, control valves), but I am looking for electronic solutions.

Thanks in advance!

 

You are gonna need to explain this a bit more.
What is driving this generator? What kind of generator? Why do you need to control it's speed?

 

Based on your other threads I am guessing this is a wind turbine application and you're looking for a way to prevent it going above max speed?

If so I think you need to consider those other options you mentioned not wanting to consider. If you wanted to limit the speed of the turbine using the generator then you would need to place a controlled load across the generator output. This would quickly burn up your generator and probably your load device as well.

 

The generator would be driven by air flow through pipe. The generator could be any kind, but likely and induction or BLDC design

I am only exploring electrical options for a small experimental project. I understand that current technology uses mechanical methods for good reason.

It seems as though an electronic alternative to the VFD would be a BLDC motor with a control pair to a battery bank/resistive load. ODrive community has been helpful in helping me here. I am curious if this community knows of other electronic methods to control speed.

 

I am curious if this community knows of other electronic methods to control speed.

Like I said

you would need to place a controlled load across the generator output.

Is there something you don't like about that suggestion?

 

I am hoping to get more specific than a "controlled load". Specifically, the different way to control the load. Is a VFD with an induction motor the best method or would the BLDC with a controller be better? Are there other methods besides those two that I can look into?

Are there methods that would no burn up the generator and load device?

 

I am hoping to get more specific than a "controlled load". Specifically, the different way to control the load. Is a VFD with an induction motor the best method or would the BLDC with a controller be better? Are there other methods besides those two that I can look into?

It's not obvious what you mean.
You seem to be on a quest to find out which is the best device to use as a generator (induction motor vs BLDC). And you also now seem to be asking the same question with regard to a controlled load. When I said

place a controlled load across the generator output.

I meant

place a controlled load across the generator output.

...not "use the generator itself as a controlled load across its own output."

What I had in mind was a resistive load bank controlled by a phase angle fired power controller that would be in control loop to load down the generator above a certain speed.

 

Thanks for your input. Could you elaborate a bit as to why you would choose a phase angle fired power controller rather than a VFD or something similar? What are the benefits?

What are the pros and cons to switching. Are there other solutions I could use?

Hopefully I am clear that I am

hoping to understand and get a full pictures of different devices that can control the speed of a generator

and the generator

could be any kind, but likely and induction or BLDC design

 

Thanks for your input. Could you elaborate a bit as to why you would choose a phase angle fired power controller rather than a VFD or something similar? What are the benefits?




Hopefully I am clear that I am



and the generator

We don't communicate on the same wavelength. It's probably my fault but I'm still confused as to what you're asking.

Do you mean (for example)
A. BLDC used as a generator
B. VFD with a separate motor as a load to limit the speed of (A) above

Or do you mean
A. VFD with a motor acting as a generator that limits its own speed

Because what I am proposing is:
A. BLDC used as a generator
B. Phase angle controller with a load bank as a load to limit the speed of (A) above

 

It is certainly difficult to communicate over the forum, especially with my lack of knowledge in this field.

A. VFD with a motor acting as a generator that limits its own speed

This would be what I mean.

Your proposal could be a solution. I am hoping to better understand why you would propose a phase angle controller rather than something similar to ODrive or a VFD as the "controller". I understand that there would need to be a load bank that would provide the load to limit the speed.

 

Let me back up a little bit.

When a generator is controlled by a variable torque prime mover such as wind/water turbine, gas engine, dude on a bike, etc. its speed (regardless of induction, brushed, BLDC, alternator, etc.) will be regulated by the electrical load across its output. Zero electrical load, it will spin very fast. Heavy electrical load, it will spin very slow (despite lots of air, water, gasoline, whatever). You can't just command a generator to go a certain speed, regardless of what technology you choose. The only way to do that is to control the mechanical input power (via variable pitch propeller/impeller, flow control valve, engine governor, mechanical brake, etc.) Which you have already stated are off the table. So that leaves one option remaining: control the electrical load to affect the generator speed. That requires an electrical load external to the generator, and that external circuit is what I'm discussing. Whether you choose induction or BLDC or brushed alternator or whatever, I don't care. I'm not talking about that. I'm talking about the separate electrical load circuit. I'm not proposing a phase angle power control instead of a VFD, I'm proposing it in addition to a VFD (or BLDC, or alternator, or whatever)

 

Thanks for backing up. It seems my error of misunderstanding a VFD is what has led us here. My understanding is (was?) a VFD is what would control the electrical load circuit going to the motor/generator. Isn't that how the speed of a motor is regulated when connected to power?

 

Thanks for backing up. It seems my error of misunderstanding a VFD is what has led us here. My understanding is (was?) a VFD is what would control the electrical load circuit going to the motor/generator. Isn't that how the speed of a motor is regulated when connected to power?

To understand the specific way that an induction motor (fed from VFD or other AC source) generates power is a topic that will require a great deal more foundational electrical knowledge than you appear to have. Please don't take that as an insult, it isn't meant that way. Just know that a VFD doesn't fundamentally offer you any way around the physics I described. What it does offer you though, is an integrated option to burn off excess power in the same manner as I described with the phase angle power controller, into a load bank (dynamic braking resistor).

The flip side to a VFD though, is that it requires a preexisting supply voltage to operate. You should think of it more as a "booster" or something to assist an existing AC source. And it is big pain in the ass to set up this way. This is absolutely not what you want as a first foray into the world of VFDs. I commission VFDs as part of my job and I do not recommend you choose an induction motor and VFD as your generator.

If I were you, I would be looking at BLDC or PMAC or AC servo motors if for some reason you are unhappy with the conventional solutions (brushed or alternator designs) typically utilized for generator heads.

 

No insult taken. Thanks for explaining!

So it sounds like I need to forget about the VFD and do research on correct solutions.

These solutions seem to be:

1. BLDC with a controller and load bank
2. PMAC with a controller and load bank
3. AC servo motor with load bank

If I have any of the above wrong please correct me. If there other options I should also be researching please feel free to let me know.

Thanks!

 

No insult taken. Thanks for explaining!

So it sounds like I need to forget about the VFD and do research on correct solutions.

These solutions seem to be:

1. BLDC with a controller and load bank
2. PMAC with a controller and load bank
3. AC servo motor with load bank

If I have any of the above wrong please correct me. If there other options I should also be researching please feel free to let me know.

Thanks!

Note that I suggested the load bank only because you ruled out what I consider to be more sensible solutions.

I don't have the specifics of your application, something about air through a tube. So my first thought would be to regulate the amount of air that gets into the generator. This would be very simple, easy to implement, and most efficient.

The load bank is going to waste a lot of energy. Energy which (I assume) comes at premium since this seems like some kind of energy scavenging arrangement.

But, if you are restricted in ways that are not obvious to me then yes, those things in that order.

 

The load bank is going to waste a lot of energy. Energy which (I assume) comes at premium since this seems like some kind of energy scavenging arrangement.

I didn't explain this very well. Let's assume for a moment that there is a gasoline engine running our generator instead of a tube full of wind.
When we first turn on the generator, the main breaker is off, so the engine uses very little gas.
We flip on one breaker, for lighting. The electrical load of the lights causes the engine speed to drop down a little bit, but the generator's governor (be it electronic or mechanical) senses that dip in speed, and immediately compensates by opening the throttle a little bit. So the extremely brief dip in speed is barely perceptible.
Now we flip on another breaker, for the oven. There's a roast in there, so now a huge load is presented to the generator, but again, the governor does its thing, opens the throttle more, and RPM stabilizes back at its original level.
5 minutes later, we turn off that breaker for the oven, and we see a reverse effect; the RPM of the engine very briefly goes over speed. But again the governor springs into action, this time closing the throttle.
Now turn the oven breaker back on, governor opens the throttle.
Now rip the governor off the engine and stomp it to pieces.
Now turn the oven breaker off again.
Now the RPM of the engine will skyrocket as there is no load to keep it at the proper RPM.
We need to either turn the oven back on, or substitute in another load which will draw as much current as the oven did, to keep the RPMs at the proper level.
So what do we do?
The way this conversation is going, it looks like we will be building an electronic phase angle fired controller connected to a load bank which will simulate the oven to keep the engine RPM at the proper level. When we turn the lights off, our controller will recognize the increase in speed, and waste even more energy to compensate for the lights being off. If we turn on the oven again, it will waste much less energy (because the oven is actually using that energy again) but always, 24/7, our generator will be generating the combined total energy of the lights and the oven, whether we are using them or not, just because we don't want to use a normal governor.

Obviously what we should be doing is building another engine governor to replace the one we destroyed. You should be regulating the amount of air through your turbine to get the desired output RPM, not regulating the load to waste enough energy to keep it at desired speed.
But, if you insist, the phase angle fired load bank is the way to do it IMO.

 

Thanks for the insight Strantor. I appreciate you walking a beginner like me through this. I follow the logic of governor and load control. The (bad) assumption I am making in this situation is that energy is plentiful.

I have been doing some (not a lot) of research on VFDs and am struggling to understand why I can't use one to control speed on an AC generator but adjusting frequency of load. Would you be able to point me to any resources that could help me better understand?

Additionally - Would you be able to give me an examples of controllers in the BLDC and PMAC scenarios. Can the same controller be used in both scenarios? Are there different types of phase angle fired load banks or alternatives?

Thanks in advance!

 

I have been doing some (not a lot) of research on VFDs and am struggling to understand why I can't use one to control speed on an AC generator but adjusting frequency of load. Would you be able to point me to any resources that could help me better understand?

Additionally - Would you be able to give me an examples of controllers in the BLDC and PMAC scenarios. Can the same controller be used in both scenarios? Are there different types of phase angle fired load banks or alternatives?

To me your thinking is that a controller, VFD or otherwise will control a generator. But those VFD's and controllers are used to control when there is already a power source. Your trying to create a power source. A generator controller controls the speed of what ever is driving a generator. Whether that is a gasoline engine or wind.

I hope that helps you to understand.

 

I do understand that, but my understanding is that there are electronic control methods that can be used along with a load source to alter (control) the rotational speed of a generator. Many seem to refer to these methods as simulated loads.

I am just trying to better understand those control methods.

 

Thanks for the insight Strantor. I appreciate you walking a beginner like me through this. I follow the logic of governor and load control. The (bad) assumption I am making in this situation is that energy is plentiful.

I have been doing some (not a lot) of research on VFDs and am struggling to understand why I can't use one to control speed on an AC generator but adjusting frequency of load. Would you be able to point me to any resources that could help me better understand?

Additionally - Would you be able to give me an examples of controllers in the BLDC and PMAC scenarios. Can the same controller be used in both scenarios? Are there different types of phase angle fired load banks or alternatives?

Thanks in advance!

Thanks for the insight Strantor. I appreciate you walking a beginner like me through this. I follow the logic of governor and load control. The (bad) assumption I am making in this situation is that energy is plentiful.

I have been doing some (not a lot) of research on VFDs and am struggling to understand why I can't use one to control speed on an AC generator but adjusting frequency of load. Would you be able to point me to any resources that could help me better understand?

Additionally - Would you be able to give me an examples of controllers in the BLDC and PMAC scenarios. Can the same controller be used in both scenarios? Are there different types of phase angle fired load banks or alternatives?

Thanks in advance!

All the motors we are talking about (induction, servo, BLDC) all work on the same principle. The rotor has a "fixed" magnetic field and the stator has a rotating magnetic field. The fixed field of the rotor is attracted to the rotating field of the stator, so it rotates. In the case of BLDC, PMAC, and servo motors, the fixed field of the rotor comes from permanent magnets affixed to the rotor. So these motors are easily used for the opposite purpose, as generators. You simply mechanically rotate the rotor, and the spinning magnets inside the coils of the stator produce electricity.

In the case of induction motors however, things are less simple. There are no magnets on the rotor. An induction motor is both a motor and a transformer. When you apply power to the stator, the rotating magnetic field induces (hence induction) a current in the rotor, and that current produces its own magnetic field, and that "fixed" magnetic field is what is then attracted to the rotating magnetic field of the stator. So one cannot simply mechanically rotate the rotor of an induction motor and get electricity output. A preexisting rotating magnetic field (supply) must be present for the induction motor to do anything electrical.

Now, an induction motor can be used as a generator if there is an existing AC supply, you simply need to spin the motor faster than the rotating magnetic field of the supply. That's why they're good solutions as boosters to aid an existing grid. You connect the motor to the grid and once it's at speed, you overspeed it with mechanical power and it starts pumping current into the grid. The specific physics behind which this is possible is a bit abstract even for one who has a good grasp of the theory but it does work.

That description was with no VFD; just a motor connected across the line. When you add a VFD in between the source and the motor it gets more complicated. Now the VFD is taking in the voltage from the grid, controlling the speed of the rotating magnetic field of the motor, a mechanical source is overspeeding it, and the generated current is absorbed by the VFD. The VFD then has to "pump up" the voltage to pump it back into the grid. Not every VFD can do this. In fact, most can't. Most VFDs are designed to take this excess power generated by the motor and dissipate it into a braking resistor as I described before. This is because 99% of VFD applications are NOT about supplying power to the grid, they are about controlling the speed of a motor. Let's say you have a motor connectrd to a machine spinning with a lot of inertia and you want to slow it with a VFD, you decrease the frequency output (speed of the rotating stator field) so that the motor starts overspeeding the magnetic field, turning into a generator, and the VFD places an electrical load on that generator, which slows it dramatically.

If you had a 4 quadrant regenerative drive then you could, instead of dissipating that recovered energy into a braking resistor, pump it back out into the grid. But there (still) must be an active AC grid to pump it into, and the range of acceptable voltages that you can use to pump it is very tight. So there is a narrow window of which you can actually regenerate back into the line. The rest will be dumped into a braking resistor.

The only way to control the speed of a generator is by controlling the mechanical input (available fuel, air, water, whatever) or by controlling the load. It is always much better to control the mechanical input. That's why engine generators have a governor, that's why hydro generators have gate controls, that's why wind turbines have variable pitch blades, etc. Without those controls the generators would spin way too fast when the source is plentiful and speed would have to be controlled using electrical dummy loads, always wasting immense amounts of power. A VFD offers no magical antidote to this physical reality. If it did, wind turbines would just have a VFD instead of the mechanical complexity of variable pitch blades, hydro plants would just install VFDs instead of the costly water gates, etc.

There is a reason things are done the way that they are, and if you aren't capable of pressing the "I believe" button, well, I get that. Neither can I. But you have a long road of learning ahead if you are going to build the knowledge necessary to understand the "why." But it sure would help if you could just press that button. Even if "just for now" and revisit it later.

Resources you should consider, well, this site. I really recommend starting at chapter 1. The level of resource you're probably asking about it more around chapter 337. This site has an ebook that is an excellent place to start learning electrical theory. Also the Navy NEETS modules which are available online.