Inverse of a VFD

[doehl1]

I have an application in which an induction motor (island mode w/ capacitors for reactive power) supplies a variable rotational speed/frequency input and I need a constant speed/frequency output. In other words, I have an input, single phase voltage signal which will vary from 0-60Hz, and I need a constant 60Hz output.

It's almost like a VFD operating in reverse. Is there such a frequency converter device on the market for this? I know the aircraft industry used VSCF (variable speed constant frequency) devices, but I'm looking for a low cost piece of power electronics that could accomplish this.

Expected power output range is 1000 to 5000W.

David

http://www.mavenpower.com

 

[davidbeach]

I don't know about low cost, depends on your definition, but what you're looking for is a four quadrant VFD.

 

[Sparweb]

If I understand what you need correctly, you could rectify your "wild" AC supply into DC (smoothing the ripple as necessary), then invert the DC into 60HZ AC at whatever voltage you require.
It sounds a lot like a wind turbine to me.
There are grid-tie inverters on the market that do the whole thing for you (Power One, Sunny Boy, etc.) in your power range.
Grid-tie inverters tend to shut down when the grid fails. There are work-arounds for that.

STF

 

[doehl1]

Spar, yes this is similar to a wind power application (actually tidal power), but on a small scale, and running in island mode (no grid tie). I am looking for a converter of this "wild" AC power to a constant frequency supply. What would you suggest for the lack of utility reactive power? We've done some research into induction motors with capacitors. Right now just need enough reactive power to run the motor - load is purely resistive. I'll investigate your vendors, we actually are set up with Power One.

David

http://www.mavenpower.com

 

[itsmoked]

Because of the varying nature of tidal power I suggest you generate the power as low voltage DC into a battery bank then use an inexpensive off-the-shelf inverter to create your AC. It's really the only way to work a variable energy supply in island mode. Anything else will drive you crazy because your AC will otherwise come and go making almost all AC loads unhappy with their power.

With a battery bank the varying energy can be 'accumulated' and the load can be trimmed, over a bit of time, to not exceed the long term average you get from your generation. If things are managed correctly the AC can be available continuously over even the slack,(nautical term), periods.

Sounds like a LOT of fun. May I ask geographically where you're doing this?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Keith's right, you need an accumulator of some sort here, otherwise the output voltage would have to be relegated to the lowest possible level and/or just turned off when it drops below useful, making it essentially unreliable as a resource.

Keith, from his website:

CONTACT INFO

134 Vintage Park Blvd., Ste A-101
Houston, Texas 77070

"Will work for (the memory of) salami"

 

[itsmoked]

Ah, thanks for pointing that out Jeff. Bit of an inconvenient drive that!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Yeah, not even a pretty one at that...

"Will work for (the memory of) salami"

 

[rhatcher]

What would you suggest for the lack of utility reactive power? We've done some research into induction motors with capacitors. Right now just need enough reactive power to run the motor...

It would seem that the problem is a lack of reactive power to excite the AC induction generator. I suggest that a permanent magnet synchronous generator may be a good solution.

 

[Sparweb]

David,
I agree, cool project.

Can you give some definition of the speed range that you want to use?
My approach to wind turbines has been to use a "synchronous generator", but basically it's been just a hobby for me, so my hacks aren't necessarily what you're looking for.
Still, using a 3-phase permanent magnet generator will allow you to operate the prime-mover at a wide speed range and simplify the power conversion by just rectifying to DC.
In a system like this, starting from a dead stop the machine speeds up to a "cut-in speed", below which current doesn't flow. Up to that point the generator has not produced enough voltage to equal the voltage blocked at the rectifiers. Once that threshold is met, then the peaks of the generator's AC is "clamped" to the DC system voltage as the machine speeds up. The current flows in proportion to the voltage drop and resistance/reactance in the generator windings. As my system stands, it cuts in at about 250 RPM and power generated increases steadily up to about 600 RPM. The speed would continue to increase if wind speed increased, but above a certain limit the wind speed could be damaging so the machine takes measures to prevent higher speed and protect itself (called furling).
I think I've posted enough of the guts of my system -here:

http://www.sparweb.ca/6_Power/Power.html

(Hope this doesn't look hokey - my WT is just a hobby project but I thought it might be relevant. There are plenty of more subtle and intelligent schemes (ie using SCR's) but I don't use them.)

Your tidal generator may operate at a roughly constant speed - I can only guess. I suggest the synchronous generator because it can allow operation at a speed range roughly 1:3 which may be flexible enough to deal with the variant speeds of your system. The more you can tell about that, the more helpful my answers can be.

My inverter can be grid-tied though I don't need it to. It's a 20th century beast with massive transformers but it may have the features you need; battery charging, inverting, generator support and grid-tie. It's flexible enough that if there is no grid, meaning it's permanently "islanded" - as long as there is NO chance it will EVER see a grid - I must emphasize - then I am told that it can be configured to BE the "grid" for its own particular island. Then other similar inverters can share the AC line on their grid input. I'm sure it gets complex very quickly, and out of my depth. Others have accomplished this - but not me. If you want to pursue this, I know exactly who to ask. Alternatively, it can accept the AC frequency input of another inverter/generator based power supply, as long as that supply is stable enough. The inverter will disconnect from any grid AC frequency or voltage input that falls outside certain parameters.

Mine is too small for your 5kW power range. Its big brother, the XW6048, may have the muscle to do what you need.



STF

 

[LiteYear]

Even though it looks like the motor is connected to the wrong end, a standard VFD will do. Your induction motor and capacitor combination is called a self-excited induction generator (SEIG). You feed that directly into the rectifier front end of a VFD and have it invert to give you the 60Hz output.

Unfortunately with this arrangement you'll have to raise your lower operating speed requirement for two reasons: firstly, self-excitation requires a certain speed to begin, and secondly, the VFD will have a minimum DC bus it can invert from and your induction motor's voltage will increase as its speed increases.

You can tune your self-excitation capacitors to give you the best excitation speed range, and there are even more complicated variable capacitor schemes that extend that range. However, the voltage produced by a SEIG is notoriously sensitive to speed, so that will probably be your limiting factor.

FWIW, I conducted a project on a 5kW three phase SEIG and found that with fixed capacitance my usable speed range was about 40-60Hz, developing about 300-700V on the DC bus. Your single phase system could well perform quite differently.

If you want power delivery over more than a 66-133% speed range (eg. more than 30-60Hz), a SEIG with fixed capacitance starts to look highly unsuitable.