Backwards Dynamometer

[toygasm4u]

Hey guys,
My counterparts and I are digging in to what it would take to test rotor assemblies for VAWTs, with the target end results being measured in kW/RPM. I'm assuming that what we'll see is an increase in RPM and Torque, up to a certain point where RPM will start to level off due to loading if I can get this working. We already have an inline torque transducer (brushed style) and a RPM/Rate Meter from FUTEK and OMEGA that are left overs from other projects. I'm kind of stuck in how I want to load this thing.

On the shelf in the shop, I have a PM 3 phase AC motor, with a theoretical 1HP operating ceiling(630 in.lbs @ 100RPM). I would like to use this motor as my variable torque load. I've read on a thread or two here about using different sizes of light bulb, but this seems extremely impractical. I've also looked into tandem rheostats, and load resistors with choppers.

I'm wondering if I can't just get a fixed 3 phase 1kW resistive load, and then use a dimmer circuit like for lighting loads. Making matters worse, we're a little tight on budget, which rules out any Hysteresis Brakes I've read about. (They look a little pricey). Any advice is much appreciated,

~Mark

 

[toygasm4u]

I have a mountain of AB Powerflex 4 and 40 series VFDs here... I appreciate everyone's feedback.

 

[LionelHutz]

Yes, I think we're confused.

Ok, Are you testing the blades to determine the best design for the alternator?? If the alternator already exists then what I posted was correct - it will be much easier to test with the expected real-world load.

Otherwise, a VFD should be able to load the motor just fine. I personally would purchase one that is oversized (which likely means 2hp) because sometimes the brake chopper likes to fail when worked continuously at rated load. Also, sometimes the brake chopper can't do rated braking and even then check that it actually has a brake chopper and can connect a braking resistor. The small hp units are usually really cheap for a reason.

From what I remember, I believe you are right. The blade available power will go up as they accelerate - to a certain point. You will likely not be able to get them to start if you have a load when stopped.

Your test would need to start with a fixed known wind speed. Then, let the rotor start spinning, apply the load and move the speed up and down recording the power as the speed changes. Then, you will find the best rpm for the wind speed you are testing at. Do it over again at another wind speed to determine best rpm there. Eventually, you'll have wind speed vs rpm and wind speed vs power curves.

 

[waross]

A question Skogs;
When using lamps for resistors, do you run them up to rated voltage or keep the voltage low? I am wondering about the effect of the 10:1 ratio of hot resistance to cold resistance in an incandescent lamp.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[toygasm4u]

LionelHutz, you are correct. A properly sized alternator does not yet exist. The goal is to maximize power capture of a blade set, with the limiting factor being wind speed and rotor diameter. Only after we settle on an optimized blade design will we attempt to design an alternator for the application.

It actually "blows" me away (intentional pun) that most of the information found in regards to wind power, illustrate that these people are building their alternators first, then trying to design blades for them. Why would I build a 1kW alternator if the required blade design requires too much diameter or RPM to keep it practical? It seems to me that the scientific methods used in the development of DIY non-commercial or small scale turbines is bass ackwards; like building a small block V8 then shopping for a car to put it in. I'm sure that budget limitations play into this method heavily. Not everyone can afford dyno's or fluid dynamics modeling software.

I've been in contact with magtrol, looking at hysteresis brakes and powder brakes. We may just skip past the redneck phase of the project and go right for the throat.

 

[Skogsgurra]

Bill.

If you design for rated voltage, things will adjust all by themselves. A constant resistor is just as good, but lamps are easier to get. And also cheaper and do not need cooling fans. (One might need sun-glasses, though). I use lamps for loads a lot. Have two boxes with twelve 300 W lamps in each of them. That is 7.2 kilowatts that I can adjust in 300 W steps. Even lower if I series connect. Invaluable for lots of measurements.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Yeah, the lamps allow easy, cheap value changes and provide some instant (idiot light) feedback.

With a test, power percentage is not an issue, but as some sort of permanent load, if you drop the voltage about 10% they will last indefinitely.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Thanks Gunnar and Keith. Re the 10% voltage drop and lamp life.
We did that once in a department store. The cost of relamping individual highlight lamps was extreme. (Difficult access) WE ran the entire panel off a three phase autotransformer bank at about 90% voltage. Lamp burnouts were extremely rare.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I think the starting point is to find the average wind speed at the install site. Then, decide on the average power output you want. Come up with a blade design that will produce that power when running in the average wind speed. Finally, try to design an alternator that will produce the power at the rpm the blades will rotate. Then, go back to the blade design and try another tip speed ratio or diameter because the alternator is not practical. At least that's the steps I was taking when I was playing with the numbers.

A lot of the manufacturers of small machines seem to be like making a rotor that will produce the maximum power at around 30mph but they end up with a machine that is almost useless in an area with an average of say 10mph winds. A bigger diameter rotor would make the machine much more useful. My test machine had a 12' diameter rotor for a machine capable of practically producing maybe 500W-600W.

 

[2dye4]

toy
Your absolutely right that it is silly to fit the turbine to the alternator. I suspect what they are doing is picking an available alternator and then trying to make the mechanics work.
Unfortunately this is totally backwards as you say.
With a wind turbine the hard part is get the system to produce energy via the aerodynamic design.
the alternator can be a last step decision relatively speaking.
The design of a wind turbine is the most deceptive design task you can imagine. Anything will spin in the wind and this inspires people to go for it and worry about the details later. Unfortunately the aerodynamic design is crucial and it needs much experience to do successfully.
Any way good suggestions have been made and I wish you the best of luck

2d