Backwards Dynamometer
Backwards Dynamometer
(OP)
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
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





RE: Backwards Dynamometer
How about a big rheostat and a wattmeter?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Backwards Dynamometer
I have Vintage Abstract Widget Trammel that desperately needs a new, tested rotor.
Hook your motor windings in as parallel as you can. Connect a 12 volt automotive battery and measure the current draw so as to calculate the effective resistance. Use that information to design a single phase Variac and bridge rectifier circuit.
Use that to dynamically brake (break?) the motor.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Backwards Dynamometer
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RE: Backwards Dynamometer
Cheers
Greg Locock
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RE: Backwards Dynamometer
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Backwards Dynamometer
My apologies for the Acronym; Vertical Axis Wind Turbine
Lol. IDK what came over me.
Waross, If I reconfigure the 3 phase stator so that the windings are in parallel, won't that prevent me from having a kinetic load that's variable from 0-100%? To me, that would seem to be setting up a magnetic tug o' war between windings.
I've never had a reason to mess with dynamic breaks, so pardon my ignorance here. If I have an old VFD with dynamic breaking as an option, then I will connect the drive to utility, install the appropriate brake resistor on that drive, and I can use a squirrel cage motor as a dynamic load? Better yet, find a brake chopper and a resistor connected to two phases of a three phase squirrel cage motor?
Am I on the right track in saying this?
RE: Backwards Dynamometer
RE: Backwards Dynamometer
If this is say a 3-phase output and you're going to connect to a rectifier and a battery the best load is a rectifier and battery with a dump load controller over the battery to maintain battery voltage. The final customer is really only interested in the output vs rpm (where rpm needs to be related to wind speed by the blade manufacturer) in the as installed configuration.
A permanent magnet generator output voltage is fairly linear with rpm. So, it won't really do anything useful until the output voltage (after the rectifier) is higher than the battery voltage. Once that voltage is reached, the output power goes up quickly, since the rectifier output voltage is then being held constant meaning the further increase in generator output voltage is basically dropped across the stator windings making the current rise quickly.
RE: Backwards Dynamometer
I think that Scotty and Greg are on the right track. Load your PM motor with cheap stove elements.
How are you going to be testing? Do you need dynamic control or can you use a fixed load at each load level?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Backwards Dynamometer
http://i35.tinypic.com/no8pol.jpg
I would like to us this as my dynamic load for the rotors under test in the dyno setup. The more I talk about this, the more I feel like I've got it licked; 3 phase rectify this PM motor, and just dump it across an appropriate load resistance / rheostat. I've seen a few rheostats that have a 60 second drive motor attached, that will slew it through the range. This fixed time base would be advantageous for correlating my data polled from the other sensors, as I will be polling data at 1/2 to 1 second intervals.
RE: Backwards Dynamometer
Keith Cress
kcress - http://www.flaminsystems.com
RE: Backwards Dynamometer
The first load has one heater.
The second has two
the third had 4
the fourth has 5
either parallel or series. to suit the load level
With these ratios you can select 2^n levels of load where n is the number of combination's by switching them in binary.
good luck
RE: Backwards Dynamometer
If you can afford a current amplifier, you can simply dial up the torque load independent of the motor speed.
RE: Backwards Dynamometer
The OP mentions a three phase induction motor and everyone starts talking about DC. Why?
Toygasm then mentions the correct way of doing it (putting an inverter before the motor and adding a chopped braking resistor to the DC link).
That is, in my view, end of discussion. Set the torque limit (the "braking power") either by using an analogue input or from the panel. Use a set of incandescent lamps for the braking resistor and let the chopper do the switching of the resistors. That's what it is there for.
You will then have a brake with a settable braking torque, which will remain constant across your speed range. Your speed setpoint should, of course, be zero RPM.
Lamps are not impractical. They are very practical. They are built to dissipate energy as heat and light radiation and they are cheap. Just make sure that you install enough watts and that your DC link voltage and the lamps' rated voltage do not differ too much. Use series connected lamps if necessary. Undervoltage does not hurt the lamps.
Yes Toygasm, you are on the right track.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Backwards Dynamometer
Or am I confused?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Backwards Dynamometer
You may be confused, Smoked, but it doesn't show
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Backwards Dynamometer
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Backwards Dynamometer
RE: Backwards Dynamometer
Some CAN get quite unstable if you run them in vector mode. Scalar is usually possible and torque or current mode is (in my experience) always possible.
Textile drives are usually PM motors and there are hundreds of them running off a common VFD. No encoders, no nothing.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Backwards Dynamometer
RE: Backwards Dynamometer
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.
RE: Backwards Dynamometer
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
RE: Backwards Dynamometer
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.
RE: Backwards Dynamometer
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Backwards Dynamometer
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
RE: Backwards Dynamometer
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
RE: Backwards Dynamometer
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.
RE: Backwards Dynamometer
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