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Backwards Dynamometer

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

RE: Backwards Dynamometer

VAWT = Virtual Alligator Weight Training?

How about a big rheostat and a wattmeter?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Backwards Dynamometer

I'm glad to hear that parts are again available.
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

There was a good thread a while ago on using a regenerative drive as a brake on a dyno. You might not get a full regernative drive at such a low power rating but a drive with a brake resistor would be practical at 1kW. That said if a 1kW heater is pushing the budget then maybe you need to consider how badly you want to do this. I can pick up a 1kW fan heater for under £10.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Backwards Dynamometer

We use electric stove elememnts, or radiant fire elements, as absorbers. They have the advantage that they are cheap as chips and absolutely bullet proof.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Backwards Dynamometer

Sorry, I didn't notice that you had a PM motor. If you can find an induction motor you can load it with DC in the windings.

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

RE: Backwards Dynamometer

(OP)
I can't believe you guys have never heard of a Variable Axis Wallaby Tilter.

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

(OP)
..in addition, I guess the only thing about the DB setup is that I will be using it at 100% duty cycle.  I should probably go ahead and dig out my safety glasses.   

RE: Backwards Dynamometer

Care to share the output of this generator? Voltage, configuration (DC, 1-phase or 3-phase) and expected current would be nice.

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

As I said, I missed the PM designation on your motor.
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

(OP)
When we connected my PM motor as a basic generator, this is what we got for performance data:

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

Get some water heater elements and stick em in water bath and switch em with relays.
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 the budget will allow you to purchase a PM DC brush type motor, 1 HP pretty inexpensive), you can use a variac and a full wave bridge to get DC current into the motor armature  This makes a pretty good electrical brake.  Use an amp meter to measure the current and you'll get a pretty good reading of the torque.

If you can afford a current amplifier, you can simply dial up the torque load independent of the motor speed.

RE: Backwards Dynamometer

I am very confused - or is the rest of the world?

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

RE: Backwards Dynamometer

I thought "Permanent Magnet 3 phase AC motor" s are not used with standard OTS VFDs.

Or am I  confused?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Backwards Dynamometer

They can very well be used. I have run many on standard VFD's. Especially well suited in a constant torque application like this, where speed is determined by the input shaft.

You may be confused, Smoked, but it doesn't show  smile

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Backwards Dynamometer

Yes, Keith. I also missed the PM part. But, luckily, PM motors can be used here. It depends somewhat on what VFD the OP has available.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Backwards Dynamometer

And is it not also true that the PM AC motor must have a shaft position encoder that is phased properly and compatible with the VFD position input?

RE: Backwards Dynamometer

No, not at all. I have run up to 11 MW synchronous motors without any encoder or rotor position indicating device.

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

RE: Backwards Dynamometer

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

RE: Backwards Dynamometer

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.

 

RE: Backwards Dynamometer

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

RE: Backwards Dynamometer

(OP)
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.    

RE: Backwards Dynamometer

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

RE: Backwards Dynamometer

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

RE: Backwards Dynamometer

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

RE: Backwards Dynamometer

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.

 

RE: Backwards Dynamometer

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

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