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Dummy load for testing turbine
By thefinis, Section Controls Posted on Fri Jan 20, 2006 at 02:50:58 PM MST
Looking for suggestions for dummy loading of turbine before attaching genny

Does anyone have suggeations for dummy loading of a turbine before it has a genny attached? I tried doing searches but kept coming up with how to load a machine after it is connected to the generator. I know about the units they use to test pto power for tractors. Dynatorque units I think is either the name or brand that I have seen but it is a little pricey to test even a tractor that is in the shop much less bring a unit out and mate it to a wind turbine. It would be nice to have something like it that would tell you the torque/power available and let you dial it up and down till you found the max power at a set wind speed.
Finis

Dummy load for testing turbine | 8 comments (8 topical)

Re: Dummy load for testing turbine (3.00 / 0) (#1)
by Laylow on Fri Jan 20, 2006 at 11:48:40 AM MST

I'm not exactly sure what you are trying to do but you can make a simple dynamometer with some thin rope and two scales.

Sorry, I don't have time to explain it all right now but maybe someone else can fill you in or you can find a plan on the web.

You just wrap a piece of rope around the shaft or sheave and hang two strain gauges off of them. You can add varying weights to one of the scales to vary the force of friction. Calculate running torque by subtracting the readings on the scales.

Sorry, got to go.

Re: Dummy load for testing turbine (3.00 / 0) (#2)
by willib on Fri Jan 20, 2006 at 02:52:22 PM MST

It would be nice if your load was varible/adjustable, so you could test your blades to the max , then take it down and retest it with a known load/weight..
as your load,i am thinking of some spinning fins in a box with oil for a viscosity load..but that would be hard to adjust.

Carpe Ventum (seize the wind)

Re: Dummy load for testing turbine (3.00 / 0) (#3)
by Ungrounded Lightning Rod on Fri Jan 20, 2006 at 07:04:11 PM MST

Basic idea is to attach an adjustable brake to the shaft, with the brake allowed to rotate with the shaft. You keep it from spinning by mounting a lever on it and tying the lever to your testbed with a spring scale - mounted so it works at right angles to the line between its attachment to the lever and the nearest point on the centerline of the shaft. (You need the length of that line for your computations.) You also need a tachometer.

The product of the spring scale reading and the length of the lever arm line gives you torque. Adjust the brake for the torque you want. The product of torque, RPM, and a constant gives you horsepower. (This is what is meant by "brake horsepower": the mechanical power you can pull from a shaft and measure with such a brake-and-tachometer setup.) The same product with a different constant gives you watts of mechanical power.

Knew a guy in high school that made such a setup for testing a hotrod engine undergoing tweaking. He made his brake assembly by cutting the rear wheel assemblies off a rear axle, welding them back-to-back, mounting the shaft to the flywheel of the engine at one end and a bearing at the other, and hooking the brake cylinders to a master cylinder on the test stand using a flexible brake line hose and other ordinary automotive brake plumbing.

You could do something similar for a prop using a motorcycle's mechanical brake mechanism.

If you want it up a tower running in real wind: Drop a rope to the ground and pull on it to apply the brake. Let the brake assembly wind another rope around a cylindrical housing, turn it with a pully at the yaw bearing, and drop THAT to the ground, too. Fasten your spring scale to a ground anchor and tie the brake rope to it with a swivel. Then you can both adjust your brake and read your torque at ground level.

If you go to the trouble to build such a brake assembly you might leave it in place and use it to stop the prop on the completed mill. B-)

It's easier to do such testing with a genny in place, though. (Especially an axial-flux genny with no iron in the stator, and thus negligible eddy-current losses.) Brake torque is proportional to (average) current (the sum of winding and eddy currents, which is why axial flux or other no-core alternators are good for this hack.) RPM is proportional to frequency. Winding current and RPM are easily measured with electrical test equipment, while brake torque is easily adjusted by varying the load.

Re: Dummy load for testing turbine (3.00 / 0) (#4)
by thefinis on Sat Jan 21, 2006 at 02:03:43 AM MST

Great this will work really well as I had planned on using a truck hub with disk brake on it as a bearing set and emergency brake. I am trying to build a vawt and with all the discussion about it I am concerned about how close my calculations are on available power at various wind speeds/rpms. I would just like to know a good estimate so as to size the real loading to where it will not overpower the genny and overspeed or stall the turbine when kick in speed is reached.

I also needed to test run the turbine for vibration and didn't think that just turning it loose without a load was a good idea.

Thanks to all for the great ideas
Finis
Texas born and bred
[ Parent ]

Re: Dummy load for testing turbine (3.00 / 0) (#5)
by finnsawyer on Sat Jan 21, 2006 at 09:22:41 AM MST

You could use an automotive type alternator in place of the PM alternator. To do this you would have to gear it up. You disable the regulator, keep the rectifiers, and bring out the rotor wires. I,ve done this on a delco alternator. It was not hard. You then use a calibrated rheostat or a number of known resistors for the load resistance and another to control the current to the rotor. You obtain the rotor current from a twelve volt battery. For a given wind speed or RPM you then adjust the rotor current to find the maximum load current and power. You need to adjust the load resistance for each test so that the load power passes through the maximum. One advantage of this set-up is that with the rotor current zero there will be no cogging, since an automotive alternator has no permanent magnets. This allows low voltage tests. A typical 60 amp alternator should be good to about 40 volts or 2400 watts. A draw back is the loss in the gearing to step up the RPMs to the alternator plus mechanical losses in the alternator. Still, it would give some useful data.
GeoM

Re: Dummy load for testing turbine (3.00 / 0) (#6)
by willib on Sat Jan 21, 2006 at 09:54:15 AM MST

thats a good idea , but you could go one step further .
since this setup is for testing wind turbine blade output .
i propose shorting the delco alt. output wires together , and applying a varible current to the field windings .
the blade output will be proportional to the applied current..
no current , no drag ..
increase the field current to increase the drag on the blades , and this should give you an indication of the blade output.

Carpe Ventum (seize the wind)
[ Parent ]

Re: Dummy load for testing turbine (3.00 / 0) (#7)
by finnsawyer on Sun Jan 22, 2006 at 09:01:08 AM MST

I suppose you could do this, but you need to know what power is being produced and dissipated by the alternator for each value of field or rotor current and each value of RPM. That is, you need to calibrate the alternator before hand and then you need to measure RPM and rotor current. You can't get the power with only one measurement. You could measure the short circuit output current instead of RPMs. The reason this is so, is that for a given wind speed the power output of the turbine depends on its RPM, and it has a maximum value for a certain RPM. It is this maximum and the associated RPM that you are trying to find. Once you have a number of these maxima you can plot the maximum output power and RPMs of the turbine versus wind speed.

You would also expect higher average diode currents in this case, which could cause diode failure.

I should point out that in the scheme that I proposed there will be gearing losses, alternator losses, and diode losses as well. Hopefully they can be ignored. If not then one can do the calibrating procedure as in your case. That is, obtain a variable speed dc motor of known efficiency and proper size. Couple that to the auto alternator with the gearing and then duplicate the measured results. That is; the measured output power versus RPM with the various values of rotor current. You measure the power into the dc motor, apply the efficiency factor and then get the total power out from the turbine for that data point.

GeoM
[ Parent ]

Re: Dummy load for testing turbine (3.00 / 0) (#8)
by willib on Sun Jan 22, 2006 at 03:49:33 PM MST

nice idea , how about this , mount the blades directly to a permenant magnet motor.
Then all you will need is a varible resistor and a current meter..

Carpe Ventum (seize the wind)
[ Parent ]

Dummy load for testing turbine | 8 comments (8 topical)

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