How Do YOU Measure Power on Large EDF Systems - RC Groups
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Oct 31, 2011, 06:56 PM
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A10pilot's Avatar

How Do YOU Measure Power on Large EDF Systems


I have been around glow ducted fans for quite some time and I am familiar with 3KW electrics (pattern planes) but I am now venturing into the 4+ KW EDf systems and I have some questions about power measurement.

I have an Astro Flight watt meter that can measure up to 50 volts, 70 amps and about 5KW. I am moving from the 70 mm EDf's to the 90- 127 mm EDf systems. I tend to be a numbers guy and I am interested in testing some of the 90-127mm fans/power systems and I am willing to buy some equipment to do the testing. I can measure volts (have a fluke digital voltmeter) but I am curious as to how some of you measure current (amps) in excess of say 70 amps, voltages above 50 and power above 3KW.

1. Are there some devices out there that some of you use to measure volts other than a dvm.
2. How do you measure amps (say in excess of 70 amps)? Do you use a clamp on meter?
3. Do you simply calculate the power using volts and amps or is there a meter out there that you use?
4. Are there any watt meters out there that you might recommend for current above about 125 amps, packs above 12s?
5. How do you measure efflux (exhaust velocity) and where do you tend to measure it (between the stators or at the back of the tail pipe)?

I do not mean to sound harsh but I am really interested in hearing from those that have actually performed these measurements (like Knife Liddle, Corsair Nut and many others).

Thanks in advance and I look forward to learing some new stuff about the larger EDF power systems.

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Oct 31, 2011, 07:48 PM
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Gaz-b's Avatar
Have a look at eagletree systems nice loggers with loads of sensors available i test edf on a set of kitchen scales with a pito tube about 3" behind the eflux tube I measure thrust.air speed.rpm.volts mainly to see if the cells are holding or will need higher c rating.amps.watts.motor and esc temp.interesting playing with eflux diameter to see air speed and thrust ratio.

Oct 31, 2011, 09:12 PM
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I have a clamp on type amp meter for measuring amps, it has a set of jaws that just close around one wire and measure through inductance I think. For volts I just hang a lipo checker off the balance tab on one pack. Watch the cell volts, multiply times cell count, multiply times amps. Beyond some point the watt meters aren't good for ESC health and are questionable as far as accuracy goes...
Oct 31, 2011, 09:28 PM
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Steve C's Avatar
Chris in right on about accuracy. When I started causing amp draws in excess of 80, I did a test to check rpm with Astro watt meter inline and then without. I saw quite a bit more rpm with it out, so I bought a clamp meter and have been using that ever since.

I'd like to know which amp meter guys are using. I bought the most expensive one I could find (it's Craftsman) and I have to give it a few good whacks to get it to zero out.

Steve C
Oct 31, 2011, 09:58 PM
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Arngeir Blakseth's Avatar
I have a v4 ET eLogger (it's good for 70V and 150A) and also a 400A clamp on meter. For my really large setups I have 2 parallell batteries so the ET logger is hooked onto one of the batteries, total amp draw is then simply the double of this, also easy to verify with the clamp on meter. The ET logger also gives me Volts, rpm, temps and efflux speed readings, the limit is really how many extra sensors you purchase.
Oct 31, 2011, 10:38 PM
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A10pilot's Avatar

thanks very much for the replies. These are the kinds of things I need to know.

Oct 31, 2011, 10:46 PM
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The one I use is by Graupner Steve, bought it off JC (aka Bidule) prbably 6 or so years ago second hand. Or maybe it was someone else <g>. I met him at a little F5B electric glider event to take an Cougar off his hands. The amp meter might have come from a different competitor. Been happy with it, it's got a little dial for zeroing - very sensitive adjustment.
Oct 31, 2011, 11:17 PM
Bajora's Avatar
Yep, the 150 amp Eagle Tree eLogger is the method I use too. Castle 6mm bullets for connectors on it.
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Oct 31, 2011, 11:44 PM
The ONLY fully accurate method is using an inductive clamp-on type meter. Shunts, like what are used in the eLoggers have a high resistance and will not give you the exact same amps when the device is removed (it will be higher).
Nov 01, 2011, 12:52 AM
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erh7771's Avatar
Thrust stand and amp meter minimum, watts mean very little without other figures to support it.

You can bust 3000 watts and get 6pds of push on bench and be wasting a lot of energy
Nov 01, 2011, 04:22 AM
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jjantti's Avatar
Shunts, like what are used in the eLoggers have a high resistance...
Adding something between ESC and battery adds resistance for sure. But there is no shunt in Eagletree (Hall-effect) and added resistance is minimal. It is somewhat the cables only. I use cables as short as possible on mine to keep that minimal.

I use that information (U-I-A-rpm-temp)to check I'm within the limits. This is interresting specially in flight. The key to me is efficiency of whole system.
Not the numbers but what it actually does IMHO.

Nov 01, 2011, 05:34 AM
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According to Eagle Tree, the E-logger don't use a shunt but a Hall sensor.
Nov 01, 2011, 09:25 AM
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JurassicJet's Avatar
Originally Posted by Gaz-b
Have a look at eagletree systems nice loggers with loads of sensors available i test edf on a set of kitchen scales with a pito tube about 3" behind the eflux tube
No disrespect but that setup looks a little scary...


If you check out EffluxRC they have a thrust testing sled that is pretty cool.
I have thought about building a setup. A person could do it cheap by using two ball bearing drawer slides (higher quality, not the cheap ones) mounted flat and horizontal with a sled attached to the slides and the slides attached to a base. The fan would be mounted to the sled. Then something as simple as a fish scale attached to the sled would give you the thrust read out. I could probably build that system for less than $30 (plus the fish scale). Just a thought....
Nov 01, 2011, 10:42 AM
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DanSavage's Avatar
To measure efflux velocity I use an 250-mph ASI I bought on eBay for $35.

Nov 01, 2011, 10:45 AM
Originally Posted by jjantti
Adding something between ESC and battery adds resistance for sure. But there is no shunt in Eagletree (Hall-effect) and added resistance is minimal.
Minimal and accurate are two different things. I designed a telemetry sensor using the same shunt that is used in the Eagletree logger. Grab the datasheet yourself and look at the stated resistance of the part. It's pretty good, but not 100% accurate. The Allegro current sensor is a metal shunt with a hall effect sensor on top of it, packaged together.

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