Measuring motor output

[Ed Kershaw]

Hi Marty,

Thank you for the generous comments.  Quite unfounded, though.  Like most of us, I just like playing with this gear.

I think what you're suggesting is that ECSs have different efficiencies.  Well, I've never tested for that - but I will - it's simple enough to do.  I've got some big and small Jeti and Scorpion ESCs (90A, 77A, 55A and 40A).

 Regarding your experience with the Castle and the Tower ESCs, I guess one possibility is that the timings were different.  Hard timing can really push the current up compared to soft.  Actually, that's another problem - I don't think I can match the Axi timings with the Scorpions because Axis are just hard and soft, whereas the Scorpions time to a figure (5 degrees;15 degrees etc).  But I'll have a go in the next few days and let you know what happens.

Ed

[Martin Roberts]

Hi Ed,

the settings Pete used because he wanted SPEEEED, were hard on both the TowerPro and the Castle which I believe are both programmable by degrees,  anyway the only thing I can think of that would cause the very different performances (and I could be way off line) would be the various manufacturers using different transistors, voltage regulators, capacitors and ..... maybe because manufacturer A wants longevity of their ESC, but manufacturer B wants performance and lots of variety of program/Cell numbers etc.... maybe it was just because the bigger unit was designed to supply a higher current, so when there is a lower draw you might get higher losses similar to voltage drop principle (see below).

I keep dreaming up little circuits and things, and have a basic understanding of IRV etc... but for example couldn't figure out why my TX kept going low, then I realised that the DX6 has a 9v reg, but uses 3v and just dissipates a lot of heat (sort of like havin a built in hand warmer) not enough to keep hands warm when your on the edge of a cliff with a 40 blowing hahaha. So I thought make a 3v reg, but instead I sourced the 3.3v reg from the states......yeah no dead Tx...

Unfortunately no one releases their circuits with a BOQ, so everything gets reverse engineered, and I imagine that give me a week or two off work and I could design my own esc with very different performance, and then get PRC to mass produce them down to a price, and we all know that a cheaper part will then get swapped for the genuine.......

For the unknowing:- Voltage drop occurs where you have to supply a relatively low voltage over a given distance. The Nation Electricity Grid for example, would not be able to supply 220volts at source because it would be far less than 220v at the recipient house, not to mention that the current draw would melt the wires that carry the electricity. Therefore they supply at say 22,000 volts and a very low amperage for a particular wattage, and uses reduction transformers just prior to the receiver, the reduction transformer reduces the voltage which then allows a higher amperage, for the same watts.

I imagine something along these lines may be the difference between the various brands, so I look forward to your results.

Marty

[Dave Fasken]

Very interested in your torque reaction method of measuring motor horsepower. I plan on making one similar to what you have and will use the Medusa power analyzer pro and Medusa scale hooked up to my laptop. I will add the appropriate calculations to give horsepower and efficiency to be displayed on the screen. So far I have mounted a 2.5" by 2.5" by 0.5" aluminum plate to an old crankhousing and will make up an adapter plate that will accept all sizes from 22mm up to 50mm.I will make sure that the reaction arm is on the centerline of the crankshaft so as to minimize any angular errors.

Before I go any further do you know of any small electric brake that could be added to this unit rather than using the prop to develop load. It would be really neat and also be considerable safer than having a large prop spinning close to you especially when you get up to the larger 40mm size motors.

Any input/ideas would be greatly appreciated.

Dave Fasken  

[Fethy Sarthe]

Nice mock-up! I was surprised that you didn't try to measure the static thrust. I noticed that you talked about a dynamometer. What do you think about mounting the motor on a CD-Rom laser's track, with a spring and an index? For sure, we miss the torque measurement!

What's your comments and suggestions for static thrust?

Rgds, Féthy. 

[Dave Fasken]

Hi Fethy;

Just to let you know I have made some progress and got the system running  last night with what seems to be reasonable numbers. I still have not come up with a brake yet but plan to make up a small pony brake where I can apply load for a short while and take readings at various loads. The big concern right now is to come up with some kind of universal coupling so that I can hook up different size motors to the brake without any vibration at high RPM's. I may have to make different size brakes as the small 22mm motors may well have difficulty running a large flywheel or at least would require assistance on startup to avoid ESC commutation problems.

I already have two different thrust scales that I use as well. One is a conventional L arm using my Medusa scale and works very well for small motors but I wouldnt trust it for larger 30 and 40mm motors. I made up a sliding type scale which uses a plate which slides on bearings bewteen two steel rods.  Seems to work OK with 40 mm motors but due to the close proximity of the propellor to the frame there may well be some interference due to windage.

You are probably wondering why I want to make a dynamometer? Well I don't really have a good answer other than I like the challenge and want to be able to say that I made one. I will definitely make some comparisons with different motors but don't  plan to make a career of it. You can tell I am retired and looking for something to do and keep my mind active. I will gladly send you some pictures when I get the unit closer to completion.

 Dave