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How to build a brushless motor test bench?
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gumleguf
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Question How to build a brushless motor test bench? - 01.21.2013, 09:28 AM

I discussed with some friends the possibility of building a test bench/test station for brushless motors. It seems to be an ongoing discussion on all forums if the more expensive motors are really better performing or not according to specifications. (Almost) no doubt they are better build quality, but what about performance?

It would be really fun to keep an updated list of the motors we come across and how they measure.

I am not too techie and can't figure out what would be required to build a "scientifically correct", fixed test station/test bench for brushless motors. Has anyone else done it and can you link to descriptions or pictures? Maybe you can even walk me through what would be a good way to build it?
   
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_paralyzed_
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01.21.2013, 02:47 PM

That's nothing new, airplane guys do it all the time using props as a load source, and a wattmeter for data.

There are much better videos out there, but this is the first one I found on a quick search:



You don't need the motor in a plane, a bench mount will suffice, and as long as you use the same prop (load) for different motors the data will show you the real world differences.


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nativepaul
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01.21.2013, 09:30 PM

A prop differs in load according to how fast it is spun. If one motor pulls 50A on a 10x6 prop and another pulls 60A on the same 10x6 prop, it tells us nothing without a tacho telling us how fast it is spinning, the 60A one could be amazingly efficient but spinning it 3000rpm faster than the 50A one, or it could be the worst motor ever and spewing 50A out as heat and spinning it 5000rpm slower.

As well as knowing what RPM they are achieving the motors will have to be of a very similar KV to draw any physical conclusions, although assumptions could be calculated despite some differences, you should also take into account where the maximum efficiency of the motor lies, ((an extreme example) you could test a Lehner 3080 and a Feigao 540xl and find they spin the same prop at 50A at the same speed and assume they are equals, but the Feiago is operating at its peak efficiency current, and the Lehner is 200A short of its and running a lot less efficiently than it can do.)


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01.21.2013, 10:42 PM

Efficiency is key when comparing motors. How to actually quantify this would be very difficult. You can easily find the input energy by simply using a watt meter. But calculating actual output energy of the motor is the hard part.


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gumleguf
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01.22.2013, 01:45 AM

Thank you, Gentlemen.

We were thinking of a real test bench/test station with a fixed power supply (to rule out differences in battery performance) and a fixed load of some sorts (perhaps using the same principle as an exercise bike). To make it "scientifically correct" and to be able to really compare the test, it would have to be a fixed test bench.

It would be interesting to us to measure Amps, Voltage drop (if any when using a proper power supply), Watts, max. RPM and Temperature. I guess as you mention brainanator, it would be difficult to measure the actual output energy, although it would be a very cool reading to have.

When you look at a brushless motor's specifications, e.g. a Neu Motors 1521 1900kv 1.5D, it has a continuous Watts reading of 1750W and a surge Watts reading of 3500W. I assume we should be able to make such Watts readings in our tests using a simple watt meter?
   
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01.22.2013, 10:39 AM

If using a power supply, be careful. The back emf of BL systems can fry PS circuits. If possible, I would add a diode across the regulator circuit so that when output voltage is higher than input (from emf pulses), that energy gets redirected. Doing this is not easy for switching PS units. Personally, I'd just use a battery but charge it fresh for each test to make sure you are doing a true apples to apples comparison.
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01.22.2013, 01:11 PM

So you want a motor dyno?


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01.22.2013, 01:26 PM

Motor dyno? Not familiar with that. What is it?

BrianG > Thanks for the advise. Duly noted!
   
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01.22.2013, 06:33 PM

You use the various BL motors to spin a big brushed motor and measure the watts you input the BL with and what you get out of the Brushed getting absolute figures is still a PITA as you have to find the efficiency of the brushed motor as a dynamo to get them, but comparative efficiencies are easy, the one with an output closest to the input wins.

It is possible to do with a BL motor for the output too as I have seen a bench setup with a 3080 for the load somewhere on the web, but a brushed motor is very easy.


Hot Bodies Lightning 2 Pro carbon, Mega 22/30/2, MMM, 4s3-5Ah.
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Lots of boats.
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01.22.2013, 07:01 PM

A dyno shows the horsepower and torque of a motor.


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gumleguf
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01.26.2013, 09:58 AM

Motor Dyno would be cool. Any recommendations for an affordable dyno setup?
   
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01.30.2013, 07:53 AM

Most interesting question I've seen in a while. I'd like to see a result but given the level of interest I think you'll be pushin' sh!1 up hill Gummy. Good luck anyway.
   
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