Watts In vs Watts Out

[d]

This one's too good to pass up, and requires no interpretation.

I recently got one of those $6 Red Motors from UH. The original wind was 15 turns of triple-strand angel hair wire. Tested it "stock" and here's what popped out with a 9050 GWS prop and 1320 TP 3s pack:

9313 rpm @ 11.02 volts 15.62 amps

Rewound it with 17 turns of 24gauge wire and got this result. Same motor, same battery and same prop on the same stand (only thing changed is winding):

9310 rpm @ 11.01 volts 13.69 amps

So that's either 151 watts in or 172 watts pushed in for the same output. Well, okay, 21 watts for 3 more rpm actually.


Dave

[Davem452]

Dave,

Very interesting result with the red motor. I couldn't understand how you could wind less turns and get the same rpm's out. After thinking it over here is my conclusion. Since the two versions of the motor turned the same prop at the same speed, in essentially the same air, we can conclude that the torque produced by two motor versions (old 17 turns & new 15 turns) is equal. It takes a certain amount of torque to turn a certain prop at a given rpm. Now we can equate the two torques in terms of the torque constants and the current applied. so we have

KTo * Io = KTn * In
( the currents in this line should be reduced by the idle currents)

KTo = torque constant of the old wind
Io = current draw of the old wind at 9310 rpm's
KTn = torque constant of the new wind
In = current draw of the new wind at 9310 rpm's

a little math leads us to ;

KTo/KTn = In/Io = 13.69/15.62 = 0.88

The torque constant of the old wind is 88% of the torque constant of the new wind, or the torque constant of the new wind is 114% of the torque constant of the old wind.

This makes sense. Since the torque constant is inversly proportional to the speed constant KV, or

KV = c/KT where c is a constant.

So now we can state;

KVn = 0.88 * KVo

or you lowered the KV of the motor by 12% by rewinding with 2 additional turns of wire.

This is what we would expect, but what troubled me was how you were able to turn the same prop at the same rpm's with a motor with a lower KV. I think what is missing here is the Rm numbers of the two winds. I wish you would have measured them. I think that the Rm of the new wind is much less than the Rm of the old wind even though you wound it with two additional winds. You said that it was 3 strands of "angle hair" wire. That must have been a very inefficient wind. Was the stator filled? Looking over the United Hobbies web pages, is this the motor?

FC2822
FC 28-22 Brushless Outrunner 1200kv

After going through all of this I realized I forgot to take into account the Idle curnent which would change my calculation of relative kV but probably not by much.

My conclusion; you are very interested in Watts out; therefore you are very interested in the motor constants KV, Io, & RM. You may look at them now and they don't have much meaning to you, but you do care very much about the effect they have. With a little time and effort I think you could get to the point where you could look at a motors weight, KV. Io. & Rm and make a good judgement on a motor. Do you want to get there? Probably not, but you already have such a good gut feel about the numbers. It is a shame you don't take it to the next level.

My apologies to Mr Badcock. After spending some time on his web pages this is my take. He uses the fact that it takes a certainn amount of torque to turn a certain propellor at a given rpm at 20C at sea level. Some people have painstakingly measured these numbers for a large number of props at range of RPM's. (He makes mention of the Hyperion people) So when you set a RPM value for a prop, his software makes use of this data. He looks up the torque required to turn the prop and multiplies this by the rpm's (and maybe a constant) to get the mechanical power out. You also input I & V so he has power in. His efficiency number is just the ratio of these two values. How accurate is his calculation? I'm not certain. It depends upon the accuracy of his prop/rpm/torque tables. It also depends upon the accuracy of the data input. I have a feeling that differences in barametric pressure and humidity could adversely affect the overall accuracy of the method. The thrust and pitch speed he outputs are not used to calculate efficiency. They are just included as additional data from the data tables he uses.

Sorry for the rambling. I wish we had the Io & Rm numbers for the De Novo and the dLRK. Do you own either of these motors? Is the dLRK commercially available?

Dave Morris

[d]

Dave,

You're quite right that the objective of the rewind was to lower the Kv and Rm simultaneously -- the former because the "weight of metal" in the stator is slight and the latter because it's always good, right?

Measuring Rm is not a simple game to play. You can use the variable load method such as:

http://peakeff.com/FindKIR.aspx

... or you can kludge up a device such as:

http://www.rcgroups.com/forums/showthread.php?t=580151

Of course another method is to use the resistance constant for the wire, along with its length.

... or you can have access to a good engineering bench. But even so, results tend to be spotty and reports often -- perhaps usually -- don't lead to numbers that relate to observed reality "at the prop."

Kv and Io are pretty simple and usually fairly accurate. But one bad input variable is, of course, enough to mess with things.

Yes, that is the correct motor. The wire is so thin it's hard to be sure what it is, but a gentleman in France estimates 34 swg. Tripling it should give roughly 31g equivalence, so it's clear that by going to 24awg single strand I'm cutting the resistance a bit (as well as the "unequal circuit load" issue).

I understand you find my lack of faith in motor constants ... disturbing. This is not the result of a disdain for the theory so much as the experience of finding that the numbers usually don't describe reality, probably because of ineffective test methods. I'm just eliminating the middle man and measuring the actual output instead -- a sort of empirical approach. If you think that's shameful, I would refer you to Richard Feynman as an example of another shameful person. I can live with it.

Mr. Badcock does not limit his work to 20C at sea level. Those are input fields and you can adjust for local conditions. One thing I don't like is the adjustments don't allow for the current air pressure. Yes, the thrust and pitch speed are not directly used; they are just the cause of the variable load (my original point was that he is _not_ just using thrust).

I do not own a deNovo, nor can I see any point in getting one other than to subject it to testing, which is not a good enough reason when I already know it's an inefficient old design.

I do own the dLRK motor, and it is not commercially available. I made it, and it's a tough piece of work at that. But a roughly equivalent motor is commercially available -- the numbers for the Axi 2204/54 are pretty darn close.


Dave

[d]

Dave,

You're quite right that the objective of the rewind was to lower the Kv and Rm simultaneously -- the former because the "weight of metal" in the stator is slight and the latter because it's always good, right?

Measuring Rm is not a simple game to play. You can use the variable load method such as:

http://peakeff.com/FindKIR.aspx

... or you can kludge up a device such as:

http://www.rcgroups.com/forums/showthread.php?t=580151

Of course another method is to use the resistance constant for the wire, along with its length.

... or you can have access to a good engineering bench. But even so, results tend to be spotty and reports often -- perhaps usually -- don't lead to numbers that relate to observed reality "at the prop."

Kv and Io are pretty simple and usually fairly accurate. But one bad input variable is, of course, enough to mess with things.

Yes, that is the correct motor. The wire is so thin it's hard to be sure what it is, but a gentleman in France estimates 34 swg. Tripling it should give roughly 31g equivalence, so it's clear that by going to 24awg single strand I'm cutting the resistance a bit (as well as the "unequal circuit load" issue).

I understand you find my lack of faith in motor constants ... disturbing. This is not the result of a disdain for the theory so much as the experience of finding that the numbers usually don't describe reality, probably because of ineffective test methods. I'm just eliminating the middle man and measuring the actual output instead -- a sort of empirical approach. If you think that's shameful, I would refer you to Richard Feynman as an example of another shameful person. I can live with it.

Mr. Badcock does not limit his work to 20C at sea level. Those are input fields and you can adjust for local conditions. One thing I don't like is the adjustments don't allow for the current air pressure. Yes, the thrust and pitch speed are not directly used; they are just the cause of the variable load (my original point was that he is _not_ just using thrust).

I do not own a deNovo, nor can I see any point in getting one other than to subject it to testing, which is not a good enough reason when I already know it's an inefficient old design.

I do own the dLRK motor, and it is not commercially available. I made it, and it's a tough piece of work at that. But a roughly equivalent motor is commercially available -- the numbers for the Axi 2204/54 are pretty darn close.


Dave

[d]

Dave,

You're quite right that the objective of the rewind was to lower the Kv and Rm simultaneously -- the former because the "weight of metal" in the stator is slight and the latter because it's always good, right?

Measuring Rm is not a simple game to play. You can use the variable load method such as:

http://peakeff.com/FindKIR.aspx

... or you can kludge up a device such as:

http://www.rcgroups.com/forums/showthread.php?t=580151

Of course another method is to use the resistance constant for the wire, along with its length.

... or you can have access to a good engineering bench. But even so, results tend to be spotty and reports often -- perhaps usually -- don't lead to numbers that relate to observed reality "at the prop."

Kv and Io are pretty simple and usually fairly accurate. But one bad input variable is, of course, enough to mess with things.

Yes, that is the correct motor. The wire is so thin it's hard to be sure what it is, but a gentleman in France estimates 34 swg. Tripling it should give roughly 31g equivalence, so it's clear that by going to 24awg single strand I'm cutting the resistance a bit (as well as the "unequal circuit load" issue).

I understand you find my lack of faith in motor constants ... disturbing. This is not the result of a disdain for the theory so much as the experience of finding that the numbers usually don't describe reality, probably because of ineffective test methods. I'm just eliminating the middle man and measuring the actual output instead -- a sort of empirical approach. If you think that's shameful, I would refer you to Richard Feynman as an example of another shameful person. I can live with it.

Mr. Badcock does not limit his work to 20C at sea level. Those are input fields and you can adjust for local conditions. One thing I don't like is the adjustments don't allow for the current air pressure. Yes, the thrust and pitch speed are not directly used; they are just the cause of the variable load (my original point was that he is _not_ just using thrust).

I do not own a deNovo, nor can I see any point in getting one other than to subject it to testing, which is not a good enough reason when I already know it's an inefficient old design.

I do own the dLRK motor, and it is not commercially available. I made it, and it's a tough piece of work at that. But a roughly equivalent motor is commercially available -- the numbers for the Axi 2204/54 are pretty darn close.


Dave

[d]

Dave,

You're quite right that the objective of the rewind was to lower the Kv and Rm simultaneously -- the former because the "weight of metal" in the stator is slight and the latter because it's always good, right?

Measuring Rm is not a simple game to play. You can use the variable load method such as:

http://peakeff.com/FindKIR.aspx

... or you can kludge up a device such as:

http://www.rcgroups.com/forums/showthread.php?t=580151

Of course another method is to use the resistance constant for the wire, along with its length.

... or you can have access to a good engineering bench. But even so, results tend to be spotty and reports often -- perhaps usually -- don't lead to numbers that relate to observed reality "at the prop."

Kv and Io are pretty simple and usually fairly accurate. But one bad input variable is, of course, enough to mess with things.

Yes, that is the correct motor. The wire is so thin it's hard to be sure what it is, but a gentleman in France estimates 34 swg. Tripling it should give roughly 31g equivalence, so it's clear that by going to 24awg single strand I'm cutting the resistance a bit (as well as the "unequal circuit load" issue).

I understand you find my lack of faith in motor constants ... disturbing. This is not the result of a disdain for the theory so much as the experience of finding that the numbers usually don't describe reality, probably because of ineffective test methods. I'm just eliminating the middle man and measuring the actual output instead -- a sort of empirical approach. If you think that's shameful, I would refer you to Richard Feynman as an example of another shameful person. I can live with it.

Mr. Badcock does not limit his work to 20C at sea level. Those are input fields and you can adjust for local conditions. One thing I don't like is the adjustments don't allow for the current air pressure. Yes, the thrust and pitch speed are not directly used; they are just the cause of the variable load (my original point was that he is _not_ just using thrust).

I do not own a deNovo, nor can I see any point in getting one other than to subject it to testing, which is not a good enough reason when I already know it's an inefficient old design.

I do own the dLRK motor, and it is not commercially available. I made it, and it's a tough piece of work at that. But a roughly equivalent motor is commercially available -- the numbers for the Axi 2204/54 are pretty darn close.


Dave

[Davem452]

Dave,

As I mentioned this afternoon, my long winded reaponse got placed in the bit bucket when I didn't realize I wasn't logged in yet. Here is the readers digest version.

Yes, the emperical approach is certainly valid.
Yes it can be difficult to get reliable measurements of the motor constants.
But measurements like thrust and RPM can also be unreliable because of variations in altitude & air pressure.

You are right about Badcock. Can't blame you about the De Novo. I would still like to see the Rm measurements of the dLRK compared to the De Novo. I believe that would give clear evidence that motor constants can be of some value.

Great work on the UH red motor. For $6 and some labor and wire it looks to be a useful motor.

I look forward to your next example of an inefficient motor.

Dave Morris