Two Brushless motors on One ESC….YES!

[m4a1usr]

In terms of "more battery mower used" please remember the measure of comparison is grams of Thrust PER WATT OF POWER. This means we are comparing performance (Thrust in this case) with the actual Volts x Amps being sucked from the 3S LiPo.

Yes. I understand you are defining a unit to measure force. Do you have a copy of "Brushless Permanent Magnet Motor Design" by Dr. Duane Hanselman? Not that you need it Joe, I was just curious as it is very informative for what you are doing currently. Many usefull formulas applicable to our hobby.

John

[TotalPackage]

where can you pick up that book ?

[TotalPackage]

Whoever said that the two would be less efficient they are right coz the motors are not gonna match exactly but you would think handwound motors that are measured could be closely matched?I mean that mistmatch inefficiency applies to the batteries not exactly matching as well right?

[m4a1usr]

where can you pick up that book ?

You can find it on Amazon but here is a link for sections or chapters. http://www.eece.maine.edu/motor/ It is spendy! The book goes for $175 so not many people own one out of their own pocket. I got lucky and inherited one from work when one of my co-workers retired. The guy is a genious at Matlab applications. Wish I had him as a consultant 5 years ago! Have since systematically been changing everything over to Labview since they are dominating the market and buying all the little guys out. Oh well. You gatta love National Instruments!


John

[questtek]

Yes. I understand you are defining a unit to measure force. Do you have a copy of "Brushless Permanent Magnet Motor Design" by Dr. Duane Hanselman? Not that you need it Joe, I was just curious as it is very informative for what you are doing currently. Many usefull formulas applicable to our hobby.

John

Thanks for the tip. I can use all the help I can get since I am in no way a motor expert but more like a slight cut above a novice. I am just trying to use them effectively in several projects and the tests are designed to find out how best to do this I will try to get of a copy of the book you recommend. I am sure it would be beneficial to me. Thanks again,

I also found a great program for Scorpion motors for evaluation. Although I am not using their specific motors, the computer program I think will help provide some guidelines. Unfortunately there is nothing on multiple motors with one ESC. Most, probably like me before the testing, thought this could never be done.
Maybe it still cannot effectively but right now that appears to be not what my test results are indicating. This is why I felt I had to include the actual test video so people did not think the data was pure vapor ware.

[m4a1usr]

At this point I'm thinking (yea, very dangerous!) you have proven that with one ESC and two motors the IR losses (wasted battery power/heat generated) is halved or something approximate. Worth every moment from my perspective and this is going to only get better.

John

[questtek]

A few posts back SAILR mad this comment........"I addressed this idea with one of the best ESC designers in China about 3 years ago. He never found time to pursue it with all the other projects he had. He said it was possible but starts and/or restarts of multiple motors would be an issue because they would get out of synch between the esc and one or more of the motors. He intended to address that issue using a different method I won't go into here because it's a bit of a trade secret."

Possibly these tests I have done may inspire them to look into it on a more serious basis. I keep trying to figure out if I did anything wrong in the tests so I went back to check if everything was in calibration and if the numbers repeated...........they did.

Project continues after Christmas...............and a Merry One to All.

[keithbradley]

No response to my posts? Im curious what you think...

[questtek]

I think when blackcat refered to the higher cost of larger escs, he was refering to escs that are higher current capable than the t-180. The t-180 is kind of an in-between. Compare a Swordfish 120 here at OSE ($69) and a Swordfish 240 ($259). He does have a piont about the more capable escs being considerably more expensive.
Steven had the 200A Swordfish's made for that very reason, but I think that esc (or the t-180) would be well suited for ONE motor like the 4074/2200kv, not two.
If you were going to run a pair of Leopard 4074's (2200kv), you would expect to use a higher Amp esc than 180, right?

I think it would be cool to use one esc if it were the more cost effective choice. I hate all the extra parts. One esc would make things a lot cleaner.

Sorry, I must have missed your post...........First, you are correct, Under high loads the SeaKing 180 is perfectly suited to the Leopard 4074 . I have recorded a high of about 120-140 Amps on my Insane Hydro with large prop on 4S.

However if you would run a milder set up and draw about 100 Amps you might be OK two Leopards on one Seaking. I will be trying this on a Mean Machine I have that is exactly set up like this. It appears that in running two motors you do not double the amps but its more like 70% of the total......at least that is what I have been told. (This is why I love the Eagle Tree and lab testing so I can actually find out)

I am not interested in using expensive ESC's. I am just happy with the $79 Seaking 180 or the $29 Hi Model or Suppro, 200 with my water cooling. The trick for me is to find motor/load combinations that permit me to use these ESC's on affordable multi, brushless motor set-ups.

There is lots of talk using 600 to 1500 watt outrunnrs in multiple motor setups. I believe that FighterCat 57 is doing this. So, theoritically a single $29, 200 AMP ESC just may possibly run 4 of these 600 watt outrunners. I am running two from one of the Seakings with no problem under loaded conditions.

It's easy to experiment with inexpensive components .....and you will notice I have no Castle HV's or multiple Leopard 5092's in my testing! However I just ordered a few of the Castle Creation SALE outrunners and will try them on one ESC.

[keithbradley]

So at 12,000RPM, a single prop makes 75grams of thrust, but 2 props make 627grams of thrust? Something doesnt make sense with the trust numbers...how are 2 props making more than twice the thrust?

I dont see how this is a comparison of efficiancy of 2 motors on 1 esc vs. 1 motor per esc...you are testing the efficiancy of 2 props vs. 1...
Why not test
2 props/2 motors/1esc
vs.
2 props/2 motors/2 escs
Then analize the wattage used. That would be a better test of efficiency. I would also monitor motor heat for both tests, that should also give you an idea of efficiancy. Either way Im not sure why 2 props are making 4X more thrust at the higher RPM range of the test and even more at the lower end. There is no compounding going on there...something doesnt make sense...

Ahh...I watched the video and a couple of things became apparent.
First, I think you would have much more accurate results measuring the actual thrust of the props rather than the lift created. The dual motor rig has the props out to the sides, where as the single motor rig has the prop in the center with the scale directly below it. The difference in available area behind the prop can have a drastic difference on the lift created. I think you should try a rig more like this where the thrust is measured at the scale rather than trying to lift an object off of it:

http://www.youtube.com/watch?v=I2WIqmDrJik

I respect what you are doing and Im not trying to bash your work. Just giving my thoughts, as I am under the impression that you are looking for input.

I was refering to these

[FighterCat57]

Yes, having excellent results using mid-large helicopter ourtunners spinning boat props. High Torque and High RPM w/high efficiency.

I have yet to try two motors from one ESC, but if I did, I would try the Suppo 200 and two 3126 outrunners. I would ask the MFR, but I don't want to confuse him any more. He's having enough trouble filling orders accurately.

[antslake]

Here are the results based on repeated tests:
1. There is over double the efficiency for two motors and one ESC VS one motor and one ESC. Specifically my measurement for comparison was grams of thrust per Watt of input.

First of all, great work! Very interesting. I am skeptical about the efficiency of a setup like this, especially when the loads start to vary on the motors.

When doing experiments like this, it is important that they be in as a controlled environment as possible. You are doing a great job. One thing I noticed though that could give this false(possibly) reading of more effceincy is the position of the prop on the stand. When you are using two props, the props backwash is not blowing on the scale that much, and the props have 50% more free space around them. When you use the single prop, the prop is blowing directly on the scale, countering the effect of lift. Here is what I would love to see you do, and you will thank me for this, turn the props around, so they push down. You will have no prop wash on the scale, and the props will not fly off and hit you either, and your family jewels will be much safer

[antslake]

I would also like to see a graph that plots rpm's vs input watts, I think that would be more accurate in determining efficiency. It would be nice to have the same exact voltage in the two tests too. When using the two motors, your voltage drops more, although I am not sure if this has an affect on it or not. But knowing that higher amps and lower volts, is less efficient than the opposite, maybe a factor.
How about using two packs with the two motors(2P), and one pack with the one motor, and maybe the voltages would stay the same. That is another variable you can eliminate. You are comparing twice the amp draw using a single pack. That could have an affect on the numbers.
Just trying to help.

[ManuelW]

Concerning efficiency:

I watched your Video of one Seaking 180 and four Leopards 4074. All the measurements where taken at about 11.000rpm as far as I can see.
Well with four motors the starting of the motors is quite rough, might be a problem to find the right commutation. But most interesting is the amp draw at 11.000rpm:
one motor: 1.38amps
two motors: 6.8amps
three motors: 8.3amps
four motors: >25amps

Doesn't really seem to increase efficiency at all. There has to be a massive error in your measurement equipment concerning thrust I think. Even the step from one motor to two motors shows a massive reduction in efficiency as I expected resulting from small manufacturing differences of each motor.

Best regards,
Manuel

[antslake]

The scale may also not be accurate determining negative numbers. Another reason to turn the props around, and make them thrust down. This is the way I use my Medusa research dyno.

[Ub Hauled]

Joe, I have a wrench to throw on your gears... how does the timing on the ESC act on the motors? Will it help with the sync?
And, how about the SENSORED ESCs/MOTORS, have you done any testing with those?
How are you?

Thanks

[questtek]

Originally Posted by keithbradley
"Ahh...I watched the video and a couple of things became apparent.
First, I think you would have much more accurate results measuring the actual thrust of the props rather than the lift created. The dual motor rig has the props out to the sides, where as the single motor rig has the prop in the center with the scale directly below it. The difference in available area behind the prop can have a drastic difference on the lift created. I think you should try a rig more like this where the thrust is measured at the scale rather than trying to lift an object off of it:"

Keith, you are an absolute wizzard and figured it out first! The area behind the prop makes an incredible difference, expecially if that is the scale. My test set up for this configuration was totally wrong and to prove it I did the testing to prove it. The following video shows what happens with the motor off to the side of the scale and then directly above the scale. I included a plot of the data at the end and comment on it.

http://www.youtube.com/watch?v=YpOq8...=youtube_gdata

I will take your excellent advice and go 2 motors 1 esc and then, IN THE EXACT SAME MOUNTING CONFIGURATION, test 2 motor each with their on ESC. As I now expect, the values should be much closer. This should go a long way to help determining the practicality of using two motors on a single ESC. It can be done but may not be feasible until after these latest tests are completed. Please keep your comments/suggestions coming!

[antslake]

That test does not prove the point we are trying to make. You added another variable by"twisting" the scale. You don't even needed the scale to determine effceincy. The props are a constant load, just measure that rpm vs input watts.

[questtek]

That test does not prove the point we are trying to make. You added another variable by"twisting" the scale. You don't even needed the scale to determine effceincy. The props are a constant load, just measure that rpm vs input watts.

Here is the data from two motors 1 ESC:
RPM WATTS
12800 229.7
14000 287.3
14700 349.3
15900 418.9
18000 539.0
18600 639.9 (This is a 600 Watt outrunner I am using)

Here is the data from one motor 1 ESC but with the motor improperly over the digital scale I use for thrust measurements:

RPM WATTS
10700 76.5
11900 93.6
13200 121.8
14050 147.0
15100 179.6
16000 192.2
17800 274.5
19500 356.1
20900 439.6

Can you make anything out of this data?

[antslake]

Yes I can! I just realized one other thing we need to know. We are assuming the second motor and prop has the same exact efficiency as the first one. If you could run a single test on the second motor and prop, and post those numbers, I can show you the math. I am typing from my phone, and will elaborate later when I get home.

[antslake]

Ok, here is how I see it. I am no expert on this, but I do know a lot, but I don't do it frequently enough and I maintain the right to be wrong, lol.

In automotive, and what I've learned from my Tekin dyno, and my medusa research dynos we can measure and compare hp/torque/efficiency of motors if the load is constant. The prop is a constant load (provided the load isn't changing when moving from 1 to 2 props due to placement on the motor mount) I think that affect is nil in the lifting setup you used. You are using 2 props of the same brand and size, but they may not be exactly the same, or are the motors exactly the same. That is why I requested the second motor test, to see if it draws around the same as the first one, which I think it will be very close.

Ok, having said that..we can simply take some numbers from the list you provided. It takes X amount of watts to turn Y amount of rpm's. We can get a number, rpm's per volt (N). Simply divide the rpm's by the watts.

2 motors, 1 ESC:
14000 / (287.3/2) =97.45 rpm's per watt (average for two motors)

1 motor, 1 ESC
14050 / 147= 95.57 rpm's per watt

At a higher rpm (which would be less accurate due to stalled props):

2 motors, 1 ESC
18000/ (539/2)=66.79 RPW

1 motor, 1 ESC
17800 / 274.5= 64.85

You are getting slightly more rpm's per volt with 2 motors. This percentage is almost negligible due to inaccuracies in the test equipment, and possibly using one battery pack, and having different input voltages. Not to mention, motor #2 may be more efficient or the prop may have less drag due to slight imperfections, or balance issues, etc.

What I see is they are even, and work fine under a constant equal load. How about trying one with a much larger prop to simulate a exaggerated uneven load?

I admit, I don't know enough about ESC and exactly how they work to give some better ideas as to why we shouldn't use this in a real setup. I do have a question about ESC's, do they sense rpm's on the poles that are not energized to know when to apply current to the energized poles? Could this be the reason why if one motor is severely loaded up different that the other, you could have major issues? I suspect the efficiency of the 2 motor setup would go down as the loads change. Or, as the load on one motor increases, the sensing part of the ESC will slow both motors down. I realize there are sensor, and sensorless ESC, but I do not know the specific differences.

[antslake]

Actually I think I found the answer:
http://www.rcgroups.com/forums/showp...77&postcount=2

This would explain why one motor would have a hard time starting if the other was already spinning, and the ESc was past it's start up phase. It also explains why in a real world application this may not work. It may also just work for a while, then the FET's will give out.

[LiPo Power]

Great tests Questtek!!! I love to read your findings.
The way you do it is straight forward....
The rest is rocket science to me.....
Cheers
Robert

[questtek]

Yes I can! I just realized one other thing we need to know. We are assuming the second motor and prop has the same exact efficiency as the first one. If you could run a single test on the second motor and prop, and post those numbers, I can show you the math. I am typing from my phone, and will elaborate later when I get home.

Great....I will run the tests in the AM pomorrow and immediately post the results. Mark F and Tony are coming over tomorrow to run but my 16' electric recovery boat...that is about 20 years old............took an untimely motor fart so I have to do a motor exchange. The 1 HP brushed moter that powers this boat weight about 30 lbs! Ugg.

[questtek]

Great tests Questtek!!! I love to read your findings.
The way you do it is straight forward....
The rest is rocket science to me.....
Cheers
Robert

Thanks but it is the guys in the forum providing the good ideas. I just have the hardware handy and love to experiment.

[questtek]

Ok, here is how I see it. I am no expert on this, but I do know a lot, but I don't do it frequently enough and I maintain the right to be wrong, lol.

In automotive, and what I've learned from my Tekin dyno, and my medusa research dynos we can measure and compare hp/torque/efficiency of motors if the load is constant. The prop is a constant load (provided the load isn't changing when moving from 1 to 2 props due to placement on the motor mount) I think that affect is nil in the lifting setup you used. You are using 2 props of the same brand and size, but they may not be exactly the same, or are the motors exactly the same. That is why I requested the second motor test, to see if it draws around the same as the first one, which I think it will be very close.

Ok, having said that..we can simply take some numbers from the list you provided. It takes X amount of watts to turn Y amount of rpm's. We can get a number, rpm's per volt (N). Simply divide the rpm's by the watts.

2 motors, 1 ESC:
14000 / (287.3/2) =97.45 rpm's per watt (average for two motors)

1 motor, 1 ESC
14050 / 147= 95.57 rpm's per watt

At a higher rpm (which would be less accurate due to stalled props):

2 motors, 1 ESC
18000/ (539/2)=66.79 RPW

1 motor, 1 ESC
17800 / 274.5= 64.85

You are getting slightly more rpm's per volt with 2 motors. This percentage is almost negligible due to inaccuracies in the test equipment, and possibly using one battery pack, and having different input voltages. Not to mention, motor #2 may be more efficient or the prop may have less drag due to slight imperfections, or balance issues, etc.

What I see is they are even, and work fine under a constant equal load. How about trying one with a much larger prop to simulate a exaggerated uneven load?

I admit, I don't know enough about ESC and exactly how they work to give some better ideas as to why we shouldn't use this in a real setup. I do have a question about ESC's, do they sense rpm's on the poles that are not energized to know when to apply current to the energized poles? Could this be the reason why if one motor is severely loaded up different that the other, you could have major issues? I suspect the efficiency of the 2 motor setup would go down as the loads change. Or, as the load on one motor increases, the sensing part of the ESC will slow both motors down. I realize there are sensor, and sensorless ESC, but I do not know the specific differences.

Good idea..............I will go over to the local hobby store and get a 7" prop. One motor on 6" and one motor on 7" props. I will use the Eagle tree but only have one RPM sensor. Which otor would you like to see that on? Or, as an option I could use my optical Cen-Tek hand-help optical tach and compare the RPM,s on each motor............but I am sure the speed control will keep them the same. Or am I sure of that?

[LiPo Power]

That is what I am saying....
You have enough motivation to keep it up with all the quastions and concerns....
Just wicked!!!!
Thanks

Thanks but it is the guys in the forum providing the good ideas. I just have the hardware handy and love to experiment.

[antslake]

Good idea..............I will go over to the local hobby store and get a 7" prop. One motor on 6" and one motor on 7" props. I will use the Eagle tree but only have one RPM sensor. Which otor would you like to see that on? Or, as an option I could use my optical Cen-Tek hand-help optical tach and compare the RPM,s on each motor............but I am sure the speed control will keep them the same. Or am I sure of that?

I guess use the optical tach, and measure both motors. Also test each motor individually with it's respective prop, so we can see the difference.

I've been doing a little reading on how ESC's work to try and understand this whole thing. It is as I assumed. I wanted to understand how the ESC knows how many pulses to send to the motor at a given throttle setting. Only any two of the three wires going to the motor are energized at a given time. The third wire is sensing the inductance created by the magnet going over the coil. Except on start up where the ESC just sends a generic signal to get the rotor going. The magnet passing over the unenergized coil does create enough current for the ESC to sense until it reaches a certain rpm.

So knowing that, everything would probably work fine if both motors were turning the same rpm(we've already kind of seen that). But if one were to get way out of phase, it would be receiving wrong timing signals from the ESC. What the affects of that would be, I don't know. Possibly higher amp draw, more stress on the FET's inside the ESC, maybe less amp draw, and a motor that starts freaking out. We already know that sometimes a stalled motor in a dual setup might not start sometimes, and you have to "reset" the throttle to zero to make them both start again. Maybe in a dual motor setup in a boat, the force of water going over a stalled motor might make it turn again?

The next step after this would be to make a setup that varies the load on one motor (variable pitch prop?) Because right now we are dealing with static setups. Just to further create a real world environment.

Great work, this thread is so interesting.

[sundog]

The next step after this would be to make a setup that varies the load on one motor (variable pitch prop?) Because right now we are dealing with static setups. Just to further create a real world environment.

Yup, that's what I was getting at on page one when I said "Try loading the unloaded motor (pinch the shaft?) while performing the same test. Will the submerged prop motor respond by slowing down to match the problem (pinched) motor?" I'm waiting for someone to try this theory in a boat. Could be tested much quicker. But it might fry an esc or motor (or two) just as quickly!

[antslake]

Maybe it would fry it quickly, or over time it would wear out the internals faster than using two speed controls.