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Author Topic: Torque data to drive propellers?  (Read 959 times)

microgyros

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Re: Torque data to drive propellers?
« Reply #25 on: January 29, 2021, 09:56:00 am »


Then I wonder why higher turn motors are quoted as "high torque".  And why have various advisors suggested a high T-number?  I understand the total power must be lower because the R is higher and the I is less (at unloaded running).  My 45T motor is way too powerful if I allow anything like full voltage (judging by the speed and volume of water displaced).  I suspect I only need a fraction of that power.  The trouble is even if I reduce the voltage to slow it down, it draws a fair amount of current.  If a higher turn motor is nearer to its unloaded rpm with my prop then I would think it would run more efficiently, or more to to the point, not heat up so much.  I understand (now, on reflection and further reading) that the stall torque will be higher in the low-turn motor, but I'm interested in the torque at the rpm I need.
Yes.  :-)) Your observations tally

Can you rotate the brush gear? That would help.
Stall torque is higher in its lower turn version at the same voltage but it is the same when the voltage is kept in in proportion to turns.

Some high turn motors in the exact same can size are better but not because a change in turns.





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Tug Fanatic

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Re: Torque data to drive propellers?
« Reply #26 on: January 29, 2021, 10:02:49 am »

Don't they have Amazon in New Zealand?
Lots of ebay sellers ship from China to everybody.
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malcolmfrary

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Re: Torque data to drive propellers?
« Reply #27 on: January 29, 2021, 10:05:45 am »

A higher turn motor will produce more torque, but it will only do so at a higher voltage.  At the higher voltage, because of its lower KV, it turns at the same speed, but produces more power, which is seen as torque.
Just dropping a high voltage motor in as a replacement for a low voltage motor will give lower rpm, but will not give more torque with the same battery.  It will run cooler because it is not dissipating as much power, but at the same time it will be offering less power to the shaft.
It's a sound engineering principle to remember that what comes out of a system, must have gone in.  What you get out of a motor, depending on its design and how it is used, is turning force, rpm and heat.  All of this comes from the charge in the battery.  A good combination gives the rpm wanted with the torque to do the job with the minimum of waste heat.  Finding a motors comfort zone does not equate to looking at a data sheet and going for "max efficiency".  Running a motor in that condition usually results in a short lived motor.  Its the point where practice trumps theory every time.
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microgyros

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Re: Torque data to drive propellers?
« Reply #28 on: January 29, 2021, 11:36:03 am »

I repeat that the higher turn, 80T in this example,will produce less torque and that the 45T motor does. It will be lower until the voltage to the 80T motor exceeds voltage of the 45T motor by the ratio 80/45 or by 1.78, rounded up.
More turns and a lower kv does not help this prop overload. To run the test prop these motors need gears or a change to the timing, and if the end stock can be rotated.  Even if the back of the motor is fixed, it may show a different kV in the reverse rotation.
I've suggested a 540 sized motor that is in the ball park. It is nearer than the 540LN
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npomeroy

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Re: Torque data to drive propellers?
« Reply #29 on: January 29, 2021, 09:39:42 pm »

...
I've suggested a 540 sized motor that is in the ball park. It is nearer than the 540LN


The motor you suggested (Mabuchi 555PH-3255) has very similar specs to the 540LN in terms of rpm @ 12v. I think both are 5-pole.  I can get the 540LN locally https://nz.rs-online.com/web/p/dc-motors/8347679/   datasheet:    https://docs.rs-online.com/cbd9/A700000007082505.pdf
Is there a good reason to prefer the 555-3255?
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Subculture

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Re: Torque data to drive propellers?
« Reply #30 on: January 29, 2021, 09:55:09 pm »

Greater torque at broadly equivalent RPM, I believe a bit over a third more. That'll help keep the prop spinning and prevent the motor bogging down.

npomeroy

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Re: Torque data to drive propellers?
« Reply #31 on: January 29, 2021, 10:34:08 pm »

Greater torque at broadly equivalent RPM, I believe a bit over a third more. That'll help keep the prop spinning and prevent the motor bogging down.


OK.  Good enough for me.  I've ordered a pair of the RS-555PH-3255 over ebay from California.  I already have a pair of HobbyWing 80A(!) ESC's.  Seemed to work fine with the 540-45T motor and I've programmed my Tx to give forward/reverse with switchable rudder mixing.


Cheers
Nelson
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roycv

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Re: Torque data to drive propellers?
« Reply #32 on: January 30, 2021, 07:18:27 am »

Hi anyone following this thread and wondering what all the numbers mean then a quick look at this website may help.
Roy

https://www.mabuchi-motor.com/product/knowledge/classification/
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npomeroy

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Re: Torque data to drive propellers?
« Reply #33 on: January 30, 2021, 08:17:52 am »

Hi anyone following this thread and wondering what all the numbers mean then a quick look at this website may help.
Roy

https://www.mabuchi-motor.com/product/knowledge/classification/


Wonderful.  Thanks.
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npomeroy

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Re: Torque data to drive propellers?
« Reply #34 on: February 10, 2021, 08:34:07 pm »

Excuse me resuming stirring this topic...
What IS IT about number of turns that makes higher turn motors recommended for applications that operate at low revs?
Is it all about kV, i.e. the (unloaded) rpm w.r.t. voltage?
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microgyros

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Re: Torque data to drive propellers?
« Reply #35 on: February 10, 2021, 09:10:10 pm »

Excuse me resuming stirring this topic...
What IS IT about number of turns that makes higher turn motors recommended for applications that operate at low revs?
Is it all about kV, i.e. the (unloaded) rpm w.r.t. voltage?
No excuses are due since it is a good question.
If you examine a servo motor you find it is wound with more turns than the equivalent sold as a propulsion motor.Rock crawlers, like servo motors operate at low revs and stalled while energised ( albeit with reduced throttle). That's where the name comes from.Many turns of smaller diameter wire effectively makes a higher voltage version of the motor which is what both applications need.
Both have a large ratio reduction gearbox.

To help with the mental picture of torque:- Magnetic strength is proportional to amp x turns.
You could wind more turns on a bobbin of wire to increase the magnetic strength of the core, limited by core saturation.
There is no space to do that on a motor.
If you wind more turns of a smaller diameter wire you have to increase the voltage proportionally for the same rpm, torque and power as before.
At the same voltage there are more turns but the amps (and amp x turns ) falls.
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npomeroy

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Re: Torque data to drive propellers?
« Reply #36 on: March 04, 2021, 03:26:02 am »

So the motors - Mabuchi RS-555PH-3255 you recommended have arrived, and seem to give ample power on a brass 2" prop drawing less than 1A.  They are very directional, i.e. weak in "reverse" as driven by my allegedly fwd/rev ESC (https://www.hobbyhangar.co.nz/hobbywing-30112750-quicrun-wp-crawler-brushed-program-card).  But if you manually swap the polarity they are strong in the reverse direction. So is this likely to be an issue with the ESC design?  You'd think with an H-bridge type circuit you could get equal power in both directions.  The throttle channels in the Tx are 0 to 100%, not -100 to 0 to + 100% but I don't see that should matter.
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npomeroy

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Re: Torque data to drive propellers?
« Reply #37 on: March 04, 2021, 03:47:41 am »

Looking a bit further it seems Rock crawler ESC's typically have a lower reverse than forward output.  I didn't expect this.  I'm sure some model tanks can rotate on their tracks and you'd think that would require equal forward and reverse power.  Maybe the ones specific for boats are like this too?  Any ideas?
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Subculture

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Re: Torque data to drive propellers?
« Reply #38 on: March 04, 2021, 08:23:08 am »

Read the instructions that come with the esc.

npomeroy

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Re: Torque data to drive propellers?
« Reply #39 on: March 04, 2021, 08:41:26 am »

Read the instructions that come with the esc.


I did this.  There is a programming box that allows different power levels to be set forward and reverse.  When both are 100% the reverse is still MUCH weaker.  The max variation that can be set is 25% fwd and 100% reverse.  This is still very weak reverse comparatively.  A google search in crawler forums suggests it may be common for these ESCs to have weaker reverse.  Please let me know if you you suspect I have overlooked some other facet.
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malcolmfrary

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Re: Torque data to drive propellers?
« Reply #40 on: March 04, 2021, 09:17:40 am »

Is this the ESC? https://www.applianceelectronics.co.uk/viewproduct/15971/hobbywing-quicrun-hw30112750-1080-80amp-wp-brushed-crawler-esc


The problem with crawler motor ESCs is the brake feature which on a boat needs to be switched off.  Some early ESCs did feature reduced reverse output, but I never found that arrangement in any way satisfactory.  It was probably done for reasons of economy in manufacture, and because "they" had to buy a truckful of the components to get the right price, never changed later when better parts became available because the skip with the original parts in still had lots left over.  If the basic ESC is designed to give 25% max in reverse, it doesn't matter how the parameters are fiddled, 100% of 25% is still 25%.  Proper marine ESCs are symmetrical in output - they give as much in reverse as forward.
Noted that in the link, the ESC is "Programmed via".  Via what is unstated.
Weak reverse (which is your brakes when you need to stop) might be made worse by the design of the prop.  Depending on the shape of the blades, it might not be able to grip and move as much water in reverse as forward.  Most land vehicles do tend to be slower in reverse, apart from the WW2 Italian tanks of legend.
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Tug Fanatic

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Re: Torque data to drive propellers?
« Reply #41 on: March 04, 2021, 09:18:06 am »

So the motors - Mabuchi RS-555PH-3255 you recommended have arrived, and seem to give ample power on a brass 2" prop drawing less than 1A.  ......................

Is that on 12v?

How many blades on the prop?
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microgyros

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Re: Torque data to drive propellers?
« Reply #42 on: March 04, 2021, 10:58:04 am »

...It seems to me that the torque required to spin a given size (and pitch, blade no. etc) propeller in still water i.e. doing a static pull, may be known.  It would be a torque-rpm curve.  While due to the number of variables it's probably too much to expect full published data, perhaps someone has experience with a motor of "x" max torque in its specs, and they find it is "too little" or "plenty" for a similar application.


It would take some of the guesswork out of motor selection if you could base it on published specifications...
Published specifications for RS-555Px-3255
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Subculture

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Re: Torque data to drive propellers?
« Reply #43 on: March 04, 2021, 03:40:58 pm »

Either your doing something wrong with the programming, or you have a defective esc. That model of esc has an option for full 100% reverse. Default setting from factor, reverse is set to 50%. The programming card is optional for ease of use- you don't need it, you can set the esc up with standard set button provided.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjX1_a--5bvAhWMTcAKHQFYDOcQFjAAegQIAxAD&url=https%3A%2F%2Fwww.hobbywing.com%2Fproducts%2Fenpdf%2FQUICRUNWPCRAWLERBRUSHED.pdf&usg=AOvVaw3vs-L3a0dOOo2LAGp4EV4w

npomeroy

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Re: Torque data to drive propellers?
« Reply #44 on: March 04, 2021, 06:54:53 pm »

Is this the ESC? https://www.applianceelectronics.co.uk/viewproduct/15971/hobbywing-quicrun-hw30112750-1080-80amp-wp-brushed-crawler-esc


The problem with crawler motor ESCs is the brake feature which on a boat needs to be switched off.  Some early ESCs did feature reduced reverse output, but I never found that arrangement in any way satisfactory.  It was probably done for reasons of economy in manufacture, and because "they" had to buy a truckful of the components to get the right price, never changed later when better parts became available because the skip with the original parts in still had lots left over.  If the basic ESC is designed to give 25% max in reverse, it doesn't matter how the parameters are fiddled, 100% of 25% is still 25%.  Proper marine ESCs are symmetrical in output - they give as much in reverse as forward.
Noted that in the link, the ESC is "Programmed via".  Via what is unstated.
Weak reverse (which is your brakes when you need to stop) might be made worse by the design of the prop.  Depending on the shape of the blades, it might not be able to grip and move as much water in reverse as forward.  Most land vehicles do tend to be slower in reverse, apart from the WW2 Italian tanks of legend.


Yes that is the ESC.  The brake feature is turned off.  It is programmed via a "box" which I have.  I understand that props are usually shaped to be directional and reverse will give less thrust but the rpm difference is so great it eclipses this effect.
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npomeroy

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Re: Torque data to drive propellers?
« Reply #45 on: March 04, 2021, 06:56:28 pm »

Either your doing something wrong with the programming, or you have a defective esc. That model of esc has an option for full 100% reverse. Default setting from factor, reverse is set to 50%. The programming card is optional for ease of use- you don't need it, you can set the esc up with standard set button provided.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjX1_a--5bvAhWMTcAKHQFYDOcQFjAAegQIAxAD&url=https%3A%2F%2Fwww.hobbywing.com%2Fproducts%2Fenpdf%2FQUICRUNWPCRAWLERBRUSHED.pdf&usg=AOvVaw3vs-L3a0dOOo2LAGp4EV4w


Yes you can set reverse and forwards between 25 and 100% in 3 steps but I'm talking about at the 100% setting.  I have the programming box.
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Re: Torque data to drive propellers?
« Reply #46 on: March 04, 2021, 07:38:33 pm »

In that case it sounds like something may be wrong. Always difficult to assess these things by internet proxy.

npomeroy

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Re: Torque data to drive propellers?
« Reply #47 on: Today at 04:35:40 am »

 :embarrassed:  Duh.  OK it's fixed now.  The ESC factory default was slow reverse, fast fwd but each was adjustable.  I thought I'd changed all combinations but hadn't pressed the correct button to save all my changed settings.  So even though it is a crawler ESC it in fact gives pretty symmetrical fwd/rev power.
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