Model Boat Mayhem
Technical, Techniques, Hints, and Tips => BRUSHLESS Motors and Speed Controllers => Topic started by: The Wizard on January 28, 2016, 04:03:30 am
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A 1400Kv motor on 11.1 volts will give 15,540 rpm. A 2100Kv motor on 7.4 volts will give 15,540 rpm.
What would be the advantages or disadvantages of either set up.
I am not considering either I'm just curious and wish to increase my knowledge.
Wizard
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The can size helps, the bigger is generally more powerful and uses more watts.
A 28mm motor with 2000kv can be 750 watts and very good in a small boat up to about 30 inches.
A 35mm motor with 1400kv can be 1600 watts and very good in larger boat from 30 to 40 inches.
Lower kv motors are for bigger boats like the Fantome a G60-500kv on 6S will pull about 30 mph(used as a club camera boat).
Hop this answer your curious mind!!!!!!!!!!!
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Canabus, your answer was interesting but doesn't really answer my question
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kind of a stream of consciousness ramble here...
watts is where its at.
for each motor to propel the same boat at the same speed (given all other motor specs are the same), they will need to use the same amount of power.
power is watts, which is amps * volts.
generally speaking the motor with the lower kv will use fewer amps to get the same watts. ie, for the motor using 7.4 volts, to get 300 watts, it will need a speed control capable of handling 41 watts. for the motor using 11.1 volts, it only needs a speed control capable of handling 28 watts. higher amp rated speed controllers cost more. higher amp rated speed controls also tend to be physically larger.
in addition, now the battery will need to be able to handle a load of 41 amps for the 2100kv motor, but only 28 amps for the 1400kv motor. higher amp rated batteries cost more. however...
generally this also means that to get the same run time out of the battery, the actual mah rating of the 7.4v battery will need to be higher than the mah rating of the 11.1v battery. this can counter-act the need for higher discharge ratings. so you might be able to buy a smaller mah rated battery, with a lower discharge rating, but with more cells. cost on the batteries will probably be a wash.
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tsenecal, thank you for that, it all begins to make sense to me.
Wizard
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kind of a stream of consciousness ramble here...
watts is where its at.
for each motor to propel the same boat at the same speed (given all other motor specs are the same), they will need to use the same amount of power.
power is watts, which is amps * volts.
generally speaking the motor with the lower kv will use fewer amps to get the same watts. ie, for the motor using 7.4 volts, to get 300 watts, it will need a speed control capable of handling 41 watts. for the motor using 11.1 volts, it only needs a speed control capable of handling 28 watts. higher amp rated speed controllers cost more. higher amp rated speed controls also tend to be physically larger.
in addition, now the battery will need to be able to handle a load of 41 amps for the 2100kv motor, but only 28 amps for the 1400kv motor. higher amp rated batteries cost more. however...
generally this also means that to get the same run time out of the battery, the actual mah rating of the 7.4v battery will need to be higher than the mah rating of the 11.1v battery. this can counter-act the need for higher discharge ratings. so you might be able to buy a smaller mah rated battery, with a lower discharge rating, but with more cells. cost on the batteries will probably be a wash.
Probably a typo error but don't get amps and watts confused.
Ned
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Amps is the amount of current used and watts the power produced,simple ohms law......Ray.
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Obviously.
I was merely pointing out that Tsenecal had
substituted the word watts for amps as the
rating for the esc's.
Ned
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Bear with me here, all this talk of 1400kv is highly confusing. Surely you don't mean 1.4 million volts, which would be instantly fatal. Obviously does not stand for kilo volts then, which is what kv usually represents.
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Bear with me here, all this talk of 1400kv is highly confusing. Surely you don't mean 1.4 million volts, which would be instantly fatal. Obviously does not stand for kilo volts then, which is what kv usually represents.
Bob
Not in this particular example. I think you'll find that 1000 volts is usually written as kV, not kv. The rating means "revs per volt", so a 450kv motor running on a 3S LiPo pack would turn at 450 x 11.1 = 4995RPM. It's actually quite a simple piece of information and a lot more useful than anything we used to get with brushed motors. The reason is that the brushless motor will always turn at that speed, (given that voltage) irrespective of whether it's lightly or highly loaded. Higher loads will just require more current. Brushed motors slow down with higher loads.
Dave M
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Hi All
Well put about brush versus brushless motors Dave M., I have found brushless motors are far more powerful and if you get the right ESC which handles the full voltage and amps of the motor plus a bit for safety you general have no problem except putting to large a prop on the boat.
It would be interesting to swap out a MFA 800 motor to a Turnigy 4258-400 kv is the same boat with the same prop to see the performance different.
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You'll probably find the higher kv motor is a bit more efficient. Less turns gives lower resistance.
Higher voltage motors tend to be kinder to batteries as they require less current for equivalent wattage, this is of lower importance with most models when using lipo batteries, as they're capable of supplying very high levels of current continuously. It would only become more important for very high powered, or very large models.
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Bob
................. I think you'll find that 1000 volts is usually written as kV, not kv. The rating means "revs per volt", so a 450kv motor running on a 3S LiPo pack would turn at 450 x 11.1 = 4995RPM. It's actually quite a simple piece of information and a lot more useful than anything we used to get with brushed motors. The reason is that the brushless motor will always turn at that speed, (given that voltage) irrespective of whether it's lightly or highly loaded. Higher loads will just require more current. Brushed motors slow down with higher loads.
Dave M
I am sorry but you are wrong. Sensorless Brushless motors do slow down as you put a load on them. For model aircraft use it is generally reckoned that they retain reasonable efficiency down to about 70% of the free running rpm (Real kv x volts). At less than 70% you are generating a lot of heat & not much go (even allowing for the cooling effect of the big fan on the front of a model aeroplane!). For model boat, use unless you are water cooling the motor, you need to keep it higher which you do by choosing a prop that loads the motor less heavily.
Whilst I agree that kv is a very useful piece of information you cannot always rely on the kv stated by the motor manufacturer nor indeed between two apparantly identical motors in the more budget ranges.
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At present I am building a cabin cruiser 37 and 1/2 inches with a XK3674-B-1900kv 1155 watt motor on 3S or 4S 5800 mah Lipo Battery and 100 amp car ESC.
Prop 40 to 50 mm 2 blade or a 3 blade 37mm all 1.4 pitch.
I also have 3639-1100kv 800 watts and a 3648-1450kv 1600 watts.
So I will see which motor and prop suit the beast.
The first motor is the longest so I started with the biggest beast first!!!
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Yes Tug
They like to rev they heads off !!!
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Sorry. Still confused. if kv is "revs per volt", then "1400 kv" is still 1.4 million rpm per battery volt. Speeds of the order of the large hadron collider?.
Surely something like "1400 rpm/V" would be more realistic and meaningfull.
Still far too blooming fast spinning for a Victorian warship with huge props. One rpm or less on minimum throttle is nice. Lots of torque.
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No. You're reading too much into it, Bob!
Forget the fact that a 'K' or a 'k' when used elsewhere signifies 1000. In this context it means "revolutions per". I don't know who chose the 'K' - probably the same bloke who invented BEC... <*<
If this was rocket-science then it would be at the "light blue touchpaper and stand clear" end of the learning curve. Now go and have a cold drink and a lie down.
DM
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Bob, don't think of the 'k' as relating to Kilo... instead it relates to a constant... in this case the number preceding it.
In actual fact the way it is presented is part of the problem since it should actually be written Kv not kv.
Not many suppliers use the correct subscript version though.
Kv = Motor velocity constant.
Kilovolt = kV not kv.
It can get a bit confusing if you are not familiar with electrics/electronics terms.
Hope that helps your thinking.
Best regards.
Sandy. :-))
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Obviously.
I was merely pointing out that Tsenecal had
substituted the word watts for amps as the
rating for the esc's.
Ned
yep, a typo... sorry for that, feel like an idiot for letting that get past. the two ratings for the speed controls should be 41 amps and 28 amps respectively,
not 41 watts and 28 watts.
Tim
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It can get a bit confusing if you are not familiar with electrics/electronics terms.
The problem is that I am very familiar with electrics/electronics terms, which is why this whole theme was so confusing. It should read rpm/V to equate to descriptions given subsequently. Using the letter "k" is daft IMHO as it means something totally different to anyone familiar with accepted terminology. Might just as well have indicated degrees Kelvin.
I only asked a very reasonable question.
Some of the replies were less than we expect from experienced Mayhemer's.
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Oh I don't know, for an initially confusing subject, the lads' done good! Don't be too hard on them :-)
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Sorry, Bob - no offence intended but it's a bit like trying to teach someone to ride a bike over the telephone - intensely frustrating for both parties. I'm sure many folk would agree that RPM/V might be a more logical abbreviation or whatever, but we weren't asked.
BTW why is the SI symbol for current I when its unit is A (amp), and why is the SI symbol for time t when its unit is s (second)?
I blame the French...
DM
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Seems a reasonable assumption......
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Well my simple question certainly got a discussion going and a heap of information.
So after all that am I right in saying more volts and lower kv will give me more run time on the pond.
Wizard
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Well my simple question certainly got a discussion going and a heap of information.
So after all that am I right in saying more volts and lower kv will give me more run time on the pond.
Wizard
nope. :)
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Perhaps I should read through these posts from start to finish to make sure I sort of understand.
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Its really kind of complex, but the simplest way to put it is you have 4 variables for a specific boat.
prop
motor
speed control
battery
all four of these inter-relate on how much run-time a boat will have. change any one of them, and you change the run-time. depending on what you change, and how you change it, run-time might increase, or it might decrease.
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1. I take all of this on-board DM,...... O0...but I think it would be near impossible to metricate or convert Roman Sundials into prime units of 10+
2. Force of habit has me writing something like .......see you @ the track @1400 %)...the response is sometimes.....'do you mean today or is that tomorrow?' {-)
3. My dear old father in law was named Kelvin...however he was a Bank manager so the temperature probably didn't bother him too much........
Derek
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Well my simple question certainly got a discussion going and a heap of information.
So after all that am I right in saying more volts and lower kv will give me more run time on the pond.
Wizard
Bigger capacity batteries (eg 4400mah instead of 2200mah but the same number of cells) will give you a longer run time provided your hull is capable of carrying the extra weight - about double the run time in my example.
More volts & lower kv where the two give an equal rpm (eg 12 volts 1000kv = 12000rpm as opposed to 24 volts 500kv which also equals 12000rpm) are likely to give you very similar run times although your 12v batteries would need double the capacity say 4400mah 12v instead of 2200mah 24v. Your motors at 12v would be using twice the amps (but the same watts - volts x amps) than they would be at 24v.
Actual real world results depend on the componets used, their efficiency and how near their limits you are using those components,
This is very simplistic but I hope that it helps and assumes that you use the model in the same way for both examples.
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Depends on the speed you are driving the boat at, the faster ewe go the faster ewe use up the battery!!!!!
I only go fast or go home!!!
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Multiple things use versions of KV etc
https://en.https://en.wikipedia.org/wiki/Motor_constantswikipedia.org/wiki/KV
Down to motor constantshttps://en.wikipedia.org/wiki/Motor_constants
Nick
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The problem is that I am very familiar with electrics/electronics terms, which is why this whole theme was so confusing. It should read rpm/V to equate to descriptions given subsequently. Using the letter "k" is daft IMHO as it means something totally different to anyone familiar with accepted terminology. Might just as well have indicated degrees Kelvin.
I only asked a very reasonable question.
Some of the replies were less than we expect from experienced Mayhemer's.
I thought I was a pretty good explanation, plenty of instances in science/engineering where term aren't totally logical and you just have to accept what they mean rather than break it down to first principles. The motor constant description pretty much nailed it.
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When I see KV - I thing of thousands of volts, i.e. power line amounts like the national grid uses, 420KV is 42000 volts, or local supply 145KV is 145000 volts, not mv millivolts or parts of a volt, so I am confused by the brushless motor terminology, I do know that static voltage is in thousands of volts say 12000v but has virtually no amperage so is not as dangerous (though that is not to say it cannot be).
So I stay away from it - I struggle with getting a 6/9/12v set up in those boats i do have running.
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An aside:
I work in a petrol station. Yesterday, a customer came in, having fuelled up his brand-new Mercedes.
"Do you know anything about tyre pressures?" he asked.
I nodded.
"The plate on the door says '200', front and rear", he said, "I haven't a clue what that means."
200?! Our machine does PSI or atm. Go much above 60 in the former or 4.0 in the latter and you'll cover the forecourt in tiny bits of rubber.
I went out to his car.
"200 kPa."
Ah! Now I don't know whether this is a New Thing, but it seems like the Germans, at least, are using SI units for tyre pressures.
Much more sensible, from an engineering point of view, but utterly unworldy, given the tyre pressure machines you'll tend to find on UK forecourts.
Andy
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An aside:
I work in a petrol station. Yesterday, a customer came in, having fuelled up his brand-new Mercedes.
"Do you know anything about tyre pressures?" he asked.
I nodded.
"The plate on the door says '200', front and rear", he said, "I haven't a clue what that means."
200?! Our machine does PSI or atm. Go much above 60 in the former or 4.0 in the latter and you'll cover the forecourt in tiny bits of rubber.
I went out to his car.
"200 kPa."
Ah! Now I don't know whether this is a New Thing, but it seems like the Germans, at least, are using SI units for tyre pressures.
Much more sensible, from an engineering point of view, but utterly unworldy, given the tyre pressure machines you'll tend to find on UK forecourts.
Andy
29psi I think.
If this is to be a world standard I support it. There is no reason that we cannot all use the same. All it takes is a few weeks to get used to the different measures. My local market still use lbs pricing with kg in very small numbers. I now find this extremely unhelpful. I have forgotten Fahrenheit.
If it isn't a world standard why use a measure that few understand?
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Never having heard of kPa thought i'd
research google it.
https://en.wikipedia.org/wiki/Pascal_(unit)
Seems it is named for some guy called Pascal, who thought a lot about such stuff. The article claims that it is a "world standard", except where either it isn't used, or it is used, but with local variations, often involving moving the decimal point a few notches either way.
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. I have forgotten Fahrenheit.
I and my skin understand the weather only in Farenheit; any mention of Celsius and my wife translates for me. But you are right about mixing standards. In work linear lengths are given in miles, chains, and yards but the tamping computer has metres as its standard for length. Track gauge and heights are in mm. Rail weight is in pounds. Oh my %%
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I and my skin understand the weather only in Farenheit; any mention of Celsius and my wife translates for me. But you are right about mixing standards. In work linear lengths are given in miles, chains, and yards but the tamping computer has metres as its standard for length. Track gauge and heights are in mm. Rail weight is in pounds. Oh my %%
Good old ALC, although gauge was always 4 foot 8 and a half, its 1435 is just the metric conversion. Are you on the tamper operator side or the pway side?
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A TQS for my sins so PWay. I am looking forward to my retirement in a couple of years. Forty years of ballast walking <:( ooh my poor feet!
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It is rumoured that the relevant Government Official :embarrassed: signing off on that countries use of the unit of pressure being kPa..was indeed an Accountant and not indeed an Engineer
Probably went to the same dingbat school where earlier graduate's invented hands, leagues, chains, cubits & other demonstrably confusing units of measurement %%............
Now, we all know that cubit was not a singular unit of measurement.....as the following will explain........Derek {-)
http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&uact=8&ved=0ahUKEwimxpbguZ7LAhXClZQKHWQXCL4QFghBMAY&url=http%3A%2F%2Fwww.convert-me.com%2Fen%2Fconvert%2Fhistory_length%2Fbibcubit.html&usg=AFQjCNGUNDoX_LDK4JIE5EJ9OgqUZm8N_g
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I have a cheapo digital tyre pressure gauge bought
from Netto, about 10 years ago that can measure
in PSI, Bar, or KPa.
Obviously in use in the rest of EU for some time. %)
Ned
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....BTW why is the SI symbol for current I when its unit is A (amp) .....?
I blame the French...
DM
Out of curiosity - alright I had nothing better to do - I tried to pin down the use of "I" for Amps and all I could find was the suggestion that it stood for 'Intensity'. I suspect it was just the choice of some theoretical scientist who had never worked in very close proximity to a live 440V switchboard with insulating gloves filling with sweat while praying that all the 'I's' didn't shoot up his arm and fry his innards.
Personally, as long as our village blacksmith still produces steam-propelled velocipedes, I shall insist on the velocimeter being calibrated in barleycorns per bat-wink as understood by our ancestors.
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I've just read all of this thread and it put a smile on my face. Sorry for going slightly off topic but have you all noticed that many of the modelling and model engineering magazines we all read now produce articles with mixed imperial and metric measurements in the same sentence. i.e. take a piece of 0.5mm brass sheet and cut to 3 inches long. etc, etc.
For all your amusement I was chastised by a teacher last year when I was doing the commentary on a very large model of the Titanic. For historic reasons I gave the dimensions of the full size ship in Feet. The teacher told me I was damaging children's education. When I responded that they will have problems when they learn to drive and encounter distances in miles and speed limits in miles per hour he huffed and puffed and walked away.
Never forget the old joke, 'Britain is going metric - inch by inch'.
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Funny how we don't use imperial capacity for car engine size though. They do in the states.
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Metrication is something the Americans have steadfastly resisted - along with democracy, good manners and eating green vegetables. Of course, you can always take that with 1/10 cup of salt...
DM