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Author Topic: Kv and volts (Brushless Motors)  (Read 8904 times)

The Wizard

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Kv and volts (Brushless Motors)
« on: January 28, 2016, 04:03:30 AM »

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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canabus

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Re: Kv and volts
« Reply #1 on: January 28, 2016, 04:36:06 AM »

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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The Wizard

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Re: Kv and volts
« Reply #2 on: January 28, 2016, 05:35:02 AM »

Canabus, your answer was interesting but doesn't really answer my question
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tsenecal

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Re: Kv and volts
« Reply #3 on: January 28, 2016, 05:44:53 AM »

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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The Wizard

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Re: Kv and volts (Brushless Motors)
« Reply #4 on: January 28, 2016, 06:25:03 AM »

tsenecal, thank you for that, it all begins to make sense to me.


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Netleyned

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Re: Kv and volts
« Reply #5 on: January 28, 2016, 06:31:05 AM »

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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sparkey

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Re: Kv and volts (Brushless Motors)
« Reply #6 on: January 28, 2016, 07:13:33 AM »

Amps is the amount of current used and watts the power produced,simple ohms law......Ray.
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Netleyned

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Re: Kv and volts (Brushless Motors)
« Reply #7 on: January 28, 2016, 07:42:59 AM »

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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Bob K

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Re: Kv and volts (Brushless Motors)
« Reply #8 on: January 28, 2016, 08:32:19 AM »

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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inertia

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Re: Kv and volts (Brushless Motors)
« Reply #9 on: January 28, 2016, 09:08:10 AM »

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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canabus

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Re: Kv and volts (Brushless Motors)
« Reply #10 on: January 28, 2016, 09:31:35 AM »

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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Subculture

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Re: Kv and volts (Brushless Motors)
« Reply #11 on: January 28, 2016, 09:36:10 AM »

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.

Tug Fanatic

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Re: Kv and volts (Brushless Motors)
« Reply #12 on: January 28, 2016, 09:58:18 AM »

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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canabus

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Re: Kv and volts (Brushless Motors)
« Reply #13 on: January 28, 2016, 10:03:55 AM »

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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canabus

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Re: Kv and volts (Brushless Motors)
« Reply #14 on: January 28, 2016, 10:06:37 AM »

Yes Tug
They like to rev they heads off !!!
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Bob K

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Re: Kv and volts (Brushless Motors)
« Reply #15 on: January 28, 2016, 12:23:49 PM »

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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inertia

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Re: Kv and volts (Brushless Motors)
« Reply #16 on: January 28, 2016, 01:51:24 PM »

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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AlexC

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Re: Kv and volts (Brushless Motors)
« Reply #17 on: January 28, 2016, 01:58:30 PM »

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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tsenecal

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Re: Kv and volts (Brushless Motors)
« Reply #18 on: January 28, 2016, 03:21:12 PM »

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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Bob K

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Re: Kv and volts (Brushless Motors)
« Reply #19 on: January 28, 2016, 03:46:47 PM »


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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unbuiltnautilus

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Re: Kv and volts (Brushless Motors)
« Reply #20 on: January 28, 2016, 04:22:30 PM »

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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inertia

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Re: Kv and volts (Brushless Motors)
« Reply #21 on: January 28, 2016, 04:32:05 PM »

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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unbuiltnautilus

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Re: Kv and volts (Brushless Motors)
« Reply #22 on: January 28, 2016, 05:34:52 PM »

Seems a reasonable assumption......
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The Wizard

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Re: Kv and volts (Brushless Motors)
« Reply #23 on: January 28, 2016, 10:46:16 PM »

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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tsenecal

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Re: Kv and volts (Brushless Motors)
« Reply #24 on: January 28, 2016, 10:51:36 PM »

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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