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Old 05-18-2010, 09:24 PM   #1
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Default Brushless motor current draw

I am wondering what the current draw difference is between different KV motors geared to reach same speed.

example losi 8 aiming for 35mph running 4s lipo

1350kv geared 45/21=34.4mph
2200kv geared 45/13=34.7mph

These two setups make the same speed but what difference of amp draw would the esc be seeing? Also whould one system see more heat than the other?

I understand that the 1350 running 6s lipo will have less amp draw and run cooler with longer runtime but I am just wondering why buy a higher kv motor and gear lower if a low kv motor can be geared for same top speed.
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Old 05-18-2010, 10:22 PM   #2
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All depends what u want it for

2200kv usually is for 4s setup & it's legal for buggy race.

1350kv setup usually is for truggy setup as if under unlimited class so anything goes.

Some ppl like to use 6s on buggies as they can get longer run time.

Some buggies can take saddle setup so 6s is possible but most can't.

Some racers hv buggies n truggies so they like to use all 4s setup can save $.

I personally prefer 4s on buggies as it's lighter n 6s on my truggy as I like more power n run cool.

4s on truggies runs hot n very heavy amp draw eg 350-400amp per min whereas 6s is abt 200-250amp per min.

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Originally Posted by adubaz7 View Post
I am wondering what the current draw difference is between different KV motors geared to reach same speed.

example losi 8 aiming for 35mph running 4s lipo

1350kv geared 45/21=34.4mph
2200kv geared 45/13=34.7mph

These two setups make the same speed but what difference of amp draw would the esc be seeing? Also whould one system see more heat than the other?

I understand that the 1350 running 6s lipo will have less amp draw and run cooler with longer runtime but I am just wondering why buy a higher kv motor and gear lower if a low kv motor can be geared for same top speed.
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Old 05-18-2010, 10:42 PM   #3
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Quote:
Originally Posted by adubaz7 View Post
I am wondering what the current draw difference is between different KV motors geared to reach same speed.

example losi 8 aiming for 35mph running 4s lipo

1350kv geared 45/21=34.4mph
2200kv geared 45/13=34.7mph

These two setups make the same speed but what difference of amp draw would the esc be seeing? Also whould one system see more heat than the other?

I understand that the 1350 running 6s lipo will have less amp draw and run cooler with longer runtime but I am just wondering why buy a higher kv motor and gear lower if a low kv motor can be geared for same top speed.
since the other guy didn't really answer your question, i'll take a stab at it.

i think they would be very close in regards to average amp draw. however, from a more specific point of view, i have a feeling they would vary in different situations. the 1350kv would probably do better on a tighter course with a lot of hairpins and not much straight away. the 2200kv would do better on a more open track.

my thinking is this, the 1350 will have an easier time winding up even when gearing up like that, but will have to keep pushing through the entire rpm range, where as the 2200 will draw more current from low down, but once it winds out a bit, the top end will come easier to it. going with a higher or lower kv motor would really depend on the track you plan on running at and finding a good middle ground for the best gearing flexibility.

i'd like to point out that i'm going purely on what makes sense to me. i don't have experience and haven't really researched this. i'm very curious what others say.
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Old 05-18-2010, 10:51 PM   #4
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Err it's a big diff in temp n amp draw I ran many setups before including 4s n 6s that y I know.

6s has more power/torque as it has more voltage so makes everything easy to drive. power on demand!

The 4s setup motor n batt will run hotter. suitable for lighter cars

For shorter race 4s is lighter but 6s I use lower Mah so d weight is d same.

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Originally Posted by LunchboX3904 View Post
since the other guy didn't really answer your question, i'll take a stab at it.

i think they would be very close in regards to average amp draw. however, from a more specific point of view, i have a feeling they would vary in different situations. the 1350kv would probably do better on a tighter course with a lot of hairpins and not much straight away. the 2200kv would do better on a more open track.

my thinking is this, the 1350 will have an easier time winding up even when gearing up like that, but will have to keep pushing through the entire rpm range, where as the 2200 will draw more current from low down, but once it winds out a bit, the top end will come easier to it. going with a higher or lower kv motor would really depend on the track you plan on running at and finding a good middle ground for the best gearing flexibility.

i'd like to point out that i'm going purely on what makes sense to me. i don't have experience and haven't really researched this. i'm very curious what others say.
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Last edited by SuperEk4; 05-18-2010 at 11:05 PM.
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Old 05-18-2010, 11:15 PM   #5
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Originally Posted by SuperEk4 View Post
Err it's a big diff in temp n amp draw I ran many setups before including 4s n 6s that y I know.

6s has more power/torque as it has more voltage so makes everything easy to drive. power on demand!

The 4s setup motor n batt will run hotter. suitable for lighter cars

For shorter race 4s is lighter but 6s I use lower Mah so d weight is d same.
i think you may have read the question wrong. it had nothing to do with the battery choice. most people here know that a low kv motor on 6s will be more efficient than the converse....he even states that in the original post. he wanted to know which setup would be more efficient on 4s and why people don't just use lower kv motors and gear them up to the appropriate speed.
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Old 05-18-2010, 11:32 PM   #6
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Originally Posted by LunchboX3904 View Post
i think you may have read the question wrong. it had nothing to do with the battery choice. most people here know that a low kv motor on 6s will be more efficient than the converse....he even states that in the original post. he wanted to know which setup would be more efficient on 4s and why people don't just use lower kv motors and gear them up to the appropriate speed.
Oic sorry man. About that 4s n low kv thingy if he gear up to the speed he want abt 35mph. The batt n motor will run excessive hot as it's overgear. Every motor kv has sweet spot gearing not undergear or overgear.
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Old 05-18-2010, 11:42 PM   #7
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Originally Posted by SuperEk4 View Post
Oic sorry man. About that 4s n low kv thingy if he gear up to the speed he want abt 35mph. The batt n motor will run excessive hot as it's overgear. Every motor kv has sweet spot gearing not undergear or overgear.
no problem dude

i've never run 1/8 so listen to this guy......now that he sees your concern
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Old 05-19-2010, 12:20 AM   #8
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The other thing to take into account is the motor internal resistance. Not everyone spec's it, but it is very important. In the same size can, and wind type ("Y" or Delta) the lower the KV means more turns of thinner wire, so higher internal resistance. So what you end up with is more heat lost to copper resistance as the motor KV goes down. When you go to a higher voltage, the current falls, but staying 4S, the lower KV motor will likely run hotter. But this higher internal resistance will also give the trigger a softer feel, it won't jump as hard for a given trigger pull, so it may be easier to drive in a tight section. A lower rpm motor has another small drawback. Assuming both 4 pole motors, the higher rpm motor will put out more power pulses per foot of travel on the ground. Almost like a V12 vs a V8. Of course the nitros only get one power pulse per engine revolution, we get 12.

Here is a little math for the geeks in here.
Using specs from the Neu 1415 series. The 2200 1Y is 0.006 ohms.
The 1500 is a 1.5 Y with a resistance os 0.011, almost double.

At 120 amps (I see this every time I get on it down the straight)

4S @ 120 amps = 1776 watts going in
2200 KV
0.006 x 120 x 120 = 86.4 watts of heat in the winding due to resistance
1689.6 watts left to turn the shaft and feed the other losses

1500 KV on 4S, pulling the same 120 due to gearing up for the same speed
0.011 x 120 x 120 = 158.4 watts of heat, OUCH!!
1617.6 watts getting to the shaft

Now if you go to 6S on the 1500, the current is just 80 amps for the same watts
22.2 x 80 = 1776
0.011 x 80 x 80 = 70.4 watts lost to heat in resistance

Almost a perfect match. Both setups turn close to the same rpm again.
2200 x 4S (14.8) = 32560 rpm
1500 x 6S (22.2) = 33300 rpm

So far, from my testing and observing the other fast guys, the old rule of targetting a motor rpm close to 30,000 seems to work very well. I was running a 3300 KV motor on 4S, it was in spec at 48,840 rpm, but I was at the end of the possible gearing in the car. It worked,m the car was rocket fast and had excellent trigger feel. Motor temp was good too. The only problem I had wih it was it had a 1/8 inch shaft. Oops. Yup, I snapped it off. I am trying to get a replacement 5mm shaft rotor for it, but it is a Medusa, and Neu is just getting some parts in now and it won't be cheap.
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Old 05-19-2010, 12:36 AM   #9
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Thx for your professor advise. Sound good! Btw my 1515/3d is with neu under repair takes damn long.

N your 1415 neu sure cost damn alot man!


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Originally Posted by GSMnow View Post
The other thing to take into account is the motor internal resistance. Not everyone spec's it, but it is very important. In the same size can, and wind type ("Y" or Delta) the lower the KV means more turns of thinner wire, so higher internal resistance. So what you end up with is more heat lost to copper resistance as the motor KV goes down. When you go to a higher voltage, the current falls, but staying 4S, the lower KV motor will likely run hotter. But this higher internal resistance will also give the trigger a softer feel, it won't jump as hard for a given trigger pull, so it may be easier to drive in a tight section. A lower rpm motor has another small drawback. Assuming both 4 pole motors, the higher rpm motor will put out more power pulses per foot of travel on the ground. Almost like a V12 vs a V8. Of course the nitros only get one power pulse per engine revolution, we get 12.

Here is a little math for the geeks in here.
Using specs from the Neu 1415 series. The 2200 1Y is 0.006 ohms.
The 1500 is a 1.5 Y with a resistance os 0.011, almost double.

At 120 amps (I see this every time I get on it down the straight)

4S @ 120 amps = 1776 watts going in
2200 KV
0.006 x 120 x 120 = 86.4 watts of heat in the winding due to resistance
1689.6 watts left to turn the shaft and feed the other losses

1500 KV on 4S, pulling the same 120 due to gearing up for the same speed
0.011 x 120 x 120 = 158.4 watts of heat, OUCH!!
1617.6 watts getting to the shaft

Now if you go to 6S on the 1500, the current is just 80 amps for the same watts
22.2 x 80 = 1776
0.011 x 80 x 80 = 70.4 watts lost to heat in resistance

Almost a perfect match. Both setups turn close to the same rpm again.
2200 x 4S (14.8) = 32560 rpm
1500 x 6S (22.2) = 33300 rpm

So far, from my testing and observing the other fast guys, the old rule of targetting a motor rpm close to 30,000 seems to work very well. I was running a 3300 KV motor on 4S, it was in spec at 48,840 rpm, but I was at the end of the possible gearing in the car. It worked,m the car was rocket fast and had excellent trigger feel. Motor temp was good too. The only problem I had wih it was it had a 1/8 inch shaft. Oops. Yup, I snapped it off. I am trying to get a replacement 5mm shaft rotor for it, but it is a Medusa, and Neu is just getting some parts in now and it won't be cheap.
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Old 05-19-2010, 04:34 AM   #10
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Originally Posted by adubaz7 View Post
I am wondering what the current draw difference is between different KV motors geared to reach same speed.

example losi 8 aiming for 35mph running 4s lipo

1350kv geared 45/21=34.4mph
2200kv geared 45/13=34.7mph

These two setups make the same speed but what difference of amp draw would the esc be seeing? Also whould one system see more heat than the other?

I understand that the 1350 running 6s lipo will have less amp draw and run cooler with longer runtime but I am just wondering why buy a higher kv motor and gear lower if a low kv motor can be geared for same top speed.
The problem is the RPMs of the 1350 will be too low and while you can achieve the same speed, you may wind up drawing more amps because the motor has to work harder to pull the taller gearing.

I have tried this and that was what I found. Most of last year I ran 5s/1550kv in my truggy which is 28.6k RPMs. It runs perfectly. Great run times and I can easily make a 15 minute main with a 5s 5000 pack on a big track.

Thinking I could get better run time, I tried a 1350kv motor on 5s and geared up to keep the top end the same, so the RPMs are down around 25k. I got worse run time! I couldn't make a 15 minute main anymore! I figured out that the low RPM crossed the threshold of the most efficient operation and power needed to really push the car effectively.

With that said, when I went to 6s with the same 1350kv motor in the truggy, my run time increased quite a bit over the 5s/1550 setup. So 5s/1350 is less efficient than 5s/1550. But 6s/1350 is more efficient than 5s/1550 from what I have seen.

The easy thing to do is to keep the RPMs around 28-33k regardless of what voltage you use and the results will be optimal.
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Old 11-07-2010, 07:50 AM   #11
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i can see that theyre some very smart people in here and your talking about what i need to know so here it is ive got a 60amp esc that bursts to 80amps running a big brushless inrunner rated for 2900kv for power ive got a 15c 4000mah 3s lipo that bursts to 100amp for 12sec with this set up running a small prop do you think i am at risk of burning something ? and fyi this application is in a boat with 1/8 flex shaft (motor esc and lipo)-will all be water cooled any help would be great
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Old 11-07-2010, 10:45 AM   #12
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i can see that theyre some very smart people in here and your talking about what i need to know so here it is ive got a 60amp esc that bursts to 80amps running a big brushless inrunner rated for 2900kv for power ive got a 15c 4000mah 3s lipo that bursts to 100amp for 12sec with this set up running a small prop do you think i am at risk of burning something ? and fyi this application is in a boat with 1/8 flex shaft (motor esc and lipo)-will all be water cooled any help would be great
the only way a battery can fry an esc or motor is if the voltage (cell count) is too high. the amp rating of a battery is what it's capable of....not what it's providing constantly. the motor will pull only the amps needed.

think of it like this.... say you've got a tank of water with a valve at the bottom. voltage is like the pressure put on the valve that will make the water run out when opened. amperage would be equivalent to how much the valve is opened. you decide how far the valve opens and ultimately how much water comes out, but there's a constant pressure on that valve. does that make sense? you can also understand how too much voltage can kill an esc/motor in the same way too much water pressure on the valve will cause it to fail.

quite possibly the best analogy i've ever come up with
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Old 11-07-2010, 01:30 PM   #13
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yea thats the best termanology ive ever heard lol but how do i determine how much water my valve can handle safely ?
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Old 11-07-2010, 02:41 PM   #14
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yea thats the best termanology ive ever heard lol but how do i determine how much water my valve can handle safely ?
There's a few template u can use but it all depends on a few things

1. Type of car
2. Run time
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Old 11-07-2010, 08:19 PM   #15
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yea thats the best termanology ive ever heard lol but how do i determine how much water my valve can handle safely ?
While a boat is technically "Off-Road" you probably won't get the answer you are looking for here, as this is a car forum. You might have more luck here.
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