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Old 11-02-2016, 05:23 AM   #1
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Default Motor Differences

I have a question about motors, say you have two identical setups the only difference is one motor is 600kv and the other is 800kv.
Will one pull more amps than the other under the exact same load? What would be the differences other than the 200kv?
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Old 11-02-2016, 07:50 AM   #2
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The difference is in RPM primarily, but it is not clear what the current would be. A low rpm (kV) motor will be slower at the same gearing, but that only holds at now load. In real life, things don't always work out that way. A lower kv motor might pull better torque, so you could gear it up and it would hit the same speed but at low RPM.

Conversely, if your gearing is wrong for the motor/track, your motor may never actually reach the top RPM and it will draw a lot of current (in other words it will be labouring).

Then you have timing thrown in if the motor allows adjustment.

Then you have different number of poles, and that changes parameters again.

Perhaps it is best to experiment with motors made by the same manufacturer to see what the difference is for yourself.

Or ask about specific motors. A lot of people here have motor dyno setups and they might have tested the motors you are interested in. Some people have put data like that on the internet as well.

What is important is to know your track and your car and the power and torque band of your motor and make sure you're going to stay in it around the track. If anyone has data they might be able to tell you about some specific motor. What you want is to have torque when you exit the corner so you can accelerate and hit peak power down the straight when you need speed.

Keep in mind, a lot of people make the mistake of gearing up in search of speed, not realising they are taking their motor out of the peak torque/power band and are surprised when they gear down and go faster.
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Old 11-02-2016, 11:03 AM   #3
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The difference is in RPM primarily, but it is not clear what the current would be. A low rpm (kV) motor will be slower at the same gearing, but that only holds at now load. In real life, things don't always work out that way. A lower kv motor might pull better torque, so you could gear it up and it would hit the same speed but at low RPM.

Conversely, if your gearing is wrong for the motor/track, your motor may never actually reach the top RPM and it will draw a lot of current (in other words it will be labouring).

Then you have timing thrown in if the motor allows adjustment.

Then you have different number of poles, and that changes parameters again.

Perhaps it is best to experiment with motors made by the same manufacturer to see what the difference is for yourself.

Or ask about specific motors. A lot of people here have motor dyno setups and they might have tested the motors you are interested in. Some people have put data like that on the internet as well.

What is important is to know your track and your car and the power and torque band of your motor and make sure you're going to stay in it around the track. If anyone has data they might be able to tell you about some specific motor. What you want is to have torque when you exit the corner so you can accelerate and hit peak power down the straight when you need speed.

Keep in mind, a lot of people make the mistake of gearing up in search of speed, not realising they are taking their motor out of the peak torque/power band and are surprised when they gear down and go faster.
Thanks so much for the reply, I should have given more info. Ok we are talking about drag racing here 132'. I am currently running the 600kv motor on 12s and just wanted to know what I can expect just going to the 800kv version of this same motor??? Will it draw more or less amp everything being the same???
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Old 11-02-2016, 01:25 PM   #4
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Same gearing ,battery, weight ?
If so it will draw more current and would be faster if your able to supply more current
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Old 11-02-2016, 01:29 PM   #5
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Same gearing ,battery, weight ?
If so it will draw more current and would be faster if your able to supply more current
Same everything just 200 more kv..
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Old 11-02-2016, 08:20 PM   #6
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And you might of course kill something.

You might have to change the gearing (I would actually be pretty sure you will), otherwise I suspect you might burn the motor. This might mean that your motor needs longer to reach the same speed, so you will need to check this against the distance you are racing on. You might reach a higher speed but it's not going to help you if you take longer than your drag race to reach it.

Experiment.

A lot of experimentation is happening in 1/8 electric on road right now with a vast range of motors as I suggested above. Sensored, sensorless, four pole, six pole, etc. What we found on our track was that at 6S, it is better to run a 1900kV motor than a 2200. Speedos tend to go, motors tend to go and so on.

Try and let us know!
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Old 11-03-2016, 04:07 AM   #7
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Well my thinking was that I could get around 8800 more rpm's with the same voltage...
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Old 11-03-2016, 06:03 AM   #8
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You will... but... the motor will draw more power, create more heat, put more strain on the ESC and battery. Also more RPMs could mean less torque. This means at the same gearing, your car could actually be slower, and like mentioned before, burn something up. It's not as simple as just throwing a higher KV motor in a car.

8800 more RPMS from just a 200kv increase? You're running a 44 volt system. Holy crap. If you aren't already pushing the limits of what ever gear you're running, you are definitely close. Then again at those numbers 200kv isn't that much more. But it could be just enough to put things over the top. Is your running gear (batts,ESC,motor) getting really hot now? If it's still cool you might have room to play. If it's really hot with what you have now, then you might be at the top of where you can go without upgrading your electronics
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Old 11-03-2016, 06:46 AM   #9
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I doubt you will see 8800 RPM extra. Like I said, what happens on paper is vastly different to what happens on the ground. I would actually hazard a guess you will start to let the smoke out of things.

If you don't believe, measure max RPM at the wheel with the car off the ground (don't use your actual wheels, because you will probably shred your tyres, use something like a disc with a marker or something so you can measure RPM). Now add another 8800 RPM at the motor and work out what the difference is going to be at the wheel. Convert that to car speed and then try to think if it seems reasonable that it will happen.

Keep in mind that air drag goes up with third power, tranny drag depends on revs (not sure how, but it's for sure worse than linear), and so on. I would say that you already are in the diminishing returns domain right now. A new motor will probably add something insignificant if not straight up burn things.

But you know what they say.

You never know until you try.
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Old 11-03-2016, 06:59 AM   #10
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Originally Posted by theproffesor View Post
You will... but... the motor will draw more power, create more heat, put more strain on the ESC and battery. Also more RPMs could mean less torque. This means at the same gearing, your car could actually be slower, and like mentioned before, burn something up. It's not as simple as just throwing a higher KV motor in a car.

8800 more RPMS from just a 200kv increase? You're running a 44 volt system. Holy crap. If you aren't already pushing the limits of what ever gear you're running, you are definitely close. Then again at those numbers 200kv isn't that much more. But it could be just enough to put things over the top. Is your running gear (batts,ESC,motor) getting really hot now? If it's still cool you might have room to play. If it's really hot with what you have now, then you might be at the top of where you can go without upgrading your electronics
Well exactly, yes I am running 12s and everything runs quite cool. The motor runs less than 120* and the ESC runs less than that, now my batteries are of a concern to me. They are pretty new about 15 cycles Thunder Power 4400 mah 70C, but I am seeing in the logging a voltage drop of 6 volts for every 132' pass. In other words from a dead stop to wide open for 2.5 seconds the voltage goes from 49v down to 43v. This is a concern to me, that's why I was thinking more kv's could help ease the packs some???
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Old 11-03-2016, 07:39 AM   #11
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More KV will be worse on the packs drawing more power. Your line of thinking seems equivalent to saying you are going to drop a v8 in a v6 car to get better mileage.

With the speeds you are running and the power you are putting down that kind of voltage drop will happen.

Look at a real dragster. They burn through several gallons of fuel in a matter of seconds in a 1/4 mile. Meanwhile my car gets 40 miles to the gallon. And goes over 400 miles on a 13 gallon tank.
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Old 11-03-2016, 07:51 AM   #12
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Originally Posted by jcflyer View Post
Well exactly, yes I am running 12s and everything runs quite cool. The motor runs less than 120* and the ESC runs less than that, now my batteries are of a concern to me. They are pretty new about 15 cycles Thunder Power 4400 mah 70C, but I am seeing in the logging a voltage drop of 6 volts for every 132' pass. In other words from a dead stop to wide open for 2.5 seconds the voltage goes from 49v down to 43v. This is a concern to me, that's why I was thinking more kv's could help ease the packs some???
Once you get to top speed.

On the way there, you are going to draw a lot more. Which probably means you're never going to get there.
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Old 11-03-2016, 08:13 AM   #13
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More KV will be worse on the packs drawing more power. Your line of thinking seems equivalent to saying you are going to drop a v8 in a v6 car to get better mileage.

With the speeds you are running and the power you are putting down that kind of voltage drop will happen.

Look at a real dragster. They burn through several gallons of fuel in a matter of seconds in a 1/4 mile. Meanwhile my car gets 40 miles to the gallon. And goes over 400 miles on a 13 gallon tank.
Ok agreed, now what if I added the 800kv motor and adjusted the gearing is it still going to draw more power???
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Old 11-03-2016, 08:15 AM   #14
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Once you get to top speed.

On the way there, you are going to draw a lot more. Which probably means you're never going to get there.
So are you sayingt hat I should stay with the 600kv motor?
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Old 11-03-2016, 08:36 AM   #15
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I am saying you should think about it before you do it (as in work out if there is some theoretical benefit), and try it if you find you can derive any gain.
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