KV, Voltage, and RPM

[Logan's_Rcs]

Hey everyone,

I was curious as to whether or not a brushless motor should always be ran at it's maximum rated voltage. I know that higher voltage geared for the same speed as lower voltage will always run cooler, because it draws less amperage. However, how does motor RPM factor in? For example, I am considering purchasing a TP Power 4060 1350kv and running it on 6s. On 6s, it would give me roughly 30k rpms. However, it would not be running on the maximum voltage of 38 volts, which gives it 51k rpm.

Would this motor run more efficiently on 6s or on 8s? I've heard that the most efficient rpm for a motor is roughly 30k, but then again, I don't know how true this is. I've also heard people state that you should always run a motor at it's maximum voltage and gear for the speed you want that way you get the most power/efficiency out of it. Again, I'm not quite sure how true this is because generally the higher rpm you run a motor on puts more stress on it, and rpm also generates heat.

So, have any of you guys experimented with this? Do you typically run your motors as close to their maximum voltage as you can, or do you have a specific rpm range that you try and shoot for when purchasing a motor?

Thanks!

 

[LibertyMKiii]

Assuming you are talking about general use / bashing then I think lower rpms are ideal.
It seems running a motor at its max makes them run very hot.

There is likely better power and torque at maximum voltage, but heat is always the enemy with electric setups.

 

[Logan's_Rcs]

Assuming you are talking about general use / bashing then I think lower rpms are ideal.
It seems running a motor at its max makes them run very hot.

There is likely better power and torque at maximum voltage, but heat is always the enemy with electric setups.

That's what I was thinking, I would assume that it's a good thing to have some headroom with rpm, but how much is the question.

 

[LibertyMKiii]

That's what I was thinking, I would assume that it's a good thing to have some headroom with rpm, but how much is the question.

There are just so many factors that it is hard to say.

A motor might run cool in a Typhon but run very hot in a Kraton because of the tires. Or possibly the way the operator drives.
Tall grass also can make it work harder.

The motor can size makes a big difference because of the surface area it has to let out the heat. (larger motors will also generate more torque)

One example I like to use is that the 6s line often runs cool in the 1600-1900kv range. It needs to be a larger can and closer to the 1600kv range on 6s if it has big heavy tires and other heavy upgrades like a M2C chassis for example. This is why the HW 4985 (1650kv) is a go to motor for those setups. That setup is running in the 30-35k rpm range.

As an FYI the BLX2050kv / spektrum 2050kv motor tested around 1917 kv on my tester and have seen several others report similar values with their motors.
Those motors run pretty good if the setup is not too heavy and the driver is not running 100% the whole time.
I had a Kraton 6s with M2C chassis running backflip LP tires and the way I drove it the motor never got too hot with the added fans blowing on it.

 

[rotauq]

Serious note:
rpm under load is much lower than theory. 10-20% of your rpm's will be lost so when you aim for a certain rpm do: voltage * kv * 0.8

 

[Garcbomber]

The manufacturer has done testing for temps at various RPM ranges and may possibly share that information if you ask. For most budget branded brushless motors, the best way is to find out yourself as every application will vary.

Here is a quick log snip of my Max 6 G2 combo, state of the art efficiency as I was back and forth full throttle on/off road with a 16lb MT over 60mph on a tired LIPO off of storage charge testing.

 

[Neelis]

 

[dure16]

The manufacturer has done testing for temps at various RPM ranges and may possibly share that information if you ask. For most budget branded brushless motors, the best way is to find out yourself as every application will vary.

Here is a quick log snip of my Max 6 G2 combo, state of the art efficiency as I was back and forth full throttle on/off road with a 16lb MT over 60mph on a tired LIPO off of storage charge testing.

View attachment 331141

HW data logging finally! Is that with the 4990 motor?

There are just so many factors that it is hard to say.

A motor might run cool in a Typhon but run very hot in a Kraton because of the tires. Or possibly the way the operator drives.
Tall grass also can make it work harder.

The motor can size makes a big difference because of the surface area it has to let out the heat. (larger motors will also generate more torque)

One example I like to use is that the 6s line often runs cool in the 1600-1900kv range. It needs to be a larger can and closer to the 1600kv range on 6s if it has big heavy tires and other heavy upgrades like a M2C chassis for example. This is why the HW 4985 (1650kv) is a go to motor for those setups. That setup is running in the 30-35k rpm range.

As an FYI the BLX2050kv / spektrum 2050kv motor tested around 1917 kv on my tester and have seen several others report similar values with their motors.
Those motors run pretty good if the setup is not too heavy and the driver is not running 100% the whole time.
I had a Kraton 6s with M2C chassis running backflip LP tires and the way I drove it the motor never got too hot with the added fans blowing on it.

Does D/Y winding type matter for heat?

It looks like TP offers a 4060 1350kv Y, 1650kv D, and 1750kv Y.

 

[rotauq]

HW data logging finally! Is that with the 4990 motor?



Does D/Y winding type matter for heat?

It looks like TP offers a 4060 1350kv Y, 1650kv D, and 1750kv Y.

A D-wind will ramp up in ampdraw a little more at full load (like speedrunning) which will also translate in heat, but for bashing you shouldnt notice any difference.

 

[Logan's_Rcs]

View attachment 331142

That's insane! Motors temps under 70 degrees?!?!? I wish hobbywing would make a g2 4092 motor for those who can't fit the larger 4990 in their rigs... That would be awesome

HW data logging finally! Is that with the 4990 motor?



Does D/Y winding type matter for heat?

It looks like TP offers a 4060 1350kv Y, 1650kv D, and 1750kv Y.

Yeah they do. I'm just trying to find an efficient setup for summer temps without the use of a fan... I may go 4070 1520kv, although I feel like that's a bit too much for an MT410. I wish I could have the power I have now, same weight, with cooler temps, although I don't think such an option exists...

 

[Garcbomber]

HW data logging finally! Is that with the 4990 motor?

Yes 4990, all instantaneous telemetry with historical graphing on your smartphone's bluetooth/app. Worth the price of admission. To be fair it was 45 degrees ambient yesterday and I have a single 40mm rocket fan running 8.4V.

 

[dure16]

I wish I could have the power I have now, same weight, with cooler temps

I don't think that's how things work, unfortunately.

Motor size is the most important indicator of its power and running temps. You probably won't notice a difference in power/temps if you're running a different motor with the same weight. You may need a 4070, 1717, or 4985 if you want high gearing without a fan.

Keep in mind that sensored motors drive much nicer in the MT410 since it pops wheelies so easily. Being able to ease into the throttle makes it more predictable. I believe you already have an MMX8S right?

That's insane! Motors temps under 70 degrees?!?!?

Look at the timestamps. That was less than a minute of hard running.

 

[SrC]

There are just so many factors that it is hard to say.

A motor might run cool in a Typhon but run very hot in a Kraton because of the tires. Or possibly the way the operator drives.
Tall grass also can make it work harder.

The motor can size makes a big difference because of the surface area it has to let out the heat. (larger motors will also generate more torque)

One example I like to use is that the 6s line often runs cool in the 1600-1900kv range. It needs to be a larger can and closer to the 1600kv range on 6s if it has big heavy tires and other heavy upgrades like a M2C chassis for example. This is why the HW 4985 (1650kv) is a go to motor for those setups. That setup is running in the 30-35k rpm range.

As an FYI the BLX2050kv / spektrum 2050kv motor tested around 1917 kv on my tester and have seen several others report similar values with their motors.
Those motors run pretty good if the setup is not too heavy and the driver is not running 100% the whole time.
I had a Kraton 6s with M2C chassis running backflip LP tires and the way I drove it the motor never got too hot with the added fans blowing on it.

+1.
Yeah I also noted that the 2050 BLX/ FIrma motors only clock out at 1900-1950 kvs. I checked many of them over the years with my BLX motor analyzer. So when calculating, numbers get skewed and less realistic. 1900 kv is probably more efficent running 6s packs anyway. Advertised KV specs are often not accurate with many brands. 100-250 kv difference's might just be splitting hairs. IDK. 1k+ kv's would be significant enough.
The reason is that there is no standard for evaluating KV specs from brand to brand. Eventual production run deviations can and often differ from the actual Whitepaper engineered specs.

 

[dure16]

1900 kv is probably more efficent running 6s

Something I've always been curious about - why do 1/8 race buggies run 1900kv on 4S?

It seems to contradict everything I read about motor RPM and efficiency, but you'd think the racing scene would have the most efficient setups.

 

[Garcbomber]

Here’s my first couple initial testing runs if you’re curious on my max specs, I was factory punch 5 for these on partially used Lipos(Hence voltages beginning).
20/46 gearing with 170mm wheels. Punch above 7 will standing backflip an XLWB without wheelie bar. Obviously more realistic compared to castle's ridiculously inflated amp draw outputs.

 

[SrC]

Something I've always been curious about - why do 1/8 race buggies run 1900kv on 4S?

It seems to contradict everything I read about motor RPM and efficiency, but you'd think the racing scene would have the most efficient setups.

Pro Racers actually have their equipment on the edge to compete hard. Dialed Efficient enough to complete a timed event.
And when running 1900-2250 kv motors on 4s (1/8 Buggies or Truggies), they are only geared for aprox. 35-40 mph tops. Any faster and they can't keep their lines on most confined groomed tracks.
The power band is always somewhere in the middle of this RPM range. Where Torque is always available. An efficent setup, based on the tracks layout.
With bashing, we are usually wide open running. Dumping on the throttle constantly, Max Rpms etc. Heat can become an issue easily. WIth Track running Throttling is very subtle and refined.
How I always seen it.

 

[parcou]

That's insane! Motors temps under 70 degrees?!?!? I wish hobbywing would make a g2 4092 motor for those who can't fit the larger 4990 in their rigs... That would be awesome

Have you looked at HobbyStar 4092 motors? Proven and run great, very happy with the one in my Fireteam. The newer 4292 is the same size can but the newer version too new to have a track record like the 4092.

 

[Logan's_Rcs]

Have you looked at HobbyStar 4092 motors? Proven and run great, very happy with the one in my Fireteam. The newer 4292 is the same size can but the newer version too new to have a track record like the 4092.

Ive thought about them, but if prefer to buy a name brand and have a bit more efficiency. Im sure they are great for the price though

 

[parcou]

Ive thought about them, but if prefer to buy a name brand and have a bit more efficiency. Im sure they are great for the price though

Excellent...

Not promoting YT'ers Rich and RC Dude81 run the heck out of them as well as great reviews from others (owner of JC Juice stands behind his stuff better than HW)...YT'ers cross it over as I do with a HW Max6 or some a Firma 160A.

Best of luck, fun days!!

 

[dure16]

Excellent...

Not promoting YT'ers Rich and RC Dude81 run the heck out of them as well as great reviews from others (owner of JC Juice stands behind his stuff better than HW)...YT'ers cross it over as I do with a HW Max6 or some a Firma 160A.

Best of luck, fun days!!

I am 187% confident Rich and RC dude use those cheap motors because they gear super, super high. I’d be surprised if their motors last five runs. And they want cheap replacements.

Don’t get me wrong though. I run some hobbystar motors. Not the best but they are great for the price.