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Old 02-17-2013, 10:55 AM   #1
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Default Motor RPM

Can someone please explain if i am on the right wave length or a bit confused, i am not sure .

To use the lower RPM range of a motor - you gear up and use less timing
To use the upper RPM range - you gear down and use more timing

So if a motor is very torquey or you are using a high torque rotor you would gear up?

and if you were using the opposite you would gear down?

So say for instance i had a said motor set at 30 degrees timing and 30 degrees esc timing on the same track at a certain gearing it would produce a certain (RPM - max) for example 50,000rpm. if i geared down so a smaller pinion would this mean that the max rpm achieved by the motor on the same track would increase? and decrease if i geared up

Hope this makes sense

Thanks

Nathan
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Old 02-17-2013, 12:38 PM   #2
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Originally Posted by Nathan Wilson View Post
Can someone please explain if i am on the right wave length or a bit confused, i am not sure .

To use the lower RPM range of a motor - you gear up and use less timing
To use the upper RPM range - you gear down and use more timing

So if a motor is very torquey or you are using a high torque rotor you would gear up?

and if you were using the opposite you would gear down?

So say for instance i had a said motor set at 30 degrees timing and 30 degrees esc timing on the same track at a certain gearing it would produce a certain (RPM - max) for example 50,000rpm. if i geared down so a smaller pinion would this mean that the max rpm achieved by the motor on the same track would increase? and decrease if i geared up

Hope this makes sense

Thanks

Nathan
try starting with this. it is confusing at times .
Understanding R/C Brushless Motor Ratings


kV Ratings
Ok, lets start with kV ratings. The letters kV stand for the the RPM of your motor per volt with no load. For example if you own a brushless motor with a kV rating of 4600 and 12V. Take the 4600, multiply by 12 to get 55,200 RPMs. This is the max RPMs that this motor can reach under no load. Once you get it inside your vehicle, this will come down due to friction.
Almost all brushless motors will have the kV ratings stamped somewhere on them. Some motors will have kV ratings on the motor can, others on the motor leads, but some you will only see on the motor�s spec sheet.
Ok. now you are this far, so what does this mean to you really?
• A motor with a higher kV will have more top end speed, but not as much acceleration/torque.
• A motor with a lower kV will not be as fast, but will accelerate faster.
So, now you can decide which one works best for your kind of racing. You have the room to really crank it up and reach top speeds? A higher kV will get you there. But maybe you are on a shorter track, and what you want is acceleration out of the corners, then look for a lower kV number. Still not sure which way to go? Try something in the middle!
Note: If motor heat is an issue then a lower kV rating with a higher voltage battery will give you the same effect.
The big thing to remember when using kV for your Brushless Motor Ratings is that your Brushless Motor and ESC will each have a maximum input voltage (battery cell count) that is allowed. So if either your motor or ESC has a lower maximum voltage then you must use this to calculate your top RPMs. If you go over the recommended voltage then you have a high chance that something will fry in your setup.

Motor Turns
Motor Turns is the same for brushed motors and brushless motors. The word turns stands for the amount of wire windings around each of the motor's rotor poles.
• The higher the number of wirings/turns means less top speed, but higher acceleration/torque.
• The lower the number of turns equals higher top end speed and lower torque/acceleration.
• A motor with a turn rating of 5.5 will have less acceleration/torque but higher top speed than a motor with a 12 turn rating.
Current Rating - Amps
It is a great idea to find an ESC that has a current rating that is higher than your motor�s by at least 20%. It will be a good safety cushion to make sure that you don�t burn up your brushless power plant.
Here's why: the max current rating is the maximum amount of current that a motor is able to handle safely. This current is measured in Amps. The continuous current rating of a motor is the Amps that a motor can handle safely over a long period of time.
The estimated current rating of a motor is usually on the factory specs sheet. However other factors affect the actual current that a Brushless motor will draw. Things like the kV rating, battery voltage, how heavy the RC vehicle is, and gear ratio or prop size. The harder a motor needs to work to reach it's top speed, the higher the Amp draw is.

Watts
Watts are the power rating or the horsepower equivalent of your brushless RC Motor. The math here is Amps x Volts = Watts. You will see a watt rating in the brushless motor specs. Your brushless motor should have a watt rating on its spec sheet, something like "180W". This is the amount of "horse power" that it should produce safely. Running anything over this rating could damage your motor, especially over a long period of time.

Motor Efficiency
The efficiency of a motor determines its quality. Higher efficiency means better design and high quality components. The higher the efficiency of the motor the more power it can produce before it overheats. A 70% efficient motor produces 70% power and 30% heat. A 85% efficient motor produces 85% power and 15% heat. If your battery is sending the ESC 180 watts, your motor will produce 153 watts (85%), the rest is gold ole heat. 27 Watts of heat will melt solder with some soldering irons, so, that is a lot of wasted watts!
A cooler running motor will give you much less trouble. To reduce heat you can change your gearing or prop size, use a more efficient motor, reduce your voltage or amps, or try a motor heat sink and motor fan. Keeping the heat down on your motor allows it to run longer, and give you the power it needs.. Efficiency matters folks.
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Old 02-18-2013, 02:25 AM   #3
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Thanks for that, so does this mean that you should neevr let your motor rev past its stated range? or is that just where its best performance is?

Thanks
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Old 02-18-2013, 04:49 AM   #4
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Quote:
Originally Posted by StevenS View Post
The higher the number of wirings/turns means less top speed, but higher acceleration/torque.

The lower the number of turns equals higher top end speed and lower torque/acceleration.

A motor with a turn rating of 5.5 will have less acceleration/torque but higher top speed than a motor with a 12 turn rating.
This is incorrect. Alll other things being equal, a motor with lower turns will have both higher speed AND higher torque than a motor with higher turns. The general rules are:

Motor speed (kV rating) is inversely proportional to the number of turns. A 5.5 turn motor will have a no-load speed about twice that of a 12 turn motor.

Motor torque is inversely proportional to the number of turns. A 5.5 turn motor will have about twice the torque as a 12 turn motor.

Motor power is inversely proportional to the number of turns squared. A 5.5 turn motor will have about four times the power of a 12 turn motor.

Quote:
Originally Posted by Nathan Wilson View Post
...so does this mean that you should neevr let your motor rev past its stated range? or is that just where its best performance is?
Very high speed motors can be damaged by free-running at maximum RPM, but most motors don't fit in this category.

The horsepower peak for a permanent magnet DC motor occurs at 1/2 of the no-load RPM, and the efficiency peak is at a somewhat higher RPM. Generally, you would gear a low-power system (high turn-count motor, low battery voltage) to keep it close to its horsepower peak for the majority of the lap, and a high-power (low turn-count motor, high battery voltage) system near its efficiency peak.

For most systems, temperature is the limiting factor, and the gearing is set to keep the motor at a safe temperature. NEVER exceed the manufacturer's specification for maximum temperature!
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Last edited by howardcano; 02-18-2013 at 11:03 AM.
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